Download Agronomy of irrigated tea in low elevation growing areas of Sri Lanka

Agronomy of Irrigated Tea in
Low Elevation Growing Areas
of
Sri Lanka
Shyamantha Nelum Bandara
Thesis submitted for the Degree of Doctor of Philosophy
School of Agriculture, Food & Wine
The University of Adelaide
2011 November
Declaration
NAME:………………………………………………………………............PROGRAM:…………………………
This work contains no material which has been accepted for the award of any other degree or diploma in
any university or other tertiary institution and, to the best of my knowledge and belief, contains no
material previously published or written by another person, except where due reference has been made
in the text.
I give consent to this copy of my thesis, when deposited in the University Library, being made available
for loan and photocopying, subject to the provisions of the Copyright Act 1968.
I also give permission for the digital version of my thesis to be made available on the web, via the
University’s digital research repository, the Library catalogue, the Australasian Digital Theses Program
(ADTP) and also through web search engines, unless permission has been granted by the university to
restrict access for a period of time.
SIGNATURE:…………………………………...........................DATE:……………………….....
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Abstract
The low elevation tea growing region of Sri Lanka produces 60% of the national production
and the most limiting factor affecting productivity in this region is the short but intense intermonsoonal dry period. Irrigation, along with other methods like cultivar selection, is likely to
be the best way to mitigate the impact of drought and generally improve productivity.
However the dearth of knowledge of the response, and financial feasibility, of low elevation
tea to irrigation retards its adoption. Therefore the Tea Research Institute of Sri Lanka
(TRISL) funded a series of trials from 2006 to 2009 with the aim to understand the agronomic
and physiological characteristics of low-grown tea in response to irrigation, and the financial
practicality of introducing irrigation in the region.
This aim was achieved through the
following objectives to evaluate: (1) the changes in physiology and yield as affected by the
drought and recovery by irrigation; (2) the environmental parameters that govern the water
use of tea in low elevation growing areas; (Li, Yang et al.) the physiological and yield
responses to different micro-irrigation methods; (4) the effect of soil moisture limitation on
young tea plant growth and physiology; and (5) the practical financial feasibility of irrigating
tea in the region.
Field trials were conducted at the TRISL’s low-country research station at Ratnapura, Sri
Lanka using mainly drip, and to a lesser extent sprinkler, irrigation to evaluate the response
using different tea cultivars. Irrigation was only applied during the inter-monsoonal dry
period of the January to March. Glasshouse experiments with different water stress levels
were conducted specially for evaluating the short term drought effect on the post-nursery
growth stages, which is critical for long term productivity.
The key results to the research objectives were as follows:
1] Key physiological process underpinning yield (Pn, El, gs, Ψ) were depressed even under
drip irrigation. Nevertheless, the drip-irrigated crop had an annual yield increase of 21%
(p<0.001) over the rain-fed control. Higher yields were observed even during non-irrigated
periods as tea plants irrigated from establishment had better root and canopy structure. Strong
cultivar differences were observed. The benchmark high-yielding, but drought-susceptible,
cultivar TRI 2023 yielded 16% more under drip irrigation than the benchmark droughttolerant cultivar TRI 3025. Evaluation of more recently developed cultivars revealed that
irrigation will be a more effective drought mitigation strategy than selection of droughttolerant cultivars.
Among a group of 5 leading new cultivars only TRI 4049 showed
physiological characters preferred for irrigation.
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2] The environmental parameters measured were rainfall, ambient temperature, solar
radiation, vapour pressure deficit and potential evapotranspiration.
Temperature is the
dominant parameter driving transpiration in the wet season (r2=0.62, P=<0.0001) and
suppressing all the other physiological processes in the dry season.
Average daily
transpiration was 2.3(±0.3) and 1.3(±0.2) mm in wet and dry seasons respectively for rain-fed
plants. Irrigated plants showed >100% increase in transpiration than rain-fed plants in dry
season.
3] During a very wet year (2008) sprinkler irrigation resulted in 15% higher assimilation rates
and produced 6% higher yield increase than drip irrigation on TRI 2023. This was due to the
maintenance of 2-40C lower leaf temperature in midday hours than under drip irrigation.
Between 6 to 8% higher water use efficiency was achieved under sprinkler than drip
irrigation.
4] Glasshouse and field experiments emphasised the importance of irrigation for the
establishment of young (<1 year) tea plants. Decreasing the irrigation frequency from daily
irrigation to a 4 day interval reduced stem growth by 20% and root growth by 46%. Deficit
irrigation of 50% for 20 days, reduced the stem and root growth by 34% and 45%
respectively.
In the field, short dry spells (<3 weeks), reduced LAI and root growth.
Establishing plants on mounds, as opposed to conventional flat ground preparation, enhanced
the effect of irrigation to facilitate the growth of more root (12 and 8% increase in fine and
coarse root) and leaf (60% increase in leaf area).
5] The on-farm feasibility of irrigating tea in low elevation areas was evaluated using Net
Present Value (NPV) analysis. The analysis was based on 10 years (1999-2009) of yield data
of TRI 2023 and TRI 3025 under drip irrigation. Long term yield data was not available for
sprinkler irrigation. Sensitivity analysis was applied using variables of wage rate, green leaf
price and discount rate (2007 as base year). Under these assumptions it is highly feasible to
establish TRI 2023 under drip irrigation but not TRI 3025 (NPVs Rs 391779 and Rs -57898
respectively. The threshold levels for feasibility of TRI 2023 are a discount rate of 15%, or a
green leaf price below Rs48. Irrigating TRI 3025 is economically viable only if green leaf
price reaches Rs 65. Cultivar selection is a key factor in the financial success of tea irrigation.
The higher water demand of a sprinkler system may limit its practical application.
The study confirms that irrigation can be used as viable option to improve productivity and to
mitigate short term drought effects in low elevation tea growing areas. It is crucial to irrigate
from establishment of young plants as early growth determines later yield potential. Cultivars
suitable for irrigation are those that are inherently faster growing and less drought tolerant.
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Further studies are encouraged comparing sprinkler and drip irrigation, soil moisture based
irrigation scheduling and different water application rates under drip irrigation.
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Table of Contents
Thesis Declaration ..................................................................................................................... ii
Abstract ..................................................................................................................................... iii
List of Tables ........................................................................................................................... xii
List of Figures ......................................................................................................................... xiii
List of Acronyms ......................................................................................................................xv
Acknowledgement .................................................................................................................. xvi
Chapter 1 Study Overview ........................................................................................................17
1.0 Introduction .....................................................................................................................17
1.2 The Aim ..........................................................................................................................20
1.3 Structure of the thesis .....................................................................................................21
Chapter 2 Sri Lanka Tea Industry–History, Production and Scope for Irrigated Cultivation ..25
2.1
Introduction ................................................................................................................25
2.2
The Emergence of “Ceylon” Tea ...............................................................................25
2.2
Cultivation of Tea ......................................................................................................25
2.2.1 Land suitability ...................................................................................................26
2.2.2 Land preparation and field planting ....................................................................26
2.2.2 Harvesting ...........................................................................................................26
2.2.3 Other agronomic operations................................................................................27
2.3
Tea Growing Areas of Sri Lanka and Productivity Constraints ................................28
2.3.1 General climate of Sri Lanka ..............................................................................28
2.3.2 Tea growing area classification ..........................................................................30
2.3.3 Adapting technology to mitigate drought ...........................................................32
2.4
Conclusion .................................................................................................................34
Chapter 3 The Botany and Physiology of Tea and its Water Relations ...................................35
3.1
Introduction ................................................................................................................35
3.2
Botany of Tea Plant ...................................................................................................36
3.21
Origin and distribution ........................................................................................36
3.2.2 Varietal difference ..............................................................................................37
3.2.3 Growth pattern ....................................................................................................39
3.3
Physiology of Tea Plant .............................................................................................40
3.3.1 Photosynthesis ....................................................................................................40
3.3.2 Plant water relations............................................................................................42
3.4
Climatic Requirement ................................................................................................43
3.4.1 Air temperature ...................................................................................................43
3.4.2 Soil temperature ..................................................................................................44
3.4.3 Vapor pressure deficit .........................................................................................45
3.4.4 Rainfall................................................................................................................45
3.5
Drought in Tea Cultivations ......................................................................................46
3.5.1 Occurrence of the drought ..................................................................................46
3.5.2 Drought in Sri Lankan tea plantations ................................................................47
3.5.3 Tea plant response to drought .............................................................................49
3.5.4 Drought mitigation ..............................................................................................50
3.5.5 Impact of climate change in low elevation tea sector .........................................51
3.6
Irrigation in Tea Plantations ......................................................................................52
3.6.1 Yield response to irrigation.................................................................................53
3.6.2 Effect on tea physiology .....................................................................................54
3.6.3 Varietal difference in response to irrigation .......................................................54
3.6.4 Growth response to irrigation .............................................................................55
3.6.5 Economic evaluation of tea irrigation .................................................................55
3.7
Conclusion .................................................................................................................56
Chapter 4 Experimental Site Description and Location ...........................................................57
4.1
Introduction ................................................................................................................57
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4.2
Experimental Overview ............................................................................................. 57
4.3
Field Experiment Site and Location .......................................................................... 59
4.3.1 Site and soil description ...................................................................................... 59
4.3.2 Climate ................................................................................................................ 62
4.3.2 Rainfall pattern ................................................................................................... 63
4.3.3 Seasonal aridity................................................................................................... 66
4.4
Significance of short rain-free periods in creating water stress Error! Bookmark not
defined.
4.4.1 Daily rainfall intensity ........................................................................................ 67
4.4.2 Water stress coefficient ...................................................................................... 68
4.5
Conclusion ................................................................................................................. 70
Chapter 5 Physiology, Water Use and Yield in Low Elevation Tea–Analyzing with Special
Reference to Dry Period of the Year ............................................................................... 71
5.1
Introduction................................................................................................................ 71
5.2 Experiment 1: Physiological and Yield Performance of Two Contrasting Tea Cultivars
in Response to Irrigation ............................................................................................ 74
5.2.1 Introduction......................................................................................................... 74
5.2.2 Method ................................................................................................................ 74
5.2.2.1
Experimental design and field layout............................................................. 74
5.2.2.2
Tea cultivar, planting and cultural practices .................................................. 75
5.2.2.2
Water application and moisture measurement ............................................... 76
5.2.2.3
Water potential measurements ....................................................................... 77
5.2.2.3
Gas exchange measurements ......................................................................... 77
5.2.2.4
Harvesting ...................................................................................................... 78
5.2.2.5
Stem canker infection..................................................................................... 78
5.2.3 Results................................................................................................................. 78
5.2.3.1
Meteorological conditions during the study period ....................................... 78
5.2.3.2
Leaf water potential ....................................................................................... 80
5.2.3.3
Photosynthesis ................................................................................................ 81
5.2.3.4
Stomatal conductance .................................................................................... 83
5.2.3.5
Leaf transpiration ........................................................................................... 84
5.2.3.6
Diurnal variation of photosynthesis, transpiration and leaf temperature ....... 84
5.2.3.7
Light response ................................................................................................ 87
5.2.3.8
Response to ambient temperature .................................................................. 89
5.2.3.9
Annual yield variation .................................................................................... 90
5.2.3.10 Yield response to climatic factors: ................................................................. 92
5.2.3.11 Stem canker infection..................................................................................... 93
5.2.4 Discussion ........................................................................................................... 94
5.2.5 Summary of Results of Experiment 1 ................................................................. 97
5.3
Experiment 2: Water Use of Tea under Rain-fed and Irrigated Conditions .............. 98
5.3.1 Introduction......................................................................................................... 98
5.3.2 Materials and Method ......................................................................................... 98
5.3.2.1
General experiment details............................................................................. 98
5.3.2.2
Instrumentation .............................................................................................. 99
5.3.2.3
Meteorological data...................................................................................... 100
5.3.2.4
Soil moisture ................................................................................................ 100
5.3.3 Results............................................................................................................... 100
5.3.3.1
Transpiration during wet season .................................................................. 100
5.3.3.1.1
Dry matter production in the wet season ................................................... 102
5.3.3.1.2
Response to environmental variables: ....................................................... 103
5.3.3.2
Transpiration of during dry season ............................................................... 104
5.3.3.2.1
Climate during the study period ............................................................... 104
5.3.3.2.2
Transpiration rate of irrigated and rain-fed tea ........................................ 105
5.3.3.2.3
Dry matter production during dry season ................................................. 106
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5.3.3.2.4
Diurnal variation in transpiration .............................................................106
5.3.4 Discussion .........................................................................................................107
5.3.4.1
Transpiration and dry matter production......................................................108
5.3.5 Summary of Results of Experiment 2 ...............................................................110
5.4
Experiment 3: Physiological Response of New Cultivars to Drought.....................111
5.4.1 Introduction .......................................................................................................111
5.4.2 Method ..............................................................................................................111
5.4.3 Results ...............................................................................................................112
5.4.3.1
Climate during the study period ...................................................................112
5.4.3.2
Soil moisture ................................................................................................112
5.4.3.3
Leaf Transpiration ........................................................................................113
5.4.3.4
Photosynthesis ..............................................................................................114
5.4.3.5
Leaf temperature ..........................................................................................115
5.4.3.6
Water use efficiency .....................................................................................116
5.4.4 Discussion .........................................................................................................118
5.4.4 Summary of Results of Experiment 3 ...............................................................118
5.5
Conclusion ...............................................................................................................118
Chapter 6 Experiment 4: Evaluation of Irrigation Technology ..............................................121
6.1
Introduction ..............................................................................................................121
6.2
Materials and Method ..............................................................................................121
6.2.1 Irrigation application.........................................................................................122
6.2.2 Harvesting and yield factors .............................................................................123
6.2.3 Physiological measurements .............................................................................123
6.2.4 Growth measurements ......................................................................................124
6.2.5 Root measurements ...........................................................................................124
6.3
Results ......................................................................................................................124
6.3.1 Weather during trial period ...............................................................................124
6.3.2 Soil moisture .....................................................................................................126
6.3.3 Photosynthesis ..................................................................................................127
6.3.4 Stomatal conductance .......................................................................................128
6.3.5 Transpiration .....................................................................................................129
6.3.6 Diurnal variation of leaf physiology .................................................................130
6.3.7 Irrigation effect on shoot weight .......................................................................133
6.3.8 Shoot extension rate and shoot count ...............................................................134
6.3.9 Tea yield ...........................................................................................................135
6.3.11 Irrigation water use efficiency ..........................................................................138
6.3.12 Total plant growth .............................................................................................138
6.3.14 Root Density .....................................................................................................139
6.4
Discussion ................................................................................................................140
6.4.1
Relationship between physiology and irrigation method ................................142
6.4.2 Yield response to environmental variables during dry season 2009 ................143
6.4.3 Yield response to irrigation...............................................................................144
6.4.4 Soil moisture variation and plant growth ..........................................................145
6.5
Summary of results ..................................................................................................145
6.6 Conclusion ....................................................................................................................146
Chapter 7 Effect of Short Term Water Stress and Raised Beds on Young Tea Plants ...........147
7.1
Introduction:.............................................................................................................147
7.2
Experiment 5: Effect of Water Stress Duration on Young Tea Plant Growth ........150
7.2.1 Introduction ..............................................................................................................150
7.2.2 Method .....................................................................................................................150
7.2.2.1
Plant material and water application ............................................................150
7.2.2.2
Measurements ..............................................................................................152
7.2.3 Results .....................................................................................................................152
7.2.3.1
Soil moisture ................................................................................................152
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7.2.3.2
Plant growth response .................................................................................. 152
7.2.3.3
Stem and root growth ................................................................................... 153
7.2.4 Discussion ................................................................................................................ 154
7.2.5 Summary of Results ................................................................................................. 155
7.3
Experiment 6: Effect of Partial Irrigation on Young Tea Growth ........................... 156
7.3.1 Introduction.............................................................................................................. 156
7.3.2 Method ..................................................................................................................... 156
7.3.3 Results...................................................................................................................... 157
7.3.3.1
Plant growth response .................................................................................. 157
7.3.3.2
Dry matter production and partition............................................................. 158
7.3.4 Discussion ................................................................................................................ 158
7.3.5 Summary of Results ................................................................................................. 159
7.4
Experiment 7: Effect of Irrigation and Raised Bed on Young Tea Growth ............ 160
7.4.1 Introduction.............................................................................................................. 160
7.4.2 Method ..................................................................................................................... 160
7.4.3 Results...................................................................................................................... 162
7.4.3.1
Rainfall and soil moisture during the study ................................................. 162
7.4.3.2
Soil bulk density........................................................................................... 163
7.4.3.2
Plant height .................................................................................................. 164
7.4.3.3
Growth of branch shoots .............................................................................. 165
7.4.3.4
Leaf area index ............................................................................................. 166
7.4.3.5
Development of stem parts and roots ........................................................... 167
7.4.4 Discussion ......................................................................................................... 168
7.4.5 Summary of results .................................................................................................. 169
7.5
Conclusion ............................................................................................................... 169
Chapter 8 Financial Feasibility of Drip Irrigation in Low Elevation Tea Growing Area ...... 171
8.1
Introduction.............................................................................................................. 171
8.2
Methodology ............................................................................................................ 172
8.2.1 Analytical methods ........................................................................................... 172
8.2.2 Irrigation system and cost estimation ............................................................... 173
8.2.3 Green leaf price and wage rate ......................................................................... 174
8.3
Results and Discussion ............................................................................................ 175
8.3.1 Drip irrigation system cost................................................................................ 175
8.3.2 Green leaf yield................................................................................................. 176
8.3.4 Net present value of installing a drip irrigation system .................................... 176
8.3.5 Internal rate of return (IRR) .............................................................................. 178
8.3.6 Variation in capital cost .................................................................................... 178
8.3.7 Sensitivity to variation in green leaf price and wage rate ................................. 179
8.4
Summary of Results ................................................................................................. 181
8.5
Conclusion .............................................................................................................. 181
Chapter 9 Discussion .............................................................................................................. 183
9.1
Introduction.............................................................................................................. 183
9.2
Tea Plant Response to Water Stress and Irrigation ................................................. 187
9.2.1 Physiological response...................................................................................... 187
9.2.2
Yield response ................................................................................................. 189
9.2.3
Tea Cultivars for irrigation and drought mitigation ........................................ 191
9.3
Water use of tea in low elevation area ..................................................................... 192
9.4
Irrigation System Selection...................................................................................... 193
9.5
Effect on Young Tea Growth .................................................................................. 195
9.5.1 Effect of water stress interval on young tea...................................................... 196
9.5.2 Effect of partial irrigation on young tea ........................................................... 197
9.5.3 Raised beds to enhance irrigation in young tea ................................................ 198
9.5.4 Final comment on irrigation in young tea plants .............................................. 199
9.6
Further agronomic considerations ........................................................................... 199
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9.6.1
Carry-over effects into the wet season ..............................................................199
9.6.2 Irrigation scheduling .........................................................................................200
9.6.3 Shade trees ........................................................................................................201
9.7
Financial Evaluation ................................................................................................202
9.8
Summary ..................................................................................................................204
Chapter 10 Conclusion............................................................................................................205
Appendix 1: Rain Partitioning in a Low Elevation Tea Field ................................................209
Appendix 2: Tea Plant Behavior under Water Stress on Different Temperature Regimes ....217
References ...............................................................................................................................229
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List of Tables
Table 2.1 Number of small holdings and extent in three census .............................................. 31
Table 2.2 Results of drought awareness survey........................................................................ 34
Table 3.1 Characters of some popular tea cultivars cultivated in Sri Lanka ........................... 38
Table 3.2 Characteristics of drought period in some tea areas ................................................ 48
Table 3.3 Characteristics of dry season and wet season in Ratnapura, Sri Lanka ................... 49
Table 4.1 An overview of the experimental parameters ........................................................... 58
Table 5.1 Major soil physical and chemical properties of Experiment 1 site........................... 75
Table 5.2 Water application rates of two cultivars ................................................................... 77
Table 5.3 Meteorological condition during first 10 weeks of 2007 ......................................... 79
Table 5.4 Total tea yield for year 2007 .................................................................................... 92
Table 5.5 Relationship between micrometeorological parameters and yield .......................... 93
Table 5.6 Climate, soil moisture content, transpiration and crop coefficient TRI 2023 ....... 106
Table 5.7 Transpiration efficiency of TRI 2023 . ................................................................... 108
Table 5.8 Climate during February–March 2009 ................................................................... 113
Table 5.9 Leaf temperature at mid-day................................................................................... 116
Table 5.10 Relationship between maximum temperature and photosynthetic rate ................ 117
Table 6.1 Number of irrigation days, amount of water applied and rainfall .......................... 123
Table 6.2 Total made tea production according to irrigation treatment ................................. 137
Table 6.3 Water use productivity during 2008 (Jan-Feb) and 2009 (Jan-Mar) ...................... 138
Table 6.4 Irrigation water use efficiency during 2008(Jan-Feb) and 2009(Jan-Mar) ............ 138
Table 7.1 Major soil chemical constituents of top 20cm soil layer in Field no 01 ................ 151
Table 7.2 Water application rate and total amount of water applied-Experiment 5 ............... 152
Table 7.3 Average volumetric moisture-Experiment 5 ......................................................... 153
Table 7.4 Plant base girth, height, leaf number and branches-Experiment 5 ......................... 153
Table 7.5 Plant stem and root weight and root:shoot ratio-Experiment 5 ............................. 154
Table 7.6 Plant growth characters-Experiment 6 .................................................................. 158
Table 7.7 Plant root and stem weight-Experiment 6 ............................................................. 158
Table 7.8 New branch growth during short dry spell 2008 .................................................... 166
Table 7.9 Dry weight of main stem and twigs ........................................................................ 167
Table 7.10 Fine and coarse root dry weight-Experiment 7 ................................................... 168
Table 8.1 Land extend distribution among tea small holder farmers ..................................... 171
Table 8.2 Investment cost of drip irrigation system ............................................................... 175
Table 8.3 Total annual operational cost ................................................................................. 175
Table 8.4 Yield response of two tea cultivars to drip irrigation ............................................. 176
Table 8.5 Net Present Value of installing drip irrigation system .......................................... 177
Table 8.6 Sensitive analysis of Net Present Value ................................................................ 178
Table 9.1 Study objectives and relevant hypothesis ............................................................... 184
Table 9.2 Experimental summary of results ........................................................................... 185
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List of Figures
Figure 1.1 Use of portable gun sprinkler in low elevation tea field .........................................19
Figure 2.1 Agro-climatic zones of Sri Lanka with district boundaries. ...................................29
Figure 2.2 Total, high , medium and low elevation tea extent ................................................31
Figure 2.3 Total tea production according to elevation ............................................................32
Figure 2.4 Farmer using small lawn sprinkler in Urubokka, Sri Lanka ...................................33
Figure 3.1 Development of tea shoot .......................................................................................40
Figure 4.1 Geographical distribution of major plantation crops in Sri Lanka ..........................60
Figure 4.2 Aerial view of main research field at St. Joachim Estate, Ratnapura .....................61
Figure 4.3 View of sprinkler operation at Experiment 4 ..........................................................61
Figure 4.4 View of drip irrigated and rain-fed TRI 3025 .........................................................62
Figure 4.5 Monthly climate average of St.Joachim Estate, Ratnapura .....................................64
Figure 4.6 Annual rainfall, Ratnapura .....................................................................................65
Figure 4.7 Percentile of annual rainfall distribution ................................................................65
Figure 4.8 Monthly variation of rainfall Ratnapura, ................................................................66
Figure 4.9 Monthly tea crop water requirement, total and effective rainfall ............................67
Figure 4.10 Average rainless days and frequency of drought duration ....................................68
Figure 4.11 Water stress coefficient for young tea ...................................................................70
Figure 5.1 Average rainfall and potential evapotranspiration in 2007 . ..................................79
Figure 5.2 Maximum potential soil water deficit from January to March, 2007 ......................80
Figure 5.3 Leaf water potential of two tea cultivars-Experiment 1 .........................................81
Figure 5.4 Mid-day photosynthetic rate-Experiment 1 .............................................................82
Figure 5.5 Mid-day stomatal conductance-Experiment 1 .........................................................83
Figure 5.6 Leaf transpiration rate-Experiment Experiment 1 ...................................................85
Figure 5.7 Diurnal air temperature, and solar radiation and physiological parameters ...........86
Figure 5.8 Diurnal air and leaf temperature-Experiment 1 .......................................................87
Figure 5.9 Light response of photosynthesis-Experiment 1 .....................................................89
Figure 5.10 Relationship of maximum air temperature and photosynthesis ............................90
Figure 5.11 Average made tea yield during 2007 ....................................................................91
Figure 5.12 Incidence of stem canker . .....................................................................................94
Figure 5.13 Sap flow sensor fixed to a drip irrigated TRI 2023 ..............................................99
Figure 5.14 Rainfall and soil moisture-Experiment 2 (Aug-Nov, 2008) ................................101
Figure 5.15 Transpiration and dry matter production-Experiment 2 (Aug-Nov, 2008) ........103
Figure 5.16 Relationships of transpiration and environmental parameters ...........................104
Figure 5.17 Rainfall, soil moisture and transpiration-Experiment 2 (Feb-Mar, 2009) ..........105
Figure 5.18 Diurnal transpiration-Experiment 2 ....................................................................107
Figure 5.19 Rainfall and soil moisture-Experiment 3(Feb-March, 2009) ..............................113
Figure 5.20 Water use of efficiency-Experiment 3.................................................................117
Figure 6.1 Rainfall during January, 2008 to March, 2009 ......................................................125
Figure 6.2 Daily rainfall and potential evapotranspiration (Janury-February, 2008) .............126
Figure 6.3 Monthly soil moisture-Experiment 4 ....................................................................127
Figure 6.4 Leaf photosynthesis vs irrigation method .............................................................128
Figure 6.5 Stomatal conductance vs irrigation method ..........................................................129
Figure 6.6 Leaf transpiration vs irrigation method .................................................................129
Figure 6.7 Diurnal climate and leaf physiology-Experiment 4 ..............................................131
Figure 6.8 Diurnal air and leaf temperature-Experiment 4 .....................................................132
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Figure 6.9 Shoot weight according to irrigation method ........................................................ 134
Figure 6.10 Yield factors vs irrigation method ....................................................................... 135
Figure 6.11 Tea yield vs irrigation method. ........................................................................... 136
Figure 6.12 Plant growth vs irrigation method ....................................................................... 139
Figure 6.13 Root density vs irrigation method ....................................................................... 140
Figure 6.14 Maximum temperature vs photosynthesis-Experiment 4 .................................... 143
Figure 6.15 Relationship of yield and climate-Experiment 4 ................................................. 144
Figure 7.1 Rainfall and potential evapotranspiration-Experiment 7 ..................................... 162
Figure 7.2 Soil water deficit-Experiment 7 ............................................................................ 163
Figure 7.3 Soil bulk density-Experiment 7 ............................................................................. 164
Figure 7.4 Plant height-Experiment 7 ..................................................................................... 165
Figure 7.5 Leaf area index-Experiment 7 .............................................................................. 167
Figure 8.1 Variation of Net Present Value ............................................................................. 180
Figure 9.1 Sprinkler irrigation in commercial tea field .......................................................... 197
Figure 9.2 Soil disturbance due to rain .................................................................................. 202
xiv
List of Acronyms
Ψ
Leaf water potential (MPa)
Φ
Quantum efficiency (µmol (PAR)-1)
ADB
Asian Development Bank
Dr
root zone water depletion (mm)
E
Transpiration (mm/day)
El
Instantaneous leaf transpiration (mmol H2O m-2s-1)
ET0
Potential evapotranspiration (mm/day)
gs
Stomatal conductance (mol H2O m-2s-1)
I
Incident light intensity (µmolm-2s-1)
IRR
Internal Rate of Return (%)
IRRi
Irrigation water applied (mm)
IWUE
Irrigation water use efficiency (kg/ha/mm)
Kc
Crop coefficient
Ks
Water stress coefficient
LCLWT
Low country live wood termite (Glyptotermes dilatatus)
MARR
Minimum Attractive Rate of Return (%)
N
Sunshine hours
NPV
Net Present Value
PAR
Photosynthetically active radiation (Wm-2)
Pn
Photosynthesis (µmol CO2 m-2s-1)
Rd
Dark respiration
RF
Rainfall (mm)
Rn
Solar radiation (MJm-2)
SHB
Shot hole borer (Xyleborus fornicatus)
SLTB
Sri Lanka Tea Board
Tavg
Average air temperature (0C)
Tmax
Maximum air temperature (0C)
Tmin
Minimum air temperature (0C)
TAW
Total available water in root zone (mm)
TE
Transpiration efficiency (g/mm)
TRISL
Tea Research Institute of Sri Lanka
TSHDA
Tea Small Holder Development Authority
Wi
Instantaneous water use efficiency
VPD
Vapor pressure deficit (kPa)
xv
Acknowledgement
I would like to thank the Government of the Republic of Sri Lanka for supporting this
research. In particular I would like to thank the Hon. Anura Yapa, Former Minister of
Plantation Industries and Dr. Sunil Jayasekara, former Chairman TRISL who activated the
scholarship. I wish to thank Dr. S.S.B.D.G. Jayawardana, Chairman and Research Board for
continuing the scholarship, and Dr. I.S.B. Abeysighe, Director and Dr. K.G. Premathilka,
Head/Agronomy Division, for their logistic and administrative support.
It is the great effort of my team of supervisors who ultimately delivered the product. Dr. Ian
Nuberg with his vast knowledge ranging from highlands of Sri Lanka to the perfect tea cup,
guided me well, and with great patience, particularly through the irrationalities and subtleties
of the English language. I also thank Prof. Janendra de Costa who was always at my rescue
during long field works in Sri Lanka; Dr. A. Ananthacumarasmamy, former scientist at
TRISL helped me with his resources and expertise during field work; and Assoc Prof. Glenn
McDonald for guiding me through critical points. I especially appreciate the moral support of
Dr. Annie McNeill. She, together with Murray, was once offered fee accommodation for my
rescue. I appreciate kind advices of Assoc Prof. Gurjeet Gill as well. A very special thank
should be delivered to Ms Jane Copeland, University of Adelaide, for advising effectively
how to recover from disastrous situations. Dr. John Golding, of the Gosford Horticultural
Institute NSW, is also acknowledged for assistance with quality analysis of tea in a parallel
experiment.
Without the kind patronage of Mr. Somapala Jayasinghe and Mr.Sunil Fernando of Hettipola,
Mr.Shiyabdeen of Asanakotuwa the scholarship would simply not have materialized. Great
thanks are also due to Mr.Palitha Jayasinghe (Dudley ayya)Ja of Chilaw High Court who
voluntarily assisted in processing the bond agreement for the scholarship. Mr.K.G.Piyasena
and Ms.Rohini Dissanayake, TRISL at Talawakalle were very helpful in administration and
financial matters.
Dr. M.A.Wijeratna and Staff-TRILCS and Manager and Staff-St. Joachim Estate, Ratnapura,
were always helpful during the hard field work time. Specially, DWV, Padmini, Noel,
Shantha, Jaliya, Samanthi, Mahinda and all others who were at my call. Nevil B, Wasantha M
and Randika gave much needed technical solutions. I am indebted specially to the field staff
at Ratnapura: Suresh, Sanju, Anura, Siva, ChandraKumar, Devaraj and all who greatly
assisted me in field even during mid night.
During my stay at Adelaide, my friends at Roseworthy Campus in South Australia – Ben,
Mick, Chris, Hugh and Malinee assisted me in employing in their own research projects. In
Adelaide, my friends Laxman, Lakshitha, Dr. Senaka, Nilmini, Pryantha, Rasika, gave much
needed encouragement.
Finally, but most importantly, my two kids, Pasindu and Sandani and loving Manju, endured
great suffering during my involvement in the research work. Manju and kids endured two
long years isolation when I was in Australia. In such instances, I highly appreciate the
assistance provided by both my parents and Manju’s parents.
xvi
Chapter 1
Study Overview
1.0 Introduction
Tea cultivation is the most important plantation industry for Sri Lanka, which is ranked
among the top three tea export nations in the world. Annual tea production of the country is
sensitive to fluctuations in weather patterns. For it to maintain this position Sri Lanka needs
to adapt to changes in climate and market. While world tea production is largely rain-fed,
some countries including Sri Lanka, are attempting to use irrigation technology to maintain
their competitive edge.
Sri Lanka’s total production of made tea was 291million Kg in 2009 while in 2010 it
increased to 329 million Kg, entirely due to favourable weather (SLTB 2011). Total export
income generated through tea export surpassed US $ 1.37 billion in 2010 (Anon. 2011). As
such it contributes to 1.1% of Sri Lanka’s Gross Domestic Production of US $ 49.5 billion
(Central Bank 2011) and provides direct employment for 220,000 people and their close
dependants of nearly a million people (Feizal 2009). This is about 5% of the population
(Central Bank 2011). Tea is grown over 222,000 hectares and the industry is broadly divided
into high-grown (>1200m.a.s.l), mid-grown (600-1200m.a.s.l) and low-grown (<600m.a.s.l)
areas, depending on the altitude of the production range. It is also divided into a plantation
sector and a small-holder sector, based on the ownership of the land. The entire southern
mountainous wet zone region of the Sri Lanka largely depends on low-grown tea as an
employment provider and income generator. This region contributes 60% of the total tea
production (Anon 2009).
The tea industry faces many uncertainties over its survival and sustainability in the face of the
changing global climate, productivity decline in mountainous agriculture lands and shrinking
worker population (Illukpitiya, Shanmugaratnam et al. 2004). Productivity decline is mainly
related with decline in fertility and ageing plant population (De Costa and Sangakkara 2006).
Global climate change is increasingly threatening the survival of present agriculture systems.
Compared with other perennial crops in Sri Lanka tea is particularly vulnerable to climate
change, mainly due to the nature of its physiology and cultivation pattern (Eriyagama,
Smakhtin et al. 2010). It is predicted under a medium global emissions scenario that the mean
temperature during the northeast monsoon and southwest seasons will increase about 2.9°C
and 2.5°C respectively, over the baseline by the year 2100 (IPCC 2001). More frequent and
more severe drought occurrences are also anticipated as the result of climate change (Pandey,
17
Gupta et al. 2003). There are reports that already rainfall patterns have changed in tea
growing areas of Sri Lanka with higher intensity rains and less rainy days (Herath and
Ratnayake 2004). So the overall effect on the local tea industry could be quite negative.
Tea needs a fairly uniform rainfall throughout the year and is very sensitive to ambient
temperature above 350C (Carr and Stephens 1992). Tea growing patterns and its geographical
locations make it difficult to mitigate the effects of climate change. Tea cultivation occupies
fragile mountain soils and is solely depend on rain for fulfilling its water requirement. Fragile
soils and rain dependent soil moisture replenishment make tea one of the most vulnerable
crops in the country in the face of climate change (Eriyagama, Smakhtin et al. 2010). Sri
Lanka experiences a bimodal monsoonal climate with two distinct dry periods. In the
southwest of the island average annual rainfall is in the order 4,000mm. However, the two
dry periods of December to March and July to August can cause severe water stress to crops.
Severe drought1 conditions devastated the Sri Lankan tea industry in 1983 and in 1992
(Fuchs 1989; Wijeratne 1994) and such conditions are expected to occur more frequently.
Even without a scenario like climate change rain-fed cultivation of tea limits its ability to
increase the crop productivity (Mongi, Majule et al. 2010).
Maintaining and improving the productivity of Sri Lanka’s tea sector has encountered
hindrances for some time (Wijeratne and Shyamalie 2009). Average yield is low and falling
(Akiyama and Trivedi 1987). While the focus of research has been in breeding programs and
improved agronomic practices, the conversion of land from rain-fed to irrigated cultivation is
recognized as pivotal for maintaining tea productivity both locally and internationally.
Consequently, the Tea Research Institute of Sri Lanka (TRISL) started preliminary trials on
sprinkler and drip irrigation of tea in 1984 (Ananthacumarswamy, Herath et al. 1985). While
the results were encouraging there was very little adoption of the technology or further
development of scientific investigation. Renewed interest in irrigation arose with the arrival
of many irrigation equipment manufacturers, like Rain Bird (USA), Netafim (Israel) and Jain
Irrigation (India) to Sri Lanka in late 1990s seeking to introduce irrigation technology among
perennial tree industries, especially among tea and coconut cultivations.
Irrigation has been part of TRISL’s long-term research agenda since 1998 (TRISL 1998).
However, the spread of irrigation technology has not occurred among large plantation estates
because of the high capital requirement, water availability and the lack of knowledge. In
1
In Australia the word “drought” is associated with rainless periods that are abnormally extended in the context of long term
averages. Droughts occur over periods of months and years. However, in the Wet Zone of Sri Lanka the word “drought” is
routinely used to refer to rainless periods of only 5 days and more Sumanasena, H. (2008). Effects of Short Dry Spells on
Productivity of some Perennial Spice and Beverage Crop Species. 63rd Annual Sessions - Sri Lanka Association for the
Advancement of Science Colombo, Sri Lanka Association for the Advancement of Science .
18
contrast, the small holder growers, especially in low elevation growing regions, show a keen
interest to adopt irrigation. In particular, they seek to increase the yields of their crops in drier
months and to protect young tea plants from drought damage (Mahinda 2009). Growers with
extent of 2.0 ha or more and with adjacent water sources are faster in adopting this new
technology due to the capital resources they have and due to the freedom of decision making
(Kuehne, Bjornlund et al. 2010). Many such farmers, who also have the luxury of nearby
water source, already practice irrigation manually. Nevertheless by 2009 less than 100
farmers had installed drip or sprinkler irrigation systems (Mahinda 2009). One of the simpler
types of installations is of the portable gun irrigation systems for watering crops during dry
seasons (Figure 1.1).
Figure 1.1 Use of portable gun sprinkler in low elevation tea field in Galle District, Sri Lanka. Farmers use nearby
perennial river (Gin Ganga) for irrigation during dry season
So while there is a very positive groundswell of interest in irrigating tea, there has been no
detailed scientific evaluation of its efficacy in Sri Lanka. Such evaluation has been made in
other tea growing regions like India or East Africa (Hudson 1991; Panda, Stephens et al.
2003; Carr 2010), but to adapt irrigation technology to Sri Lankan conditions we need to
undertake local research. Some of the issues to consider are: the physiological response of tea
to irrigation under Sri Lankan conditions, how other agronomic factors interact with
irrigation, and the financial viability of investing in irrigation technology.
It is already known that drought impacts photosynthesis and transpiration resulting in lower
dry matter production and final made tea yield (Smith, Burgess et al. 1994; Ng'etich and
Stephens 2001). The physiological response of the plant to irrigation will also depend on the
19
tea cultivar, and cultivar differences are a significant factor in determining the final yield
(Burgess and Carr 1996). A wide range of drought tolerant cultivars of tea have been bred for
the Sri Lankan industry (TRISL 2002). However, non-drought tolerant lines are still grown by
the growers, because of faster growth rate and productivity under ideal climatic conditions. It
is important for us to understand the physiological basis of the cultivar differences in response
to irrigation.
Ground management practices are very important in ensuring that water, either as rain or
applied as irrigation, gets to the crop. The root and shoot ratio of the tea plant is helpful in
understanding optimal growth of the plant that ensure long sustainability (Bannerjee 1993). In
this regard, ground management as well the type of irrigation used also needs evaluation to
underline the strategies to achieve best optimal performance of the irrigation systems.
Financial considerations are very important when implementing irrigation in tea growing
areas where bimodal rain pattern prevails (Carr 2010). Drought mitigation by irrigation may
or may not be cost effective. Though there are many unpublished data and industry
communications (TRISL 2003; TRISL 2004) in Sri Lanka claiming yield increases ranging
from 20% to 60%, the return to the investment may vary according to the age of the bush,
severity of the drought, tea cultivar, soil and other management practices.
The appropriate adaptation of irrigation systems to the low-grown tea sector requires a range
of detailed studies.
This thesis presents a collection of such studies in the attempt to
understand the long term viability of establishing irrigation systems in low elevation tea
growing regions of Sri Lanka.
1.2 The Aim
Therefore the aim of this study is to evaluate the effect of short-term water stress on the
agronomic and physiological characteristics of low-grown tea, the responses to irrigation, and
the financial practicality of introducing irrigation in the low elevation tea growing areas of Sri
Lanka.
The specific objectives to achieve this aim are as follows:
A. To quantify the changes in physiology and yield as affected by the water stress and
recovery by irrigation
B. To evaluate the water use of tea and environmental parameters that govern the water
use in low elevation tea growing areas
C. To evaluate plant performance in response to different micro-irrigation methods
D. To quantify the effect of soil moisture limitation on young tea plant growth
E. To undertake a simple valuation of the practical financial feasibility of irrigating tea.
20
1.3 Structure of the thesis
To achieve these objectives this thesis has the following structure.
Chapter 2 provides an essential background to the Sri Lankan tea industry and the
development of interest in irrigation. In particular it presents the history, cultivation pattern,
tea growing areas, productivity constraints and farmer efforts to adapt irrigation to there tea
cultivation.
Chapter 3 provides a review of botany of tea, its physiology and international literature on tea
irrigation. This review indicates that though there are studies related to water stress and
irrigation in tea, there is still a gap in understanding of how tea behaves in hot humid
condition, when subjected to short-term water stress and possible recovery through irrigation.
Chapter 4 describes the field site where the experimental work was undertaken, the St
Joachim Tea Estate of the Sri Lanka Tea Research Institute at Ratnapura. It presents an
overview of all the experiments; their timing, cultivars involved, and measurements
undertaken. This chapter also presents information on long-term weather data at the site,
particularly rainfall variability and its effectiveness to fulfil plant water requirement. It
presents the results of applying a water stress coefficient to rainfall data of 2009 and 2010.
This serves the purpose of showing that water stress is a real and regular occurrence even in
an environment with annual average rainfall of 3824 mm.
Chapters 5 to 8 are collectively the experimental and analytical body of the thesis. Chapter 5
focuses on objectives A and B, while Chapter 6 is objective C, Chapter 7 is objective D and
finally Chapter 8 achieves objective E.
Chapter 5, focusing on the physiology and yield of mature tea stands, is the largest section of
the thesis. It presents in logical (but not temporal) sequence the results of the 3 experiments
(Experiments 1-3) as follows. The hypotheses of each experiment are presented in italics.
Exp 1: Physiological and yield performance of two contrasting tea cultivars in response
to irrigation
H1 – There is a cultivar difference in physiological and yield response to irrigation
H2 - Air temperature is the main environmental factor determining yield
Exp 2: Water use of tea under rain-fed and irrigated conditions
H3 - Transpiration is closely related to the plant productivity and air temperature is
the key environmental factor controlling transpiration
Exp 3: Physiological response of new cultivars to water stress
H4 – Cultivar selection is, by itself, an inadequate strategy to cope with water stress.
21
Chapter 6 focuses on irrigation technology.
Exp 4: Evaluation of irrigation technology where the performance of tea physiology and
yield are evaluated under drip and sprinkler irrigation
H5 - Different micro-irrigation methods differ in their effect on tea physiology and
productivity
Chapter 7 comprises three experiments involving young tea plants where the short dry spell
effect on young tea growth is evaluated in field and glass house conditions.
Exp 5: Effect of water stress duration on young tea plant growth
H6 - For young tea, even short duration water stress retard the plant growth
Exp 6: Effect of partial irrigation on young tea growth
H7 – Optimal growth of young tea can be maintained under partial irrigation
Exp 7: Effect of irrigation and raised bed on young tea growth
H8 - Effect of irrigation on plant growth can be enhanced by lowering the soil
compaction in growth bed.
Chapter 8 is a financial evaluation of drip irrigation, using the yield response of two
contrasting tea cultivars over a ten year period. The question being asked is:
Under what financial conditions is the irrigation of two contrasting tea cultivars
feasible in the low elevation growing areas of Sri Lanka?
Another way of expressing the logical connections between the hypotheses and question listed
above is as follows.
Short-term water stress will have a negative impact on production related key
physiological parameters such as photosynthesis, stomatal conductance, transpiration and
instantaneous water use efficiency and differences will be observed between different
cultivars to their ability to withstand drought (H1).
However, completely drought
tolerant cultivars are not available (H4). Not only long term drought causes significant
yield and plant losses, but even very short duration rain-less periods can cause significant
growth retardation in early growth stages (H6). Even partial irrigation can maintain good
plant growth of young establishing tea (H7)
Ambient temperature is the main driver behind transpiration which determines dry matter
production and increased dry matter production in irrigated tea is related to increased
transpiration (H3). Increase in air temperature is also the main negative environmental
factor for tea yield, though irrigation has the ability to increase dry season tea yield (H2).
In terms of growth response, irrigation effect can be enhanced by removing other
constraints like soil compaction (H8). Type of irrigation technology used (i.e. sprinkler
22
vs drip) also will affect the yield response by nature of different physiological
performance (H5). The important practical question, which is not presented as a
hypothesis, is whether establishment of irrigated tea in low-grown areas of Sri Lanka is
worth the investment by land holders.
Chapter 9 draws all the conclusions from the experimental chapters together and discusses
them in the light of the overall research aim and objectives.
Chapter 10 summarizes the main results and conclusions from the whole thesis.
There are two appendices describing two parallel experiments.
They are relevant to
understanding the climate and tea physiology, especially in relation to the environment of low
elevation tea growing areas of Sri Lanka. However, since they were not directly related to the
research objectives they are not included in the body of the thesis.
23
24
Chapter 2
Sri Lanka Tea Industry – History, Production and Scope for Irrigated
Cultivation
2.1
Introduction
The scope of this thesis is to evaluate the response of tea cultivation to irrigation in low
elevation tea growing areas of Sri Lanka. As the cultivation and industry structure of tea
varies greatly among the tea producing countries of the world, it is important to understand
the specific nature of tea production in this country. This chapter briefly outlines the history
of tea cultivation, cultivation method, recent shifts in production areas and the drought faced
by the low elevation tea growers and their efforts and wishes to test micro-irrigation.
2.2
The Emergence of “Ceylon” Tea
The export of perennial crops like cinnamon from Sri Lanka began with the Portuguese
invaders in 1505. The earlier introduction of perennial cultivations spread mainly around the
coastal areas of the country. After the British took control of the entire country then called
“Ceylon” in 1815 and crops like coffee and rubber were introduced to the inland mountain
areas. Tea plants were first planted in Sri Lanka at the Royal Botanical Garden, Peradeniya in
1839 with seeds were imported from India. James Taylor, who was a Scotch planter and
considered to be the pioneer of Ceylon tea, planted 19 acres (8 ha) at Loolcondera Estate,
Hewaheta. Then curator of Royal Botanical Garden, H.K.Thwaites supplied the Assam origin
seeds. The spread of Coffee Rust (a fungal infection caused by Hemileia vastatrix), identified
in 1869 at a coffee estate in Gampola, paved the way for replacing coffee estates with tea.
The first central tea factory was commenced at Fairyland (Siles, Rey et al.) Estate, Nuwara
Eliya. Tea cultivation exceeded coffee cultivation in 1888 and growing to nearly 166,000 ha
in 1899. From the small beginning of 8 ha, the present tea cultivation in the country is
221,969 ha in 2009 (Forbes & Walker 2010).
2.2
Cultivation of Tea
From the inception of seedling cultivation in 1869, tea is cultivated as a rain-fed cultivation.
Seedling planting was later replaced with vegetative propagated (VP) cuttings raised in a
nursery.
For a successful cultivation suitable land selection, preparation and correct
agronomic practices, including harvesting are very important.
25
2.2.1
Land suitability
Tea is a shade loving plant which requires an annual minimum rainfall of at least 1200mm.
Under Sri Lankan conditions uniform distribution of rainfall throughout the year is also
important for successful cultivation. Due to the necessity of year round precipitation, tea is
cultivated in the central highlands of Sri Lanka and to the wetter western slopes of the
country. Tea plant prefers slight acid soil of pH 4.5-5.5. Well drained soils are preferred for
planting tea. Since the cultivation is entirely ranging in mountainous or at least hilly areas,
land preparation and other cultural operations are mainly done by manual labor.
2.2.2
Land preparation and field planting
As it is not easy to use machinery for the land preparation for planting, the usual method for
land preparation is the cultivation of a grass crop as a soil rehabilitation crop for 1½ to 2 year
period. Either Mana (Cymbopogon confertiflorus) or Guatemala (Tripsacum laxum) grass is
cultivated during the soil rehabilitation period. The grass is periodically cut and lopping is
added to soil as mulch. The usual practice is to plant grasses with onset of the monsoon.
Prior to planting grasses, land is prepared by constructing contour and main drains, and stone
terraces where it is necessary, to prevent the soil erosion.
Field planting of the young plants is done using vegetative propagated (VP) plants, raised in a
nursery for 9-10 months, after the soil rehabilitation period. Nursery plants can be raised
either from a seed or single node cutting. Earlier practice was the planting of tea seedlings or
seeds in the field, but now VP plants are the preferred option. Since 1934, VP plants are used
extensively as they produce higher yield and more uniform growth in the field. The standard
spacing for the planting is 60cm within row and 120cm in between rows. Field planting is
done in accordance with the onset of monsoon rainfall in the country. The majority of tea
growing areas receive the south west monsoon during April and July of the year. Generally,
field planting is started during May after the soil receives enough moisture. However, in the
eastern slopes of hill country areas like Badulla and Passara, field planting is done during
October, where the north east monsoon prevails. After 3 months in the field the plant is
pruned to a height of 30cm to encourage the side branches to grow into a bush.
2.2.2
Harvesting
Harvest is usually commenced after 3 years in field planting.
In cooler environments
harvesting may take more than 3 years of field planting because of the slower growth rate. In
Sri Lanka, manual harvesting, known as plucking, is practiced entirely. The main reason for
manual harvesting is the better selection of fresh young tea shoots leaving mature leaves and
26
immature buds in the plant. This ensures the high quality for which Sri Lankan tea is known.
The usual harvesting period of the plant is 7 days.
The standard harvesting practice is to pluck the shoot with two leaves and the unfurled leaf
(bud). However in practice the harvest can consist of a mixture of one leaf plus bud, two
leaves plus bud or three leaves plus bud. Depending on the weather condition and location of
the shoot in the plant (either in top surface or inside the bush), either single leaf plus bud or 3
leaf plus bud could be harvested. During the high rain months like May, three leaves and
shoots are harvested. Similarly during the dry period, the harvest may be limited to single
leaf and bud largely. During the harvest care is taken so that fresh leaves are at the correct
maturity, proportions and composition. Hard fibrous material develops in tea leaves with
maturity. Hence if the shoots are overgrown, it will decrease the quality of made tea.
2.2.3
Other agronomic operations
Tea is a plant that grows into medium to tall shrub when it is grown freely. However in tea
plantations, the plant is pruned to form a bush. Plants are maintained at a height of around 1m
height. There are three objectives for pruning: (1) To stimulate vegetative growth phase of
the plant to produce more active shoots; (2) To maintain an easy plant height for easy
harvesting and (Li, Yang et al.) To clean infect branches (e.g. shot hole borer infection)
(Danthanarayana 1966).
The pruning period or cycle differs according to the geographic location. In low elevation tea
growing areas, and in many other areas of Sri Lanka, the tea plant is pruned usually every
three years time. But in the very cool climates of high altitudes, the pruning cycle may be
extended, up to 5 years.
Like the field planting, the time of the pruning coincides with the onset of monsoonal rains.
Soil moisture stress causes poor recovery after pruning, sometimes causing death of the plant
as well. Prior to pruning, plants are rested for 4-6 weeks, without plucking as a mean of
preparing the plant for the pruning. During the rest period, dry matter partitioning to roots is
encouraged. Care is taken to protect the pruned plant from high solar radiation by covering it
with pruned branches. Some farmers in low elevation growing areas also apply water to the
recovering pruned plants.
During pruning, it is recommended to fork the soil to reduce the compaction and to increase
rainwater infiltration. Pruned lopping is buried in soil to enhance the soil nutrient and also to
ease the penetration of rain water to the soil. But the practice is largely not followed as it
require high amount of labor.
27
The cultivation of shade trees is widely practiced in Sri Lanka tea fields. Apart from shade
these trees provide some additional advantages such as organic matter addition and soil
moisture conservation during dry periods. Shade trees can be classified as either medium
shade or high shade trees, depending on the canopy height of the shade plant. Albizia
(Falcataria moluccana formerly known as Albicia molucana (ILDIS 2005)) and silver oak
(Grevillea robusta) are two common high shade species, while gliricidia (Gliricidia
maculata) and dadap (Erythrina variegata (syn. E.indica)) are commonly used as medium
shade trees.
2.3
Tea Growing Areas of Sri Lanka and Productivity Constraints
2.3.1
General climate of Sri Lanka
Sri Lanka climate varies strongly over space and time particularly with respect to rainfall.
This is largely due to Sri Lanka’s, location near the equator and the influence of monsoonal
circulation over south Asia (Punyawardena 2004). Among the various climatic regions, south
western quadrant of Sri Lanka has a sharp climatic contrast between rest of the country due to
the presence of central mountainous region, with a peak elevation of 2524m. This is the area
where most of the low elevation tea is located, in the main administrative districts of
Ratnapura, Galle Matara and Kalutara (Figure 2.1).
Sri Lanka has been classified into three climatic zones based on the rainfall distribution
(Figure 2.1). The south western part including central highlands is the Wet Zone. The Dry
Zone predominately covers the northern and eastern parts of the island. The Intermediate
Zone, skirting the central hills, except south and the west, separates the Wet Zone and Dry
Zone. The annual rainfall of the Wet Zone is over 2500mm, while the Dry Zone receives a
mean annual rainfall of less than 1750mm with a distinct dry season from May to September
(Punyawardena 2004). The Intermediate Zone receives a mean annual rainfall between 1750
to 2500mm with a short and less dry season.
28
A
NOTE:
This figure/table/image has been removed
to comply with copyright regulations.
It is included in the print copy of the thesis
held by the University of Adelaide Library.
Figure 2.1 Agro-climatic zones of Sri Lanka with district boundaries. Main low elevation tea growing districts are
Galle, Ratnapura and Matara (Punyawardena 2004)
Temperature regimes in Sri Lanka are also characterized by significant decreases in the
temperature with altitude. There is vertical lapse of temperature, approximately around 5-70C
for every 100m rise in elevation (Punyawardena 2004). There is a considerable variation in
temperature in the Wet Zone. As low temperature is an important factor affecting plant
growth in Wet and Intermediate Zones another classification has been created based on
elevation to classify agro climatic zones within each climatic zone (Figure 2.1). In this
climatic classification, Low country is demarcated as the land below 300m of mean sea level
29
(msl), Mid-country is the area with elevation between, 300-900m, and Up-country is the area
above 900m msl elevation.
2.3.2
Tea growing area classification
Tea growing areas of Sri Lanka are also classified according to the elevation above mean sea
level (amsl). This differs slightly from the agro-climate classification outlined in Figure 2.1.
Tea fields up to 600m amsl are classified as low elevation/country tea. Tea fields in the
elevation range between 600 – 1200m amsl are called mid elevation/country tea. The rest of
the tea fields above 1200m amsl are classified as up country tea. The demarcation of the tea
growing areas by the elevation is not only related to the geographic location, but and for
management purposes as these areas based on elevation creates distinct features of the made
tea quality. Also as the climatic parameters vary with the elevation, so do the growth rate and
the level of environmental stress vary with the growing elevation. In particular relevance to
the low elevation tea areas, it experiences severe drought stress during some part of the year.
When tea was being established as an industry in days of Ceylon the focus was in the mid and
high elevation tea growing areas and to a much lesser extent in low elevation tea growing
areas. Because of this history these larger mid and high elevation estates belong to either
companies or wealthy families and the resident managers of the properties not the owners.
Since the latter part of the 20th century, more tea growing areas emerged in low elevation tea
growing areas of Sri Lanka. In 1992 there was a significant decline in total tea area in Sri
Lanka as a part of a program of privatization and registration of tea fields (Figure 2.2). This
period also witnessed a significant increase in the area of low-grown tea. In 2010, 60% of
total tea production was produced from low elevation tea fields as shown in Figure 2.2 (Anon.
2011).
An important feature of the low elevation tea fields is the higher percentage of land belonging
to small holder tea farmers. Small holder tea farmers are land owners who own less than 20
ha according to Land Reform Act of 1972 (ADB 2000). There has been a steady increase in
the number of small holders and the cultivation extent since 1983. Table 2.1 shows the
increase in land extent and ownership in three main tea growing districts of low elevation
area. With the expansion of the area and ownership, small holder farmers have become the
main producers of the national tea production. For example, in first six months of 2008 small
holder farmers produced 74% of Sri Lanka’s tea (MPISL 2008).
30
A
NOTE:
This figure/table/image has been removed
to comply with copyright regulations.
It is included in the print copy of the thesis
held by the University of Adelaide Library.
Figure 2.2 Total, high , medium and low elevation tea extent from 1959 - 2000 (there was a drastic reduction in
high and medium sector, while increase in low sector after privatization of tea estates in 1992) (Holsinger
2002)
Table 2.1 Number of and extent tea small holdings over three census periods in the Wet Zone of Sri Lanka (FAO
2010)
1983 Census
District
Number
Extent
(ha)
1994 Census
Number
Extent
(ha)
2005 Census
Number
Extent
(ha)
Galle
36,479
13,603
56,547
17,855
90,524
25,325
Matara
27,964
13,342
44,051
16,886
67,613
22,971
Ratnapura
17,713
9,818
49,161
17,789
97,984
28,232
The total tea production of Sri Lanka is largely supplied by low elevation tea growing areas,
throughout the year. As shown in monthly tea production data of 2010 (Figure 2.3), total
production largely followed the production fluctuation of low elevation sector in most
months. Monthly largest tea production in May 2010, was however contributed by the peak
monthly production of mid elevation and high elevation tea growing areas.
31
high
medium
low
Total
40000
Production (made tea '000kg)
35000
30000
25000
20000
15000
10000
5000
0
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Month
Figure 2.3 Total tea production in Sri Lanka classified according to elevation during 2010 (SLTB 2011)
Though there has been this increase in total tea production in low-grown tea areas, yields have
not been consistent with other areas.
A comparison of yields between 2004 and 2008
indicates that production is stagnant at Galle district and there was a 15% reduction of
productivity in small holder tea fields in Ratnapura (SLTB 2010). The reason for loss of the
production in low elevation tea growing areas can be attributed to the age of tea fields
(Wijeratne and Shyamalie 2009), high soil erosion in low elevation tea fields (Ananda and
Herath 2001) and high damage to low elevation tea fields during drought periods (Fuchs
1989).
Raising productivity of small holder fields in low elevation tea growing areas, can be
accomplished by introducing new technology, including improved cultivar and better
management practices.
However, there is a limit to increase the productivity through
improved cultivar introduction, since tea inherently has a limitation to increase productivity,
because of its nature of photosynthesis mechanism (Raghavendra 2003).
Irrigation
technology can be considered as an alternative to cultivar selection for drought mitigation in
the mean time.
2.3.3
Adapting technology to mitigate drought
The technology dissemination of the small holding sector is satisfactory, yet the adaptation of
present soil and moisture conservation practices (without irrigation as a component) is at
32
around 60% adoption level (Jayamanne, Wijeratne et al. 2002). However, it was observed
during the recent short term droughts
that considerable number of farmers
practiced
irrigation as a mean to prevent drought damage and to increase the productivity (Gopal,
Shilpakar et al. 2010). Figure 2.4 indicates the resourcefulness and effort of some farmers to
adapt available technology in an attempt to irrigate tea under stress.
Figure 2.4 Farmer using small lawn sprinkler to irrigate mature tea field during a dry spell (February, 2010) in
Urubokka, Sri Lanka
Also surveys indicate a strong willingness among farmer groups to consider irrigation. Table
2.2 shows the answers of participants at an advisory workshop during a drought awareness
program conducted for tea advisory officers and representatives of small holder tea growing
societies in the low elevation tea growing area (Mahindapala and Bandara 2005). In this
survey 80% of the participants believe that drought affects tea fields and there is a need for
further study about irrigation. Another revelation of this survey was that among the 9% of
crops (mainly young fields) under severe drought stress most farmers used traditional surface
irrigation by directly pouring water to fields using rubber hoses. Only one respondent replied
with using sprinkler and another mentioned about soil injectors.
33
Table 2.2 Results of drought awareness survey. The survey was conducted among tea growers and advisory
officers (n=41) (Bandara and Mahindapala 2009)
A
NOTE:
This figure/table/image has been removed
to comply with copyright regulations.
It is included in the print copy of the thesis
held by the University of Adelaide Library.
2.4
Conclusion
This chapter has outlined the historical, geographical and agronomic specifics of the Sri
Lankan tea industry. It shows that the production frontier of the Sri Lankan tea industry has
shifted from high and mid elevation tea growing areas to low elevation areas. Also the
industry in these areas is dominated by small holder production sytems.
Productivity
constraints in this area are strongly related to short seasonal droughts and with expected
climate change these problems will increase, as shown in Figure 2.3. Efforts are needed do
address the main constraints faced by the tea growers in those areas. Already farmers are
adapting irrigation at a small scale to combat drought. However considerable research is
needed to inform the best ways for farmers to use such technology, if indeed it is worth the
effort. This thesis evaluates the feasibility of introducing irrigation to low elevation tea
growing areas by evaluating the physiological and financial performance of tea. The
following chapter lays out the literature that underpins this research.
34
Chapter 3
The Botany and Physiology of Tea and its Water Relations
3.1
Introduction
Tea growers in all tea growing countries face different types of plant biotic and abiotic
stresses while keeping the cultivation in an economically sustainable level. The plant
experiences several such stresses either concurrently or at different times through growing
season that frequently limit the growth and productivity (Tester and Bacic 2005). Common
abiotic stresses include decreased availability of water, extremes in temperature, decreased
availability of soil nutrients, extreme light or hardness of soil that restrict root growth
(Verslues, Agarwal et al. 2006). It is estimated that abiotic stresses represent serious
limitations to agriculture, more than halving average yields for major crops (DaMatta and
Ramalho 2006). Among them drought is the most important abiotic stress that affects field
grown crops. The tea plant is also subjected to drought under various growing conditions in
different growing environments. Now, drought has become a factor that determine the total
tea production in Sri Lanka as well the long term prevailing of the tea in some areas of the
country (Gunasekara 2010).
Sri Lankan tea cultivation is predominantly rain dependant. In contrast some competitive
countries in Asia and East African countries, to some extent have converted their tea lands
into irrigated cultivations to mitigate drought and to enhance productivity. Up to now drought
mitigation of Sri Lankan cultivation was largely dependent on screening of drought tolerant
cultivars, improvement and implementation of soil conservation measures, proper shade
management and expansion of multi cropping system (Wijeratne 1996; Eriyagama, Smakhtin
et al. 2010). Sri Lanka has invested relatively little into water management and irrigation
compared with east African tea growing areas and some Central Asian tea growing countries
(Carr 2010). This is largely because the Sri Lankan industry has until the last decade been
dominated by mid and high elevation large estates in relatively cooler climes. Now the
industry has a significant small holder sector in low elevation areas where a hot humid
climate prevails with high bimodal rainfall distribution associated with hot short duration dry
spells.
The aim of this chapter is to review the irrigation research in the international arena,
particularly regarding tea. It will identify key issues on which irrigation research in the low
elevation tea sector of Sri Lanka should focus.
35
The opening section summarizes the botany of tea plant including the origin and distribution,
different varieties and growth pattern. The second section will review the physiology of tea
plant particularly relating to the important physiological parameters and their relationship
with water stress. The third section of the chapter will then review the climatic requirements
of the plant. It will summarize the key environmental parameters that affect the productivity
and survival of the plant. The fourth section will review the drought experienced in different
tea growing regions of the world and it will shed light on the unique characters among the
drought occurrence in low elevation tea growing areas of Sri Lanka and also impeding
drought that could occur as a result to global climate change in this region. The final section
will review different research done on tea irrigation and related irrigation on some
horticultural crops and future directions for irrigation studies in the study area.
3.2
Botany of Tea Plant
3.21
Origin and distribution
The tea plant belongs to genus Camellia, family Theaceae and tribe Gordonieae. After several
revisions, the name of the tea plant has now become Camellia sinensis (L.) O. Kuntze (Paul,
Wachira et al. 1997; Hajra 2001). The evergreen plant naturally grows to a height of 10-15m.
Tea plant was originated in south east Asia, specifically around the intersection of latitude
290N and longitude 980E, the point of confluence of the lands of northeast India, north Burma
and south west China and Tibet (Kingdon Ward 1950; Mondal 2007). The climate of this
area is a monsoon type with a warm, wet summer and cool dry winter (Carr and Stephens
1992). Most tea growing lands are associated with high precipitation regimes. Global tea
cultivation now extends from Mediterranean type climates to hot humid tropics in 31
countries (Hajra 2001). The geographical area is from 490N, from the Outer Carpathians
mountain region in former Soviet Russia to Natal, South Africa, 33 0 S (Shoubo 1989).
Altitudes of tea cultivations can vary from sea level in Japan to over 2700m in Kenya and
Rwanda (Ng'etich and Stephens 2001; Owuor, Obanda et al. 2008) . Due to worldwide
demand, tea cultivation is widely spread, while the major tea producing countries in the world
are India, Kenya, China, Japan and Sri Lanka (FAOSTAT 2010).
The Chinese had dominated the art of tea cultivation for many centuries. Tea was then
introduced to Japan from China in the early part of eighth century. From Japan, tea cultivation
was spread to Indonesia in the 17th century. Meanwhile in India, commercial tea cultivation
was started in early 19th century. Commercial tea cultivation started in Sri Lanka in late 19th
century. Tea cultivation started in the USSR in the end of 19th century and in East African
countries during early 20th century.
36
3.2.2
Varietal difference
There are mainly three types of tea (also known as jats in South Asia). They are called China
(Camellia sinensis var. sinensis), Assam (Camellia sinensis var. assamica) and Cambod
(Camellia sinensis var. cambod) types. The China type is a shrub (1-3m tall), with many
stems arising from the base. The relatively small, thick and leathery leaves have stomata
sunken in the lamina. Short and stout petioles gives leaves an erect pose and are usually 3-7
in number. It grows in open spaces. The Cambod type plant is an upright tree (6-10m tall).
It has several almost developed branches and more or less erect, glossy light green to
coppery-yellow or pinkish red leaves. Leaf size is intermediate. The Assam type is a 10-15m
high tree with a trunk and robust branching system, with large, light-green to yellowish
leaves, thin glossy leaves, grown under the canopy of large trees in natural habitats (Mondal
2009; Nair 2010). Cross pollination of tea gives vigorous better quality progeny than parents.
Bio types cross freely with each other.
In commercial tea plantations, the crop consist of mixed hybridized populations of China,
Assam and Cambod types. Tea cultivars found in tea plantations are mostly a mix of China
and Assam tea traits. Tolerance of the tea plant to environment stress depends on the variety,
from which the particular cultivar was derived.
Tea cultivars derived from China type
varieties are slow growing but tolerant to drought, with small semi-erect leaves, whereas
cultivars derived from Assam type are fast growing with less tolerance to drought. The
varietal characters are important in selecting plants for drought tolerance and for irrigation.
Localized varieties of tea have been given vernacular names in different tea-growing
countries. The commercially available tea cultivars are also widely known as tea clones in the
industry. Therefore the term cultivar is used throughout this document and cultivar is exactly
to clone in the tea industry nomenclature,
Table 3.1 shows the characters of some tea cultivars released by tea breeders in Sri Lanka.
The degree of tolerance to drought varies among cultivars (Nagarajah and Ratnasuriya 1981)
and harvesting yield as well. These two characters mainly determine the selection of
particular cultivar suitability for potential irrigated cultivation. Among all, cultivar TRI 2023
is the highest yielding tea cultivar in the tea breeding yield books in Sri Lanka (Piyasundara
2009). However, the particular cultivar had to be withdrawn from the suitability list for low
elevation tea growing areas, due to high infestation of stem canker disease, during drought.
The cultivar may have an ability to respond with high yields under irrigation during drought
periods. Hence it is necessary to evaluate different cultivar to irrigation and their ability to
withstand drought.
37
38
Table 3.1 Characters of some popular tea cultivars cultivated in Sri Lanka (Piyasundara 2008)
A
NOTE:
This figure/table/image has been removed
to comply with copyright regulations.
It is included in the print copy of the thesis
held by the University of Adelaide Library.
3.2.3
Growth pattern
Under natural conditions tea plants are medium sized trees, but under cultivation the tea plant
is grown as a bush of 1.2-1.5m height for ease of harvest and to increase productivity. To
maintain the appropriate height the plant is pruned at regular intervals of 3-5 years, removing
some leaf bearing branches of the plant (Nissanka, Anandacoomaraswamy et al. 2004). In tea,
the part harvested to produce green and black tea is a short, vegetative shoot consisting of a
bud with two or three immature leaves (Figure 3. 1). This is different from many other tree
crops where reproductive parts, like flowers and seeds, are harvested. Hence, keeping the
plant in the vegetative phase during its life span is important for profitability.
The growth of tea shoots also important for high productivity. The natural growth pattern of
the tea shoot, which contain a tea bud, has alternative dormant and active phases (Tanton
1981). Four to seven foliages leaves grow alternatively, above two scale leaves and then
become dormant. When the plant is under stress from low ambient temperature of water
stress, dormancy occurs after production of only two or three leaves. In a tea bush, there are
always active and dormant (banji) shoots, both of which are included in the harvest. Inclusion
of a higher proportion of dormant buds in the harvest is disadvantageous for producing good
quality tea, also lower the productivity, because dormant buds usually weigh less and have
coarse leaves. The proportion of plants containing dormant buds at the harvest depends on
the age of the bush, stage of the pruning cycle and water stress. Plants tend to produce more
dormant buds when water stressed (Stephens and Carr 1991; Wijeratne and Fordham 1996).
Maintaining a favourable soil water balance is important to keep the plants in the vegetative
phase.
The harvesting interval of tea, i.e. the duration between two successive harvests, depends on
the time taken for a young bud to come to the harvesting stage of 10-15cm. This duration is
known as the shoot replacement cycle, which may vary from 30 days to 490 days (Carr &
Stephen, 1992), but the harvesting interval usually varies from 7-25 days (Stephens and Carr
1994; Wijeratne and Fordham 1996). The main factor determining the harvest interval is
temperature (Carr and Stephens 1992). At any point in time there are shoots of different
stages of development on a bush. Tea is harvested at seven-day intervals in Sri Lanka. The
yield, at each weekly plucking reflects the level of environmental stress during shoot
development period. Regular tea yield (weekly, monthly or seasonally) is a good parameter to
reflect seasonal environmental conditions.
39
A
NOTE:
This figure/table/image has been removed
to comply with copyright regulations.
It is included in the print copy of the thesis
held by the University of Adelaide Library.
Figure 3. 1 Development of a tea shoot (Burgess & Carr, 1998)
3.3
Physiology of Tea Plant
Early identification of the physiological and morphological characters suitable for irrigated
tea cultivation reduces the potential risk of investing for a cultivar that respond poorly to the
irrigation (Squire 1985; Smith, Burgess et al. 1994). In a recent review, physiological activity
of tea plant and their relationship with environmental parameters has been discussed by De
Costa et al (2007).
3.3.1
Photosynthesis
Photosynthesis is the process which produce carbon assimilates to yield formation and growth
of maintenance foliage, stem and branch structure and the roots of the tea plant (De Costa,
Mohotti et al. 2007). Tea yield is determined by the photosynthetic rate of the maintenance
foliage and the shoot extension rate (Manivel and Hussain 1982; Okano, Matsuo et al. 1996).
The tea plant exhibits C3 type photosynthesis pathway (Roberts and Keys 1978). While the
major organs of the tea plant photosynthesis are leaves, mature brown stems also assimilate
CO2 but with low efficiency (Sivapalan 1975). In pruned tea, newly emerging shoots
assimilate CO2 through brown stems. Tea leaves shows significant photorespiration. Under
normal atmospheric conditions, photorespiration accounts for 19% of net photosynthesis (De
Costa, Mohotti et al. 2007).
40
In comparison to other crops, little work has been done on the study of the tea photosynthesis
(Mohotti and Lawlor 2002). C3 photosynthesis has three main phases (Raghavendra 2003),
viz: (i) During the first phase of carboxylation, carbon dioxide is accepted by ribulose-1, 5biphosphate (RuBP) to give two molecules to 3-phosphoglycerate (PGA); (ii) next phase is
called reduction where PGA is reduced to triose phosphate (triose-P) using adenosine 5
triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH); (iii) last
phase is the regeneration of the primary acceptor of CO2, RuBP from triose phosphate
through a series of reactions.
Low rates of net photosynthesis and stomatal conductance have been reported for tea (Smith,
Stephens et al. 1993; Mohotti and Lawlor 2002). In Sri Lankan conditions a maximum
stomatal conductance value was reported as 0.8mol H2O m-2s-1
under field conditions
(Mohotti and Lawlor 2002). Much lower values of 0.015-0.098mol H2O m-2s-1 have been
reported in South India (Joshi and Palni 1998). In mature tea when the stomatal conductance
lowers during morning hours and photosynthesis increases, it reduces the intercellular carbon
dioxide concentration. Intercellular carbon dioxide concentration regulates the photosynthesis
process in the tea plant (De Costa, Mohotti et al. 2007).
Photosynthesis is controlled by many external and internal factors which determine the final
productivity of the plant. The internal factors controlling photosynthesis are stomatal
conductance (Smith, Stephens et al. 1993), leaf age and position in the plant (Okano and
Matsuo 1994; Raj Kumar, Manivel et al. 1998), varietal difference (Smith, Burgess et al.
1994; Joshi and Palni 1998) and pest and disease occurrences (Ponmurugan, Baby et al.
2007). The external factors controlling the photosynthesis are light intensity and shade
(Mohotti and Lawlor 2002; Karunaratne, Mohotti et al. 2003), temperature (Joshi and Palni
1998; Netto, Jayaram et al. 2005), CO2 concentration (Kumar, Venkatesalu et al. 1993),
mineral nutrition (Smith, Stephens et al. 1993) and water deficit (Smith, Burgess et al. 1994;
Luo and Pan 1996; Marimuthu and Kumar 1998).
Irrigation has the ability to control many external factors affecting the photosynthesis of tea.
Irrigation also increases the stomatal conductance, photosynthetic rate and ratio between
stomatal conductance and photosynthesis. It also maintains favourable leaf temperature level
optimum for photosynthesis and reduced photo-inhibition at high luminance (Smith, Burgess
et al. 1994). However, there is a factor associated with the age of the tea plant, where old tea
plants do not increase the photosynthesis rate of tea as compared with young tea as a response
to irrigation (Smith, Burgess et al. 1994). The cultivar effect of tea is also visible in the
response to photosynthetic rate to the irrigation effect. Photosynthesis has been selected as a
tool to investigate drought effects primarily because of its role in determining productivity of
41
tea (Bannerjee 1993), sensitivity to drought (Berry and Bjo¨rkman 1980) and its reflection of
the temperature effect (Joshi and Palni 1998) which is critical during drought at low elevation.
As the present planting recommendation include many cultivars for low elevation tea growing
areas (Table 3.1), it would be advisable to evaluate their physiological response to irrigation
to minimize the time consumption for field selection procedure. Otherwise, unlike for an
annual crop, the financial cost of installing a irrigation system in a new planting field and
harvesting lower yield after 2 or 3 years would be a high economic cost on the farmer.
3.3.2
Plant water relations
Tea crop water use can be expressed as its transpiration, since completely grown tea canopy
covers ground surface, there is usually minimal soil evaporation. Nevertheless there are only
few studies that directly measure transpiration in tea crops viz: Kigalu, 2007 and
Anandacoomaraswamy et al 2000. Average daily transpiration from 1.9 to 5.5mmday-1 were
reported from Mufindi, Tanzania (Kigalu 2007) from well watered soils. Water use per unit
leaf area for young tea varied with cultivar and crop density. Higher transpiration rates were
observed from the plants with lower plant densities. In Sri Lanka, transpiration rates ranging
from
1.6-3.3mm
per
day
were
reported
(Ananthacumaraswamy, De Costa et al. 2000).
under
low
temperature
conditions
High transpiration in tea plants cause
significant water loss from the soil suppressing shoot growth (Stephens and Carr 1993).
Stephen and Carr (1993) found this interaction while conducting experiment at relatively a
lower air temperature value of 15-200C in Tanzania. So the studies related to tea transpiration
has been done entirely in low elevation regimes as compared to the high temperature (>30 0C
in dry months) prevail in Ratnapura, Sri Lanka. So the transpiration of tea in such
environment is affected by the behaviour of stomata, as influenced by soil water stress as well
as the high temperature.
Tea plant canopies have high transpiration rates which causes soil water deficits leading to
decreased leaf expansion rates (Squire 1990; Stephens and Carr 1993). The reduction of leaf
expansion rate is directly inverse to the green leaf yield. Excessive transpiration occurs even
under wet conditions, when soil contains adequate soil moisture, due to high level of
irradiance and saturation deficit (Smith, Burgess et al. 1994). Temporary water deficits occur
within the plant as a result. Productivity is directly affected by the water deficits of the plant.
Apart from irradiance and saturation deficit, there is a chance for high ambient temperature to
increase transpiration mostly in rainless periods in hot humid environment, which is not
apparent in the studies of cool dry environments like East African tea growing regions.
42
Tea plant water use has previously been estimated from the grass reference evapotranspiration
and multiplied with a crop coefficient. The crop coefficient adjusts the difference between the
selected crop species (tea) and the grass, by considering the species physiology, age and
method of training (Allen, Pereira et al. 1998). However, reference crop cannot be evaluated
as representing a tea crop in estimating the water use (Dragoni, Lakso et al. 2006). Water use
of tea has to be evaluated separately specially in the humid conditions (Annandale and
Stockle 1994). Unlike in short grass conditions, in tree crops transpiration is more controlled
by bulk air conditions than net radiation (McNaughton and Jarvis 1991).
Stomata close partially during the day even when the soil is wet (Williams 1971; Carr 1977;
Squire and Callander 1981). Stomatal closure is a response to an internal water deficit in the
shoot. Due to possible specific characteristics in the absorbing region of the root system and/
or the xylem vessels, the rate of root water absorption and subsequent transfer through xylem
is not very efficient (De Costa, Mohotti et al. 2007). Xylem water potential is more sensitive
to air temperature and vapour pressure deficit when soil is wet, than when soil is dry. Low
stomatal conductance is associated with drought tolerance, and it is a useful selection
parameter for selecting the tea cultivar for drought tolerance (Saikia and Dey 1984). When
there are drought conditions in Ratnapura, plants experience not only soil water stress but
high temperature stress as well. The behaviour of the different cultivars under drought
condition predicts the better quality cultivar selection for irrigated tea cultivation.
3.4
Climatic Requirement
3.4.1
Air temperature
Ambient temperature is one of the major climatic factors which determines the shoot
extension rate and shoot weight of tea, and hence its geographical distribution (Squire and
Callander 1981; Wijeratne and Fordham 1996). The temperature at which shoot extension
ceases is known as the base temperature. Several authors have reported various values for the
base temperature, according to cultivar and location. A base temperature value of 12.50C has
been reported for cultivar SFS 204 in Malawi (Tanton 1982), which was validated by Carr
(1992) and used to calculate the shoot replacement cycle in many locations. However, in
Mufindi, Tanzania, base temperature values ranging from 8.9 to 11.30C have been reported
for four different tea cultivars (Burgess and Carr 1997). Though growth ceases in tea shoots
during cool winter periods, the plant survives to start shoot development when the warmer
season arrives. The response of tea to irrigation during cool dry months is less than in warmer
months due to the lower air temperature that restricts shoot growth.
43
The upper, or ceiling, temperature above which shoot growth is restricted has not been
determined. Wijeratne and Fordham (1996) maintain that it is higher vapour pressure deficit
rather than higher ambient temperature that restricts growth (Wijeratne and Fordham 1996).
Nevertheless, temperature above 300C is generally considered not suitable for tea growth (Das
and Barua 1987). Wijeratne (1996) found that the extension rate of tea shoots was reduced at
mean temperatures above 260C. Under controlled environment conditions, it has been found
that the optimal temperature for leaf photosynthesis is 350C and it decreases rapidly above
370C (Hadfield, 1975 cited in Hajra, 2001). Where ambient temperature in tea areas exceeds
300C cultivation of shade trees like Falcataria mollucana or Gliricidia maculata is advised
(Das and Barua 1987).
Prevailing ambient air temperature, above a base temperature of 12.50C, can be used to
calculate the time taken for a bud to reach a harvestable stage of 10-15cm long shoot (Carr
and Stephens 1992). Shoot replacement cycles vary from 160 days in the winter months for
cooler tea growing areas to 30 days in warmer regions during the hot season when other
factors are not limiting the growth rate. Under field conditions at low elevations in Sri Lanka,
it takes 42-49 days in the wet season and 56-64 days in the dry season for the shoot
replacement cycle (Wijeratne and Fordham 1996). This can be considered as a base to
evaluate the low response even during wet season for high ambient temperature prevailing in
the area, when soil moisture is not limiting.
3.4.2
Soil temperature
Soil temperature influences growth and yield of tea and also has a great influence on plant
survival (Hajra 2001). The growth rate of tea shoots decreases at soil temperatures above
250C at 0.3m (Fordham 1971). The lower limit of soil temperature for tea plant growth is
considered to be as 200C (Carr and Stephens 1992). Measurement of lowest soil temperature
for tea plant growth is difficult as isolation between low soil moisture and high air
temperature is not practicable.
Application of mulches and planting of shade trees are practised in tea plantations to reduce
the adverse effects of high soil temperature during dry seasons. There are two types of
mulches that are used in tea fields frequently. Organic mulches derived from plant materials
and synthetic mulches derived from plastic, polythene or other suitable material. Organic
mulches such as grass reduces the soil moisture level, whereas mulching with polythene
increases the soil temperature (Othieno, Stephens et al. 1992). Cultivation of shade trees is
important in controlling soil moisture levels in dry periods as well as providing mulching
materials. Further experiments are necessary to study the effect of high soil temperature
without soil moisture stress conditions.
44
3.4.3
Vapor pressure deficit
Vapour pressure deficit (VPD) is mostly related with high ambient temperature. Squire
(1979) was the first to report an inverse linear relationship between VPD and shoot growth
rate of tea between 0.8-3.2kPa. Tanton (1982) reported that when VPD of air exceeded 2.3
kPa, it depressed the growth of tea shoots. This value is widely used as the critical level of
VPD for predicting tea yield. In contrast, a critical value of 1.2kPa has been reported at low
elevations of Sri Lanka (Wijeratne and Fordham 1996). This is significant as the relative
humidity of low-grown tea areas do not fall to very low levels of 40-50%. This result
suggests the possible effect of high air temperature on shoot growth. In controlled conditions
with 280C constant day temperature value, Balasuriya (1997) found a reduction in shoot
extension rate as VPD increased from 1.6 to 2.1kPa. All these differences in response of
shoot growth rate can be due to different air temperature levels that prevailed in experimental
periods. However, high saturation deficit of even more than 2.0kP lasting for short durations,
for few hours in a day, does not have any effect on plant yield (Rahman and Nath 1994).
The negative effect of saturation deficit is due to inductance of high leaf water potential,
which is a result of the high rate of transpiration that occurs under dry conditions. The effect
of saturation deficit was observed in irrigation trials of Malawi and Tanzania. The yield
response to irrigation during dry seasons in Malawi was lower than that in Tanzania, because
of the higher saturation deficit that prevails in Malawi, during the dry season, as compared to
Tanzania (Tanton 1982). In an irrigation experiment by Carr et al (1987), it was found that
tea yield was limited by the high VPD (2-4kPa) during the month of October and November
after removal of major limiting factors of air temperature and soil water deficit
Although it is not practical in tea cultivations, application of mist irrigation was successful to
control the adverse effects of high VPD (Tanton 1982). Response to the two main irrigation
types, sprinkler and drip irrigation, will be important under low elevation conditions in Sri
Lanka, because, sprinkler irrigation has the ability to modify the microclimate of the tea bush.
3.4.4
Rainfall
Tea is grown as a rain-fed crop in Sri Lanka. Even distribution of annual rainfall is very
important in determining the geographical distribution of the tea plant, especially in hot
climates. Tea is grown in areas receiving a rainfall of 700mm (Chipinga, Zimbabwe) to
5000mm (Sri Lanka Tea Board) per annum. Supplementary irrigation is considered necessary
for areas receiving less than 1150mm (Carr and Stephens 1992). But this widely claimed
rainfall requirement has been calculated for eastern African countries, where the air
temperature does not increase 300C often and drought period is known as winter drought,
45
where low temperature prevails. Under low-grown Sri Lanka conditions, total monthly
rainfall and its distribution within months are also important factors, because total monthly
rainfall value does not reflect the climatic factors of the month (Wadasinghe and Peiris 1987).
In Low elevation tea growing areas of Sri Lanka, ambient air temperature generally increases
with the onset of consecutive rainless days. Hence regular rainfall throughout the month or
year is important for a uniform production in the year. (Wijeratne 2010) In contrast, in many
tea growing areas in Africa, distribution of rainfall is not very important for the survival of
plants, mainly because of the low temperature prevailing in the dry season. Due to the low
temperature water loss from plants is lower compared to warm climate. For example, in
Kenya, 90% of the 2100 mm annual rainfall is received between mid March and mid
November (Hajra 2001). Due to relatively uniform distribution of monthly ambient air
temperature, except spikes in dry spells, yield of tea plantations in the low- grown region of
Sri Lanka is highly correlated with rainfall.
3.5
Drought in Tea Cultivations
In meteorological terms drought refers to a period in which rainfall falls below potential
evapotranspiration (Smakhtin and Hughes 2004). However, regarding agriculture drought
particularly in Tropics, drought has to be considered as a multidimensional stress, since
drought is aggravated by solar radiation and high temperature (DaMatta and Ramalho 2006).
Since the tea plant is sensitive to temperature stress, and dryness of air, drought occurrence in
tea cultivations is sometimes masked, if analysed in meteorological terms. Many studies
concerning drought effects on tea plant physiology has been done on container grown plants
in controlled or semi-controlled environments. DeMatta and Rmalho (2006) suggest some
limitations in such experiments to compare with field conditions, such as: (i) root growth is
particularly restricted in pots; (ii) soil substrates in pot experiments usually creates short, local
water deficit in roots; (iii) transpiration is decoupling, since air surrounding the plant is
isolated from external atmosphere; and (iv) if humidity and temperature is not controlled,
evaporative demand may rise. Thus, it arises the need for field studies of drought related
physiology.
3.5.1
Occurrence of the drought
Drought is the main abiotic stress that reduces productivity and occur in all tea growing
regions of the world (Mondal 2007). Barua (1989) reported that 60-65% of the total world tea
area is subjected to periods of drought varying from 3 to 20 or more weeks.
It is a
phenomenon that occurs in any temperature or precipitation regime (Fuchs 1989). Even
though tea is grown in regions where the annual rainfall can be high, many tea plantations are
46
subjected to dry spell, which may ranges from only a few weeks to up to six months, due to
the annual cycle of tropical weather (Squire and Callander 1981).
The drought season often consists with combination of dry soil, dry air and high temperature
(Squire and Callander 1981). The severity of drought and its effect on the growth of the tea
bush varies with air temperature and humidity. For example, in Mufindi, Tanzania, there is a
six-month dry period with low air temperature.
Low temperature does not allow the
saturation deficit to reach critical values and the plants do not undergo much stress. In
contrast, in Mulanje, Malawi, the dry season is shorter than Mufindi, Tanzania but the high
saturation deficit, associated with high air temperature, results in more severe water stress
(Squire and Callander 1981). Table 3.2 shows the characters of drought season in different
tea growing areas of the world. Except Ratnapura, Sri Lanka, all other regions have low
rainfall and maximum temperature is less than 300C during dry season. Even though the
average monthly rainfall during the dry season is also more than 150mm, distribution of the
rainfall within the month is not uniform. The number of rainy days is limited to a few, leaving
large number of consecutive rainless days.
3.5.2
Drought in Sri Lankan tea plantations
In Sri Lanka, periods of drought are considered to be typical, although irregular and not wide
spread in monsoon climate (Domros 1977). In an analysis of rainfall and tea yields in Sri
Lanka, Fuchs (1989) reported that any month having less than 50 mm of rainfall causes
reduction in yield and growth retardation of young plants. Even within the high rainfall
months, 15 consecutive rainless days can induce losses in production from drought
(Wadasinghe and Peiris 1987).
The climate of Sri Lanka has distinct wet and dry seasons, which are associated with the
monsoons. There are two monsoons, the southwest and northeast monsoons, both of which
bring rains to lowland tea areas of the island. The southwest monsoon occurs between AprilJuly and the northeast monsoon occurs between October and December. The period following
the southeast monsoon is the dry spell for low-grown tea region (Wadasinghe and Peiris
1987), but the inter monsoon period of July-September also creates a relatively dry
environment, lasting for 4 to 5 weeks. In addition to the annual dry spells, recurrent droughts
which usually have 10 year intervals mainly result in widespread plant death, as happened in
1983 and 1992 (Fuchs 1989; Wijeratne 1996). The greatest losses were reported in low-grown
tea areas during the1989 drought spell, though the duration was comparatively shorter (Fuchs
1989).
47
Table 3.2 Characteristics of drought period in some tea areas (Squire and Callander 1981; Wijeratne and Fordham
1996)
Site
Location
8 33’S
(Tanzania)
35 10’E
Kericho
0 22’S
0
0
35 21’E
Mulanje
16 05’S
(Malawi)
35 37’E
Tocklai
26 17’N
(India)
94 12’E
0
Tmax
0
( C)
ET0
(mm/day
)
1890
May-Oct
<10mm
1-2
18-24
3-5
Variable
2.5-3.5
23-27
4-6
15-30mm
3-4
26-33
5-7
2178
650
Decmid Mar
mid Augmid Nov
0
0
80
Dec-Feb
10-30mm
1.5
23-26
5
1060
Jan-March
<10mm
1.5-3.0
25-30
-
100
Jun-Aug
2.0-2.5
30
-
2.5
31-33
3-5
0
10 N
(India)
77 E
Krasnodar
45 N
Board)
VPD
(kPa)
0
Annamali,
(Sri Lanka Tea
Monthly RF
(mm)
0
(Kenya)
Ratnapura,
Main Dry
Period
0
Mufindi
(former USSR)
Altitude
(m)
0
0
0
39 E
0
80 25’E
weeks
Variable
0
6 40’N
No rain 2-6
29
Jan-Mar
Avg
>135mm
The drought season in January –March of each year usually associated with a maximum
temperature of >340C. The plants are subjected to heat stress in addition to drought stress.
Continuous exposure of coffee plants to temperature as high as 300C, has resulted in
decreased growth, leaf abnormalities and growth of tumours at the base of the stem (DaMatta
and Ramalho 2006). It was found that both canopy photosynthetic rate and photo chemical
efficiency was reduced when the plants are subjected to both drought and heat stress together.
The canopy photosynthetic rate fell nearly zero, when subjected to heat and drought stress
together as compared to 20 days in drought alone and 34 days of heat stress in Kentucky blue
grass (Jiang and Huang 2000).
The usual weather pattern in the southwest of Sri Lanka is to have maximum temperature
around 1400hrs and followed by rain showers in the evening. This prevents increasing the
leaf temperature, and opens stomata, which closes during the mid-day. During the drought
period of January-March, incidence of solar radiation, sunshine hours and maximum
temperature increases respectively up to 19.1 MJ/m2/d, 6.8hrs and 32.80C (Table 3.3). This
results in having multiple stress factors on plant growth.
There is an interaction hence between ambient air, light variation and depletion of plant
available water in the drought. During a day, leaf temperature increases relative to the air
48
above the leaf. When leaf temperature increases, internal vapour pressure increases in
comparison to that in the air, causing vapour pressure gradient to increase (Nilsen and Orcutt
1996). In response, transpiration increases and induces stomatal closure and wilting at the end
of the day.
Table 3.3 Characteristics of dry season (Jan-March) and wet season (Apr-Dec) in Ratnapura, Sri Lanka (FAO,
2005) (ET0 – Potential evapotranspiration)
3.5.3
Season
N
(hours)
Rn
-2
-1
(MJm day )
VPD
(kPa)
Tmax
0
( C)
Dry
5.4
17.1
1.0
34.1
Wet
4.4
15.5
0.9
32.6
Tea plant response to drought
Visible indicators of the effects of drought conditions on tea plants are leaf wilting, shedding
of leaves and eventual die back (Fuchs 1989) and yield reduction (Burgess and Carr 1996). In
Tanzania, after a 16-week drought treatment, it was found that means light interception was
reduced by 25% and mean radiation efficiency was reduced by 78%. Further drought reduced
the dry matter partition to leaves, stems and shoots by 80-95 % (Burgess and Carr 1996). Bud
break and shoot growth is inhibited by moderate water deficits (Borchert 1994). There was a
reduction in relative extension and development rates of tea shoots due to drought (Burgess
and Carr 1997). It has been found in other crops also that shoot growth is very sensitive to
water deficit induced by drought (Chartzoulakis, Noitsakis et al. 1993). In a thorough analysis
of drought conditions in low-grown tea areas of Sri Lanka, Wijeratne (1996) reported a
reduction of shoot population density, shoot extension rates and weight of tea shoots in two
drought tolerant and drought susceptible cultivars.
Jeyaramaja et al (2005) observed a 50% reduction of photosynthesis under a relatively
moderate water deficit of 1.5 KPa. When soil water potential reaches 2.0 KPa, or severe water
stress, there was a complete reduction in photosynthesis (Jeyaramraja, Meenakshi et al. 2005).
The time taken for soil to shift from mild to severe stress depends on the rate of water loss
from the plant. Hence in high temperature zones like Ratnapura, plants can undergo severe
stress even during short dry spells
Fully grown tea canopy covers the ground completely allowing little solar radiation to
penetrate down to the soil surface. Hence evapotranspiration is almost equal to transpiration
in tea (De Costa, Mohotti et al. 2007). Tea transpiration rate is sensitive to soil water
49
availability.
The transpiration reduction is attributed to gradual stomatal closure and
consequent reduction in stomatal conductance (De Costa, Mohotti et al. 2007)
While lack of available water is the major cause of drought stress, disease may exacerbate the
effects of water stress. For example, stem canker disease of tea is associated with drought
conditions (Carr and Stephens 1992). Heat stress causes cracks in the stem and this reduces
the ability of the plant to transport water to the shoot. The water supply from soil to canopy
has an effect on the daily trunk shrinkage (Ortuno, Alarcon et al. 2005), which may have an
effect on stem cracks. The cultivar TRI 2023, while being a high dry matter producer, is
highly susceptible to stem canker and hence is not considered suitable for cultivation. It will
be important to study the swelling and shrinkage of the stem to evaluate the relationship
between drought and stem canker disease occurrence. Further, the high incidence of stem
canker in fields with gravel and stone could be associated with high soil temperature and low
water holding capacity during dry periods.
3.5.4
Drought mitigation
There are various methods applied in tea plantations to mitigate the effects of drought,
including the selection of drought tolerant cultivars, application of plant growth regulators
(Wei 2004), grafting (Luo and Liu 2000) application of soil mulches (Othieno 1980),
maintenance of shade plants (Wijeratne and Ekanayake 1990) and supplementary irrigation
(Carr, Dale et al. 1987). The main reasons for poor adoption of drought mitigation strategies
in the field are their additional cost and that many of them, apart from irrigation, have a
history of failure. Mulching is practiced commonly to mitigate drought, using either grass or
plastic mulch. Plastic mulch however reduces the soil water infiltration during the rainy
season and effect of grass mulch on soil moisture conservation depend on nature and/or rate
of decay (Othieno 1980). Root stock selection for grafting tea to mitigate drought stress is
very laborious (Prakash, Sood et al. 1999). Chemical application for drought mitigation has to
be carried our prior to ceasing the rainfall and which is mostly very difficult to predict with
nature of rainfall.
When selecting drought mitigation strategies, growers are mostly concerned with the
investment cost and the return on investment with increased effect on yields. Selection of
drought tolerant cultivars appears to be an attractive option, but most of the drought tolerant
cultivars have low yields (Wijeratne 1996). Widespread use of vegetatively propagated plants
has resulted in a lower ability to withstand drought in tea plantations as compared to seedling
tea. Seedling tea usually have a deeper root system, sometimes more than 5.0m (Carr, Dale et
al. 1987). During dry season seedling tea maintained a higher xylem water potential and
higher degree of stomatal opening, showing a lower drought stress than vegetative propagated
50
tea (Carr 1977). It is advised to include drought tolerant tea cultivars in the planting program.
However, when selecting new plant varieties though seedlings are not normally included
(Wijeratne and Ekanayake 1990). Though application of plant growth regulators is an easy
and popular method in some tea growing countries, it is not recommended under Sri Lanka
conditions, as application of chemicals may have a negative effect on consumer demand.
High yielding cultivar scions grafted either to seedling or drought tolerant vegetative
propagated tea stock is termed a composite plant. Use of composite plants is a cost effective
solution to convert low yielding, drought resistant tea plantations into high yielding
plantations, with scions selected from high yielding tea cultivars. However, this technique is
not popular except in China and some African countries because it is a complex process and
some root stock x scion combinations are not giving promising results (Luo, Qian et al. 1999;
Mizambwa 2002).
3.5.5
Impact of climate change in low elevation tea sector
There are growing indications that the effect of climate change has already been experienced
in Sri Lanka. Scientists attribute that the country is experiencing both a global change effect
and local heat island effect, caused by rapid urbanization (Emmanuel 2001; Basnayake,
Fernando et al. 2002; Fernando and Basnayake 2002).
Annual temperature is on an
increasing trend and rainfall is on a decreasing trend. During the 1961-1990 period, mean air
temperatures of the country increased by 0.0160C per year.
Mean annual precipitation
decreased by 134mm (7%) compared to 1931-1960 (Chandrapala 1996). Verifying this claim,
Herath and Ratnayake (2004) found a clear sign of decreasing rainfall in tea growing regions
and reduction in rainy days, after analysing 60 observation point rainfall data. And further to
this, Madduma Bandara and Wickramagamage (2004) observed that the decline in rainfall in
western slopes of Central Highlands is particularly significant because it is the catchment area
for low elevation tea growing areas. The significance of this for Sri Lankan tea production is
that we know for every 100mm reduction in annual rainfall we can expect a reduction of 3080 kg of made tea per hectare (Wijeratna, Ananthacoomaraswamy et al. 2007).
Climate models predict that mean annual temperature of Sri Lanka will increased by 0.94.00C over the base line 1960-1990, by the year 2100 (Eriyagama, Smakhtin et al. 2010).
While the effect of such changes is effective in all over the island, there are some climate hot
spots where effects are more significant than other areas. Also there are some crops that are
more vulnerable to the climate change. Low elevation grown tea is more vulnerable to
changes in climate changes than high elevation grown tea (Wijeratna, Ananthacoomaraswamy
et al. 2007). The above findings suggest that studying the drought related stress in tea
cultivation is vital for long term survival of the industry.
51
3.6
Irrigation in Tea Plantations
Tea is most commonly grown as a rain-fed crop. The earliest investigations into tea irrigation
in the world were made in Sri Lanka in the 1950s (Rogers 1959). The irrigation idea did not
take root in Ceylon (as Sri Lanka was called before 1972) but was practiced in many tea
growing countries, like Iran (Salardini 1978), Azerbaijan (Rekvava 1986; Kuliev 1988),
Malavi (Willatt 1970), Tanzania (Carr 1974), Kenya (Othieno 1978) and India (Hajra 2001) .
The main types of the irrigation practised were sprinkler irrigation largely but surface
irrigation is also practiced in some tea growing regions (Tkebuchava 1988; Dabral and Rao
1997). Recently there has been much interest on drip/trickle irrigation in Tanzania and
Malawi (Möller and Weatherhead 2006; Kigalu, Kimamboa et al. 2008) as well. Indeed,
nearly 20% of Tanzania’s tea plantations are under irrigation (Möller and Weatherhead 2006).
When compared to other major crops research on tea irrigation has been limited. Many
experimental details were based on the research done on East African tea growing countries.
Adaptation of the irrigation in the Asian region has been limited to some Indian plantations,
both in northern India and southern India, Iran and some former Soviet Union countries. Even
though Sri Lanka is one world’s leading tea exporters, there has been little interest in
irrigation technology until recently. The lack of interest is mainly due to lack of water
sources, cost of irrigation systems, significant profit even under rain-fed cultivation and lack
of knowledge about the irrigation among growers.
Response of the tea plant to supplemental irrigation has been analysed on yield, physiological,
growth and financial basis. The response largely depends on climate of the study period and
temperature and associated saturation deficit of the dry period being the most significant co
factors (Carr, Dale et al. 1987). Irrigation has been mostly successful in the tea growing
regions where there is only one rainy season and a prolonged dry season that can be lasted for
4 to 6 months (Carr 2010). For other tea growing regions, where bimodal rainfall pattern
prevails, drought mitigation methods are generally advised. However, when the short term
drought is coupled with high temperature stress as well, it may need more robust drought
mitigation methods (Fuchs 1989). One of the main factors facilitating the application of
irrigation and subsequent research on irrigation is facilitated by the close proximity of the
cultivation to water sources. In many tea growing locations there are no water sources for
irrigation. When such drought occurrences happen growers either shift the cultivation to some
other crop (e.g. rubber is such an alternative crop, used by growers in low elevation tea
growing areas of Sri Lanka) that resist drought or start a new planting program.
52
3.6.1
Yield response to irrigation
The early work in Ceylon of Rogers (1959) reported yield increases of more than 50% in six
year old tea plants under overhead sprinkler irrigation. In Malawi yield increases from 1000
to 2000 kgha-1, were reported from six year old heterogeneous seedling tea during the period
1967-1970 (Carr 1974). Part of that annual yield increase was associated with reduced yield
fluctuations across the seasons. The water productivity (which will be referred to as ‘water
use efficiency’ in this thesis) of that irrigation treatment was reported as1.4 kg(ha.mm)-1.
However seasonal yield elevation was not observed when the ambient temperature fell below
base temperature of 12.50 C required for plant shoot growth (Carr 1971). But the yield
increase once the limiting factor is eliminated in the subsequent raining months. Also when
the air temperature reached above 300C for 30 consecutive days and dry air (saturation
deficits >2.0kPa) conditions experienced, the tea for the irrigation treatment was reduced to
0.3kg(ha.mm)-1 of water applied (Carr, Dale et al. 1987). This is equal to less than one third
in a normal year. Modifications of the microclimate have been recommended in such
instances to control air temperature and humidity. This has been observed in field trials done
in Malawi and Tanzania. Application of mist irrigation at short time intervals increased tea
yield during the dry season, as compared to sprinkler irrigation in Malawi (Tanton 1982).
In Malawi, vegetatively propagated tea yielded twice more than heterogeneous seedling tea
under sprinkler irrigation, Some varieties gave a very high response to irrigation water
applied, for cultivar, 6/8, gave a water productivity of 1.9-2.9 kg(ha.mm)-1 (Stephens and
Carr 1991).
However in Tanzania seedling tea gave a comparable yield increase with
vegetatively propagated tea, when the saturation deficit was controlled by micro jet mist
irrigation, in addition to sprinkler irrigation (Clowes and Starch 1988).
The response of different tea cultivars to irrigation is not the same. Some cultivars produce
very high response, while some other cultivars produce low response (Burgess and Carr
1996), and this difference is attributed to the dry matter partitioning of different cultivars
(Burgess and Carr 1996). Also in the areas where drought prevails during winter periods, the
difference in the shoot basal temperature for different cultivars has an effect on tea yield
(Burgess and Carr 1997; Carr 2010). In Kenya, Smith et al (1993) found no interaction
between cultivar and irrigation.
So when implementing an irrigation system on tea
plantations, it is imperative to assess the available tea varieties to make the best of the
irrigation investment.
53
3.6.2
Effect on tea physiology
Studies have been undertaken to measure the irrigation effects on physiology of tea plants.
The relative significance of photosynthesis on yield is sometimes confusing in tea. Some
researchers suggest that since tea have a low harvesting index, photosynthesis does not have
effect on tea yield (Squire and Callander 1981). However, others have shown that
photosynthesis and yield are not independent (Stephens and Carr 1991; Smith, Stephens et al.
1993). Even if there are low yields from irrigated plants due to climatic limitations other than
moisture stress, an increase in photosynthesis has an effect on the later yield. So the enhanced
photosynthesis of the plant due to irrigation during low yielding months, either very hot or
cool, can accumulate carbohydrate reserves and mobilized to promote emerging shoots
(Hakamata and Sakai 1980).
Water stress reduces photosynthesis through direct influence of metabolic or photochemical
process in the leaf, or indirect influence on stomatal closure and cessation of leaf growth
which results in decreased leaf surface (Dejong 1996). When the leaf water potential reached
-2.0MPa under growth-room experiment, there was a nearly complete inhibition of
photosynthesis (Jeyaramraja, Meenakshi et al. 2005). Irrigation increases the photosynthesis
by increasing photosynthetic rate per unit leaf area and by increasing the proportion of light
intercepted by photosynthetically efficient leaves. Increase in photosynthetic rate could be
accounted for by increases in stomatal conductance and associated reduction in leaf
temperature (Carr 2010). Irrigation also prevents the photosynthetic rate decreasing under
high luminance (Smith, Burgess et al. 1994). Otherwise tea photosynthetic rate decreases
when the photon flux density increases beyond 1000 µmolm-2s-1 (Squire 1977).
3.6.3
Varietal difference in response to irrigation
Irrigation increased the photosynthesis rate and stomatal conductance while reducing leaf
temperature of cultivar 6/8 in Tanzania (Smith, Stephens et al. 1993).
In the same
experiment, it was shown that there was no cultivar interaction with irrigation treatment.
However in later studies done on young tea plants Burgess (1996) found that there was a
cultivar difference in response to irrigation.
The botanical basis of yield increase in tea is based on changes in basal shoot population,
advanced peak population density with onset of rains and an increase in shoot weight
(Stephens and Carr 1994). As most of the results are based on sprinkler irrigation trials it will
be important to evaluate the yield increment of drip irrigation, in dry season as compared to
wet season.
54
3.6.4
Growth response to irrigation
Difference in dry matter production in different cultivars as response to irrigation was visible
when analysing the pruning weight of irrigated and drought affected plants. Dry weight of
foliage removed at pruning were greatest for cultivar TN 1403(7 years of age), is 34 t/ha
(well-irrigated) and 22 t/ha (droughted). For the same age cultivar, PC81, well irrigated
treatment produced 27t/ha (Carr 2010). In comparison to other popular drought mitigation
method of grass mulch, irrigation has resulted in higher root depth and distribution (Willatt
1970).
As against the popular belief that irrigation results in lower root volume and
distribution, there are some evidence that irrigation has increased the root development (Carr
2010). But there was no difference in canopy root:shoot ratio among irrigation and rain-fed
plants (Nixon and Sanga 1995). The above findings are however based on experiments
conducted at Kenya and Tanzania, where soils are deep and root depth of more than 5m have
been reported. In those sites, winter climate is the mostly limiting factor for root growth. But
in contrast, low elevation tea fields in Sri Lanka, soils are shallower (Panabokke 1996) and
favourable temperature prevail throughout the year for root growth.
3.6.5
Economic evaluation of tea irrigation
There are mainly two types of irrigation systems, drip irrigation and sprinkler irrigation. On a
commercial level in India, sprinkler irrigation of tea produces higher yields than drip
irrigation (Hudson 1991). But in other crops, orange in humid climates and cashew in
Australia, drip irrigation has been able to produce similar yields as with sprinkler irrigation
(Myers and Harrison 1978; Blaikie, Chacko et al. 2001). The importance of maintaining
vegetative growth in contrast to the balance between vegetative and reproductive phase in
most tree crops, can be a decisive factor between selection of drip and sprinkler irrigation.
Renewed interest has been placed in African tea growing countries due to the pressure applied
on the existing surface water sources (Möller and Weatherhead 2006; Kigalu, Kimamboa et
al. 2008). Benefits of drip irrigation include better plant survival, greater yields, more
efficient distribution of nutrients and less plant stress (Çetin, Yazgan et al. 2004; Kigalu,
Kimamboa et al. 2008). Drip irrigation would be an ideal tool for the tea producing region
like low elevation tea growing areas of Sri Lanka, where water sources are scarce for
irrigation during the drought periods. The recent introduction of in-line drip tubes for closed
orchard cultivation has been an alternative for the high cost associated with in-line drip
emitter installation for a high density crop like tea. Industrial communications and some
unpublished
data
also
suggest
high
yield
with
drip
irrigation
in
Sri
Lanka
(Ananthacumarswamy, Herath et al. 1985).
55
The conventional drought mitigation method of using drought-tolerant tea cultivars is
associated with lower productivity and the success of the methods mostly depends on the
severity of the drought, unlike irrigation. The characteristics that confer drought-tolerance,
and consequently lower productivity, are smaller leaves, thicker cuticle, lower transpiration
and photosynthesis rates.
Application of irrigation with other inputs like fertilizer and
pesticides could increase the productivity and income (Carruthers and Clark 1983). Irrigation
system selection is affected by water source, land to be irrigated, plant and soil type (Çetin,
Yazgan et al. 2004). An early and isolated tea irrigation experiment in Sri Lanka from the
1950s, before the introduction of drip systems, illustrated the relative profitability of sprinkler
irrigation compared to rain-fed cultivation (Rogers 1959). Trials with drip irrigation in low
temperature, high elevation tea growing area of Sri Lanka showed positive benefit/cost ratio
(BCR) of 7.7 for irrigation (Ananthacumarswamy, Herath et al. 1985). These earlier trials
from the highlands have encouraged the exploration of drip irrigation in low elevation areas.
However the growing environments are very different between highland and lowland. Also
the TRISL breeding program has focussed on cultivar performance under rain-fed conditions.
So the financial viability of drip irrigation systems in low elevation areas is going to depend
not only on the costs of production (pump and line installation, wage rates) and tea leaf price,
but very much on the yield response of the chosen cultivar to irrigation.
3.7
Conclusion
This chapter has reviewed the botany and physiology of the tea plant particularly with respect
to its water relations. Even though the tea plant originated in a cool humid environment, it is
cultivated across a wide range of environments; of particular interest here is its performance
in the hot humid low elevation tea growing areas of Sri Lanka. In such an environment the
tea plant experiences significant water stress on a seasonal basis. Irrigation has been of
international interest in tea for 50 years, but only recently has it been seriously considered in
Sri Lanka. Strong cultivar differences in response to irrigation were observed in East Africa
(Smith, Burgess et al. 1994) and such differences are likely in Sri Lanka as well. Also in East
Africa low ambient temperature effect was found to be the main limiting factor in irrigation
trials. This is unlikely to be a problem in the low elevation areas of Sri Lanka. However, high
air temperatures are characteristic of this environment and the impact of upper temperatures
has not been defined. Therefore this review supports the need for research on the response of
tea to irrigation to mitigate water stress under hot humid conditions.
56
Chapter 4
Experimental Site Description and Location
4.1
Introduction
This thesis is concerned with understanding the behaviour of tea plants during hot humid
drought in the low elevation tea growing areas of Sri Lanka and their response to irrigation. It
includes five field trials and two glass house experiments.
The field and glass house
experiments were carried out at the St. Joachim Estate, Tea Research Institute, Ratnapura, Sri
Lanka. The results of these experiments and investigations are presented in the sequence of
Chapters 5 to 7. As the materials and methods of many of the experiments overlap they are
presented here for the sake of brevity and clarity. This chapter begins with a brief summary of
the parameters of each experiment in the form of a table including objectives, location,
duration and measurements (Table 4.1). This is followed by a description of the main field
research site, its long term climatic conditions and description of the methods used across the
experiments.
The characteristics of the different tea cultivars tested and environmental
conditions during the experiments will be discussed in the Chapter 5 to 7 separately.
4.2
Experimental Overview
Table 4.1 presents an overview of the key parameters of each experiment, particularly the
‘where’, ‘when’, ‘with what’ and ‘how’ of each experiment.
Field experiments were
conducted with the object of revealing the performance of the tea plant during hot humid dry
period and inferring specially the weather relations. Experiments 1 and 2 in Chapter 5 and
experiment 6 in Chapter 7 are based on an in-line drip irrigation trial. Meanwhile experiment
3 of Chapter 5 evaluates the performance of the newly released tea clones during drought in
same field conditions, but not under irrigation. Glass house experiments (Chapter 7) were
conducted to simulate the conditions that could not be achieved in the field. The maximum
dry period observed in recent years at Ratnapura is 32 days in 1992. Because of this it was
necessary to simulate drought with two experiments involving young tea plants in a glass
house. In total, the experimental work occurred over the period 2007-2009. This table may
help to explain why the explication of the experimental work in the results chapters follows a
logical, rather than temporal, sequence.
57
Experiment
1) Physiological and yield
performance of two contrasting
tea cultivars in response to
irrigation
2) Water use of tea under rainfed and irrigated conditions
3) Physiological response of
new cultivars to water stress
4) Evaluation of irrigation
technology
5) Effect of water stress duration
on young tea plant growth
6) Effect of partial irrigation on
young tea growth
7) Effect of irrigation and raised
bed on young tea growth
8) Financial evaluation of drip
irrigation
Chapter
section
5.2
5.3
5.4
6.0
7.2
7.3
7.4
8.0
Description
for low elevation tea growing area
Economic analysis of drip irrigation
beds
Ratnapura
Estate
St.Joachim
Ratnapura
Estate
St.Joachim
Growth response of young tea in
field to drip irrigation and raised
Ratnapura
Glass House
Ratnapura
2001-2009
2009 Mar
2008 May-
2007 Jul-Aug
2007 Jul-Nov
2009 Mar
Estate Ratnapura
Glass House
2008 Jan-
2009 Jan-Mar
2009 Mar
2008 June-
2007 Jan-Mar
Period
St.Joachim
Ratnapura
Estate
St.Joachim
Ratnapura,
Estate
St.Joachim
Ratnapura.
Estate
St.Joachim
Location / Period
young tea plant growth and duration
Effect of water application rate on
physiology
on young plant growth and
Effect on duration of stress period
drip and sprinkler irrigation
Physiological and yield response to
released tea cultivars to drought
Physiological response of newly
and dry seasons
Transpiration measurement in wet
parameters , water potential
Yield related physiological
Table 4.1 An overview of the experimental parameters
TRI 2023
TRI 3025
TRI 2023
TRI 4042
TRI 4042
TRI 2023
TRI 3014
TRI 3025
TRI 4053
TRI 4047
TRI 4049
TRI 2023
TRI 2023
TRI 3025
Cultivars
Rate of Return
Net Present Value, Internal
dry matter production
growth parameters
dry matter production
growth parameters
dry matter production
growth parameters
yield
Pn, gs, El, Tl
Pn, El, Tl
dry matter production
Sap flow measurement
Pn, gs, El, Tl
Measurements
58
4.3
Field Experiment Site and Location
4.3.1
Site and soil description
The field and glasshouse experiments were carried out at Field no 01, St. Joachim Estate, Tea
Research Institute, Ratnapura Sri Lanka (latitude: 6040’N; longitude: 80025’E, altitude 29m
amsl). St.Joachim Estate is a research estate managed by Tea Research Institute of Sri Lanka.
In addition to the main crop of tea, there are some divisions with rubber cultivation in the
estate. Tea fields in low elevation areas are scattered, unlike tea estates in up country Sri
Lanka (Figure 4.1). The estate landscape is flat to undulating. Field no 01 is generally a flat
area and represent the general climate of low elevation tea growing area (Figure 4.2 and 4.3)
In addition to main crops like tea, rubber and rice (in flat alluvial plains), there is perennial
vegetation, planted on homesteads or growing naturally. This is a very different landscape
from high-grown tea, where there is little perennial vegetation other than trees regularly
planted as shade trees. Perennial vegetation improves the environment substantially as a
whole (Wickramasinghe 1992).
Presence of perennial vegetation provides shade and
coolness, lessening the impact of tropical sun (Renault, Hemakumara et al. 2001). Cultivation
of shade trees as either high or medium shade is an approved method in the area, however, as
the planted perennial trees too consume large portion of water in the area (Renault,
Hemakumara et al. 2001), shade trees were not planted in the irrigation research site. In the
small holder fields, it is rare to find proper high shade trees, as it is difficult to maintain high
shade plants in a small plot of land. The other assumption is that during wet season of the
year (Figure 4.4), shade plants could limit the sunshine hours available limiting the
productivity of irrigated plants.
The soil group of the site belongs to Red Yellow Podzol (Panabokke 1996), according to local
classification. According to the FAO/UNESCO classification, it is classified as Haplic Alisol
(Mapa, Somasiri et al. 1999). Surface soil layers are sandy clay loam and the sub surface soil
layers are clay loam soil. In agricultural terms, the soil is shallow and weathered rock
fragments are redundantly available in soil. The shallow soil depth is responsible for shallow
rooting of tea plants in the area. In the research field, there is an impermeable soil layer
sometimes as shallow as 1-2m. The volumetric moisture content of the soil at field capacity
and permanent wilting point are 27% and 14% (v/v) respectively. The water holding capacity
of the soil is 130mm/m. Mature tea plants in the Field no 01 were affected severely in a
major drought in 1992. Since then, a replanting program was launched with cultivar TRI
2026. Experimental section of the drip and sprinkler irrigation trials were planted in 1999 and
2000 (Details are given in Chapter 5.0 and 6.0).
59
A
NOTE:
This figure/table/image has been removed
to comply with copyright regulations.
It is included in the print copy of the thesis
held by the University of Adelaide Library.
Figure 4.1 Geographical distribution of major plantation crops in Sri Lanka. Ratnaura geographical location
6040’N, 80025’E (Department of Census & Statistics, Colombo)
60
A
NOTE:
This figure/table/image has been removed
to comply with copyright regulations.
It is included in the print copy of the thesis
held by the University of Adelaide Library.
Figure 4.2 Aerial view of main research field at St. Joachim Estate, Ratnapura, Sri Lanka (a) Drip irrigation site
(Experiments 1 and 2) (b) Sprinkler irrigation site (Experiment 4) (Source : map.google.com accessed on
26.09.2011)
Figure 4.3 View of sprinkler operation at Experiment 4 site. (Rain Bird SW 2000 sidewinder)
61
Figure 4.4 View of drip irrigated and rain-fed TRI 3025 plots in experiment 1. The scattered line shows the
boundary between irrigated and rain-fed plants. The rain-fed plants show defoliation during drought in 2009
March. (White plastic was to cover sap flow sensors during heavy rains)
4.3.2
Climate
The climate of the St Joachim Estate, Ratnapura can be described as hot humid climate, where
average air temperature is 28.0(±0.1)0C and relative humidity is 77(±0.4)%.
Highest
maximum air temperature are observed in February (34.5(±0.1)0C) and March (34.8(±0.2)0C).
The rest of the months are relatively cooler. The lowest air temperature of 22.1(±0.1)0C are
recorded in the months of January and February. The minimum air temperature is always
above the basal minimum temperature that supposedly needed to maintain the shoot growth in
tea (Burgess and Carr 1997). The initial 3 months of January to March receives highest
sunshine hours of more than 5 hours a day and the gloomiest month is October, where
average sun-shine hours is only 3.7(±0.2) hrs a day. The availability of solar radiation is
15.9MJm-2s-1 and again the months of January to March receive highest solar radiation.
Vapour Pressure deficit (VPD) is an important component in tea drought physiology and VPD
above 2.0kPa limits the shoot growth of tea (Squire 1979). The average VPD in the estate is
0.9(±0.02) kPa. Only in the months of February and March is there a 10% increase of the
VPD. The daily average plant water requirement according to FAO modified PenmanMonteith calculation (Allen, Pereira et al. 1998) is 2.9(±0.06)mm. Nevertheless, February
and March are high water demanding months, where crop water requirement average is 3.2
and 3.3mm/day respectively.
62
4.3.2
Rainfall pattern
The area belongs to WL1 agro ecological region (Wet zone, Low elevation), where 75%
expectancy of annual rainfall is 3200mm (Punyawardana 2008). The average annual rainfall
is 3824(±41) mm, with a bimodal rainfall pattern with two monsoonal rain peaks. The long
term annual rainfall data of more than 140 years of data showed no significant trend either
increasing or decreasing, despite concern about global climate change (Figure 4.5). The
variation of the annual rainfall is minimal (CV=12.6%). There is a chance of one in five
years of the annual rainfall varying either 3392 or 4254mm (Figure 4.6). It means that
extreme low or high rainfall years are rare. However, variation within the year was not
analysed. The South-West monsoon is very moist and variable, (mid May to September), and
North- East monsoon from December to February, which is dry and consistent (Fuchs 1989).
In addition, there are two inter-monsoonal periods also bringing rain.
The first inter-
monsoonal rains occur between March to April and second inter-monsoonal period occurs
between October to November (Eriyagama, Smakhtin et al. 2010).
There is a high variation in monthly rainfall, May and October are the highest rain months,
with mean monthly rainfall of 477(±18) and 474(±13) mm respectively. Average monthly
rain of January and February is lower, each month only receiving 4% of annual rainfall
(Figure 4.7).
Figure 4.8 shows the monthly tea crop water requirement (FAO modified Penman-Monteith
method) and monthly rainfall availability. The average water requirement in a month is
88(±1.7) mm/month. March is the highest water consuming month, with evapotranspiration
demand of 100mm. The average total rainfall availability in a month is always above the crop
water requirement. However as rainfall is lost from the root zone as run-off or drainage,
effective rainfall is lower than total rainfall (Burman, Cuenca et al. 1983; Allen, Pereira et al.
1998). Effective rainfall calculated for each month according to USDA soil conservation
method (USDA 1970; Allen, Pereira et al. 1998) and field based method are given in Figure
4.8. The effective rainfall of January and February, according to USDA (1970) method, are 25
and 19% higher than the crop water requirement, which is a marginal increase. The USDA
method of evaluating effective rainfall was based on the long-term data analysis. However,
according to field based estimation, effective rainfall is lower than plant water requirement in
the period from January to March. Field based methods are more accurate as they based on
field and soil characters rather than just monthly rainfall.
63
36
maximum
minimum
average
(a)
Temperature (0C)
34
32
30
28
26
24
22
Vapor pressure deificit (kPa)
sunshine
radiation
6
16
5
14
4
12
3
1.2
Crop evapotranspiration (mm/day)
18
(b)
Net Radiation (MJm-2day-1)
Sunshine (hours)
20
7
10
(c)
1.1
1.0
0.9
0.8
0.7
3.4
(d)
3.2
3.0
2.8
2.6
2.4
Jan
Feb Mar Apr May Jun July Aug Sep Oct Nov Dec
Month
Figure 4.5 Monthly climate average of St. Joachim Estate, Ratnapura (a)air temperature (0C) (b)sun shine hours
and solar radiation (MJm—2day-1) (c)vapour pressure deficit (kPa) (d)tea crop evapotranspiration (mm/day).
(Data shown based on 10 year weekly average, except tea crop evapotranspiration. Error bars show the
standard error)
64
A
NOTE:
This figure/table/image has been removed
to comply with copyright regulations.
It is included in the print copy of the thesis
held by the University of Adelaide Library.
Figure 4.6 Annual rainfall (mm) Ratnapura, Sri Lanka 1869-2010 (DeCosta 2011). Dashed line shows the mean
annual rainfall
A
NOTE:
This figure/table/image has been removed
to comply with copyright regulations.
It is included in the print copy of the thesis
held by the University of Adelaide Library.
Figure 4.7 Percentile of annual rainfall distribution 1869-2010 (DeCosta 2011)
65
1200
minimum
mean
maximum
25%
75%
Rainfall (mm/month)
1000
800
600
400
200
0
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Month
Figure 4.8 Monthly variation of rainfall Ratnapura, Sri Lanka 1869-2010. Monthly rainfall values of mean,
maximum, minimum, 25% and 75% expectant rainfall values are shown
4.3.3
Seasonal aridity
In the field tea plants experience water stress due to short rainless periods. Such short dry
spells may not be termed as conventional agricultural drought in countries other than Sri
Lanka. Drought is defined as a temporary lack of water, caused by abnormal climate,
damaging the environment (Wilk and Hughes 2002). More precisely, such short dry spells
can be termed as a seasonal aridity (Kallis 2008), i.e. a recurrent dry period after monsoon
rains. Nevertheless, the word “drought” is commonly used to refer to rainless periods greater
than 5 days for perennial crops in Wet zone of Sri Lanka (Sumanasena 2008). Even though
similar seasonal aridity occurs after the South West monsoon in August–September, and in
January – March, after North East monsoon, severity is high in the January–March period,
due to low monthly rainfall as well as high ambient temperature. (Figure 4.5 & 4.8).
4.4
Significance of short rain-free periods in creating water stress
Long term climate averages for Ratnapura were presented earlier to indicate the general
climate of the area and to identify water stress periods. However, combining the actual
weather data and plant water use enables us to identify the level of stress plants actually
experience in the field, especially during the establishment period. This section presents the
results of a desk-top analysis of long term rainfall data for Ratnapura to:
1. determine the frequency of various rain-free periods, and
66
2. to use a water stress coefficient to two case study seasons to illustrate the distribution
of water stress.
700
ET0
total RF
effective RF (USDA)
effective RF (field)
Water use & Rainfall (mm/month)
600
500
400
300
200
100
0
Jan
Feb
Mar
Apr
May
Jun
July
Aug
Sep
Oct
Nov
Dec
Month
Figure 4.9 Monthly tea crop water requirement (FAO modified Penman-Monteith method), monthly total rainfall
and effective rainfall according to USDA and field estimated method using long-term (>140 years) monthly
rainfall data
4.4.1
Daily rainfall intensity
The average annual rainfall in Ratnapura area for the period 1869-2010 period is 3822(±41)
mm. This rain is however not evenly distributed across the year. The high rain is mainly
caused by two monsoonal peaks which were described earlier. Based on the daily rainfall
data of 1986-2010, the average number of rainless days in a year is 156(±2.1) days (Figure
4.10a). In tea, to consider as a wet day, daily rainfall of at least 3mm should be received
(Ananthacumaraswamy and Prematunga 2008). The number of days, receiving rainfall less
than 3mm, is 200(±2.4).
Figure 4.10b shows the average duration of different frequency rain-free periods from 5 days
to 30 days during the period 1986-2010. The most frequent rain-free periods across the year
are of 6 to 7 days duration. On average there are between 3 to 4 such rain-free periods in a
year. During the measurement period, there were two major duration drought events of more
than 30 days. These were in 1989 and 1992. The 1992 drought lasted for 32 days and
average production of tea dropped by 25% due to this period
67
250
(a)
200
Days
150
100
50
0
0
>2
>3
Rain (mm/day)
4
(b)
Frequency
3
2
1
0
5
6-7
8-10
11-14
15-20
20-30
>30
Drought length (days)
Figure 4.10 (a) Average number of days without rain, >2mm/day or >3mm/day and (b) Frequency of drought
duration from 5 days to >30 during 1986-2010 period
4.4.2
Water stress coefficient (Ks)
Figure 4.10 in the above paragraph shows that on average tea plants experience 6-7 day
drought at least 3 times a year. Nevertheless, it does not represent that the tea plant is
experiencing severe drought stress in each of such occurrence due to (1) age of the plant mature plants has an ability to withstand drought. (2) soil factors, like water retention (Li,
Yang et al.) climatic factors – specially previous rainfall and air temperature during the
rainless period. It is a common factor that young tea plants are responsive for even short
duration drought. An attempt was made to quantify the drought effect one year old tea plant
could experience in low elevation tea growing area.
Water stress coefficient (Ks) for young tea was calculated based on the following formula
(Allen, Pereira et al. 1998).
68
‫ ݏܭ‬ൌ ܶ‫ ܹܣ‬െ ‫ݎܦ‬
ሺͳ െ ‫݌‬ሻܶ‫ܹܣ‬
Where, Dr is the root zone depletion (mm), and TAW is the total available soil water in the
root zone (mm). Dr is calculated using Instat climate software (Version 3.036), for a root
zone depth of 35cm. Dr was calculated based on the effective rainfall in an open tea field,
based on run off, drainage, stem fall and through fall (Appendix 1). Root zone depth was
based on sampling root length of large number (n=48) of one year old tea plants. A field
survey of rooting depths of mature tea bushes in the low country wet zone of Sri Lanka
showed that in this environment the root system was confined to a depth of 30-35cm. It was
further observed that beyond the above depth the root system moves laterally (Vithana 2003).
Water retention of the soil was calculated earlier for rain-fed fields as 130mm/m.
p is the fraction of TAW that a crop can extract from the root zone without suffering water
stress. For tea, p is given as 0.40 for non-shaded tree for evapotranspiration of 5.0mm/day.
Hence for Ratnapura p is calculated as follows (Allen, Pereira et al. 1998)
‫ ݌‬ൌ ͲǤͶ ൅ ͲǤͲͶሺͷ െ ‫ܶܧ‬ሻ
Where, ET is the tea evapotranspiration (mm/day).
Tea evapotranspiration was calculated based on the FAO modified Penman-Monteith
equation, with a crop coefficient of 0.85 (Allen, Pereira et al. 1998). There were some days,
where some climate variables were missing. In such days, ET0 was calculated based on the
Class A pan evaporation value and multiplying with crop coefficient of 0.85 (Laycock 1964).
The daily water stress coefficients for 2009 and 2010 are shown as the broken line in Figure
4.11, while the vertical bars are daily rainfall events. This figure shows that young plants are
experiencing severe water stress condition due to lack of rainfall specially in the first 4
months of the year. In addition, similar serious water stress conditions were observed in May
2009 and August 2010 a time of year normally considered as the wet season. The maximum
rain-free period in 2009 was 22 days and in 2010 it was 16 days. Overall 2009 was a very
wet year and it received annual rainfall of 4969mm, which is 30% higher than long term
annual average of 3824(±41) mm. It shows that even in very wet year, there is a chance for
young tea plants to experience severe water stress in the field.
69
Rain
KS
(a)
1.0
160
Rainfall (mm/day)
140
0.5
120
0.0
100
-0.5
80
60
-1.0
Water stress coefficient (KS)
180
40
-1.5
20
0
-2.0
(b)
1.0
160
Rainfall (mm/day)
140
0.5
120
0.0
100
-0.5
80
60
-1.0
Water stress coefficient (KS)
180
40
-1.5
20
0
Jan
-2.0
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Month
Figure 4.11 Water stress coefficient (Ks) for young tea, based on daily crop water requirement and daily effective
rainfall (a) year 2009 (b) year 2010
4.5
Conclusion
The field experiment site at St. Joachim Estate Ratnapura, belongs to the WL1a agro-climate
zone and shows a hot humid climate. While the annual rainfall far exceeds the plant water
requirement, the temporal distribution of rainfall within a year and lower rainfall availability
in January-March period, indicate distinct periods of water stress. These periods also receive
the maximum temperatures in the year. This analysis based on daily rainfall, crop water use
and soil water retention, supports the claim that plants experience severe water stress even
within a year with very high annual rainfall.
70
Chapter 5
Physiology, Water Use and Yield in Low Elevation Tea – Analyzing with
Special Reference to Dry Period of the Year
5.1
Introduction
In low elevation tea growing areas of Sri Lanka, drought causes significant yield reductions
and sometimes death of tea plants during the comparatively short dry period between January
and March. One prominent feature associated with drought in these areas is the high ambient
temperature which sometimes reaches more than 350C (Fuchs 1989). The effect of high
vapour pressure deficit (VPD) during drought is the main factor in lowering yield in other tea
growing regions, such as central and east Africa (Squire and Callander 1981). VPD does not
exceed 2.0 kPa in the low country of Sri Lanka and is not so much a problem. However,
exposure to high temperature is commonly recognised to adversely affect growth within a few
days, subsequently to impact on yield and sometimes the death of the plants.
Like water stress, high temperature stress strongly affects photosynthesis, growth and the
survival of plants (Chaves, Pereira et al. 2002).
In tea, photosynthesis and related
physiological parameters such as stomatal conductance, transpiration rate and transpiration
efficiency are used to evaluate the effects of drought (Lin 1998; Marimuthu and Kumar 1998;
Ajayakumar, Jayakumar et al. 2001). Among them, photosynthesis is particularly useful as it
is a direct measure of productivity (Bannerjee 1993), sensitivity to drought (Berry and
Bjo¨rkman 1980) and strongly influenced by temperature (Joshi and Palni 1998).
The influence of drought has been well recorded in the low elevation tea areas of Sri Lanka,
where monthly tea yields closely follow the bimodal rainfall pattern (Wijeratne and Fordham
1996). The relative responses of different tea cultivars have also been recorded and one of the
main strategies in controlling drought damage is the selection of drought-resistant cultivars
(TRISL 2002). Nevertheless, considerable drought-induced yield reduction still occurs even
with drought-resistant cultivars. This has prompted the need to find alternative ways to
minimize the effect of drought other than cultivar selection. Irrigation is one such method
applied in other tea growing regions such as in Tanzania (Möller and Weatherhead 2006;
Kigalu, Kimamboa et al. 2008).
There has been only one limited exploration of tea irrigation in Sri Lanka by
Ananthacumaraswamy et al, (1985). He applied drip irrigation to a young and mature stand
of TRI 2026, a drought-susceptible cultivar, for one season. The degree of the resultant yield
increases were such that suggested irrigation could be economically feasible. However, this
71
was conducted at Talawakelle which is a high elevation location. In this part of Sri Lanka the
tea estates are large and are resistant to invest in irrigation infrastructure. Also the impact of
irrigation in preventing plant death is minimal in this relatively cooler region.
As the dry period in low elevation areas is different to that at Talawakelle, and as the low
elevation growing areas are an increasingly significant contributor to Sri Lanka’s national tea
production, it is important to study the physiological performance, yield and water use of the
plant in response to irrigation in this region.
Even though irrigation has the potential to emerge as an alternative to cultivar selection as a
drought mitigation strategy, the existing cultivar breeding program still remains the key
mitigating drought strategy for the Sri Lankan tea industry. Progressing from previous TRI
2000 and TRI 3000 series cultivars, now the industry is using TRI 4000 series cultivars
released in the late 1990s. Nevertheless, the drought-susceptible cultivar TRI 2023 remains a
benchmark because of its high yield potential. Similarly TRI 3025 is considered a benchmark
for drought tolerance. So it will be necessary to evaluate some of the later releases against
these benchmarks.
This first step in understanding the feasibility of irrigation in low elevation areas of Sri Lanka
begins with the aims to:
1. describe the effect of irrigation on the physiology, water use and yield specifically in
the dry period of the year, and
2. assess some of the cultivar variation against benchmark cultivars.
It does this with the objectives of
a) understanding the environmental limitations to productivity, and
b) suggesting improved management practices for this environment.
Accordingly, this chapter describes three separate field experiments, conducted during the
2007-2009 period, which evaluate the physiology, productivity, water use performance in low
elevation tea growing area as response to drought and irrigation.
Experiment 1 evaluates the physiological and yield response of two benchmark tea cultivars
under irrigation.
Experiment 2, examines the water use in the wet and dry period of the most productive
cultivar (TRI 2023) in low elevation tea areas.
Experiment 3 evaluates the physiological performance of the present generation new cultivars
against the benchmark drought-resistant cultivar used in Experiment 1.
72
Each of these experiments is presented with their own methods and results and interpretation
sections. A plenary discussion section will draw together the interpretations from each of the
experiments to satisfy the chapter’s research aim and objectives.
73
5.2 Experiment 1: Physiological and Yield Performance of Two Contrasting Tea
Cultivars in Response to Irrigation
5.2.1
Introduction
This experiment evaluates the production-related physiology of two tea cultivars, TRI 2023
and TRI 3025 during the drought months of 2007.
It also evaluates the ability of
supplementary drip irrigation to compensate for the effect of drought on physiological
processes. The measurement of physiological parameters was made only over the drought
months of January and February when irrigation was applied.
However weekly yield
measurements were made over the whole year to see if this short irrigation period had a
meaningful effect on annual yield.
5.2.2
5.2.2.1
Method
Experimental design and field layout
The trial was conducted in an established drip irrigation plot in Field no 01 of St. Joachim
Estate, Ratnapura. Details about location, and average climatic conditions were given in
Chapter 4. The experimental design was a split-split plot. The system was laid out as two
sections, irrigated and non-irrigated with two cultivar subsections. Statistical analysis was
done using SAS statistical software package – version 9.0 (SAS Inc., USA).
Ideally, the irrigated and non-irrigated sections would have been broken up into blocks
randomly placed over the site. However, this facility was the only mature drip irrigated tea in
the area. There was no space on the estate, and no time, (another 3 years), to establish a
mature irrigated system. Given this limitation many measures were taken to limit the field
variation. Prior to planting, the field was excavated and all boulders and large stones were
removed. Also water proof plastic sheet was inserted to a 1m depth, in between the two
treatment plots to prevent sub surface water flow between plots. An aerial view of the
research field is shown on Figure 4.2. The main chemical and physical soil parameters that
could give rise to variation in yield were analysed. There were no significant discrepancies
between the irrigation and non irrigation plots in these parameters (see Table 5.1). One of the
main fertility parameter in the soil that determines tea yield is organic carbon content
(Wijeratne and Shyamalie 2009). Organic carbon content in the both irrigated and non
irrigated plots were similar except for 20cm depth. There were no significant differences
among two physical parameters of water holding capacity and bulk density.
74
Table 5.1 Major soil physical and chemical properties of Experiment 1 site at different soil depths
Parameter
Organic Carbon (%)
Potassium (ppm)
Phosphorous (ppm)
Manganese (ppm)
Water holding Capacity (mm/m)
-3
Bulk density (g/cm )
Depth
Non Irrigated
Irrigated
10
2.2(±0.05)
2.2(±0.06)
20
1.8(±0.06)
2.0(±0.04)
30
1.8(±0.05)
1.8(±0.05)
10
137.7(±12.5)
159.1(±13.3)
20
105.4(±10.5)
123.1(±9.7)
30
92.4(±9.0)
107.8(±8.2)
10
121.8(±9.4)
136.6(±8.4)
20
81.4(±7.9)
85.8(±8.4)
30
72.7(±8.8)
65.3(±5.9)
10
41.6(±5.7)
43.9(±3.5)
20
38.6(±4.38)
44.3(±4.5)
30
37.5(4.3)
41.9(±4.8)
130(±7.3)
127(7.4)
1.57(±0.02)
1.54(±0.03)
(cm)
(Organic carbon –(Walkley and Black 1934), K, P & Mn - (Westerman 1990))
5.2.2.2
Tea cultivar, planting and cultural practices
Vegetatively propagated plants of cultivar TRI 2023 and TRI 3025 were used for the
experiment. The plants were planted in the field in May 1999 and plucking was commenced
in October 2001. The spacing of the planting was according to a double hedgerow system.
Two plant rows were grown at 90cm spacing apart in a raised bed of 30cm height. Row
spacing was 60cm, where as distance between two raised beds was 150cm. The plant density
of the planting system is 12,500 plants/ha, which is the recommended plant density for tea
crops in Sri Lanka. Shade trees are conventionally planted in tea plantations. However, in this
situation they were not planted to avoid confounding factors like uneven solar radiation and
competition for water. The plants were pruned to a height of 60cm on May 2004 and May
2006 respectively. After each pruning, harvesting was commenced following a 4-5 months of
recovery.
Both cultivars are of Assam origin. TRI 2023 is a broad leaf variety which has a higher
susceptibility to drought. However, this cultivar has vigorous growth and high yielding
ability. The cultivar is the highest producing cultivar in the region but its productivity is very
75
vulnerable to drought (Piyasundara 2009). Commercial cultivation of this cultivar is not
advised in the region under normal rain-fed systems. TRI 3025 cultivar is also classified as a
high yielding variety, but it has an ability to withstand drought as well (TRISL 2002).
5.2.2.2
Water application and moisture measurement
Drip irrigation system was installed and commissioned in April 1999. The system consisted
of a pump unit, head control system with filtration and fertilizer tank, block valves and
laterals. Drip laterals were made up of low density 16 mm diameter polyethylene, laid on the
surface. The lateral drip lines were placed on each row of tea. Integral pressure compensated
drip lines (RAM 17D, Netafim, Israel) with a design discharge of 1.6L/dripper/h were spaced
at 60cm distance along the laterals. Accordingly, each plant had a dripper for irrigation.
Irrigation was specifically practised during the inter-monsoon dry spell between January and
March each year. The water requirement was calculated based on the Pan evaporation rate of
the previous day. A crop coefficient value of 0.85 has been used to estimate the actual water
requirement of tea (Laycock 1964). Pan evaporation based irrigation scheduling was used
earlier in other tea irrigation trials (Stephens and Carr 1991). Based on the water holding
capacity of the soil and temperature stress (when maximum air temperature exceeds 350C) on
non rainy days in the area, irrigation was commenced daily after 5 days of rainless period. In
wet zone of Sri Lanka, a 5 day rainless period causes significant drought stress to perennial
crops (Sumanasena 2008).
Though the design discharge of the dripper is 1.6L/hr, after a study of measuring the
uniformity of application, using 32 catch cans placed evenly for each cultivar 10 plots, it was
found that there was a variation in water application uniformity. The water application rates
for each cultivar are given in Table 5.2.
When operating the system, average water
application was assumed to be 1.4L/hr. However, irrigation water received for two cultivars
did not differ significantly. As a precautionary measure to prevent plants from dying, all rainfed plants were manually irrigated with 9L/plant, on 23 February 2007. By this time, there
was a very high defoliation of rain-fed plants and high number of dead twigs was visible on
canopy. This was equivalent for 13mm/plant irrigation. The maximum potential water
deficits of the irrigated and rain-fed plots were calculated as a difference between the rainfall
or irrigation and potential evapotranspiration of tea (ET), using Instat Climate Analysis
Software (Instat for Windows, 3.036, Statistical Services Centre, University of Reading, UK).
Maximum ET0 was calculated based on FAO modified Penman Monteith calculation. To
convert the potential evapotranspiration value for tea, 0.85 was used as the crop coefficient
(Allen, Pereira et al. 1998).
76
Table 5.2 Water application rates of two cultivars. (Two application rates are not significantly different as shown
by high standard error in parenthesis)
Cultivar
5.2.2.3
Water application
rate(L/hr)
TRI 2023
1.4(±0.2)
TRI 3025
1.3(±0.2)
Water potential measurements (Ψ)
Leaf water potential (Ψ) of the plants was measured at pre-dawn - Ψdawn (0530hrs) and midday - Ψnoon (1200hrs), after the fifth week. Measurements were taken at the field itself using a
Scholander pressure bomb (Soil Moisture Corp.). At least 10 fully grown top-most leaves
were selected from each treatment. Selected leaves were detached from the stem using a sharp
knife and enclosed in a sealed polythene bag prior to taking to the instrument.
5.2.2.3
Gas exchange measurements
Instantaneous net photosynthetic rate (Pn) Instantaneous transpiration rate (El), leaf
temperature (Tl) and stomatal conductance (gs) were measured in the topmost dark green
mature leaves, exposed to full sunlight, using a portable infra-red gas analyser (LCA-4, ADC
BioScientific, UK). Measurements were made under saturating light intensities when there
was no cloud cover, between 1200-1400 hours of the sampling date except in days where we
tested the diurnal variation of physiological activities. This was the time of the day when
water and heat stresses on plants were expected to be at their maximum. At least five plants
were measured in each treatment. The diurnal variation of Pn, El, gs and Tl were measured on
26 February 2007 at two-hour intervals.
The light response of photosynthesis was measured on a 27 March 2007, a cloudless day,
between 1100 and 1300 hrs by varying the light intensity incident on top of the leaf using
different layers of shade cloth to cover the leaf chamber. Following asymptotic exponential
function was used to fit the photosynthetic light response of each treatment (Boote and
Loomis 1991).
ܲ݊ ൌ ܲ݉ܽ‫ ݔ‬െ ሺܲ݉ܽ‫ ݔ‬൅ ܴ݀ ሻ݁ െ‫ܫ׎‬Ȁሺܲ݉ܽ‫ ݔ‬൅ܴ݀ ሻ
Where Pn is net photosynthetic rate (µmolCO2m-2s-1), I is the light intensity incident on the
leaf (µmolm-2s-1), Pmax is the light-saturated maximum Pn and Rd is the dark respiration rate.
Φ is the quantum efficiency (μmol(PAR)-1). Parameters were estimated using the PROC
NLIN procedure of SAS statistical package.
77
5.2.2.4
Harvesting
Manual harvesting was practised at 7 day intervals. The standard plant part used for plucking
is 2 leaves and the young unfurled bud. However, during the rainy period, the third leaf was
also harvested if it was found to be in a tender stage. Harvesting was carried out by regular
workers (tea pluckers) attached to research fields of the research station. Hence, chances for
the variation of yield, due to harvesting errors by the tea harvesters were minimal during all
experimental periods. Weight measurements were taken in the field, just after harvesting to
minimize the error of weight loss in transportation. Results are expressed in terms of made
tea by multiplying the fresh weight by 22.2% (Stephens and Carr 1991).
5.2.2.5
Stem canker infection
The incidence of stem canker, a fungal disease caused by Phomopsis theae, of irrigated and
rain-fed plots of either cultivar was assessed in June 2008. The evaluation ranked the incident
of stem canker in each treatment by a visual score of the disease. Examination was conducted
by two staff members of Plant Pathology Division, Tea Research Institute, Talawakelle, Sri
Lanka. Scoring was according to severity of infection, highest being 4 for severely infected
and 1 for non-infected plants.
5.2.3
5.2.3.1
Results
Meteorological conditions during the study period
The 10-week dry period, during which the irrigation treatment was applied, received a total of
221 mm of rainfall which is only 6% of the annual rainfall on this site. Moreover, monthly
rainfall throughout this period was lower than the minimum of 100 mm month-1 that is
necessary for successful tea production (Fuchs 1989). The site received more than 5 hours of
sunshine a day, except for the second week, in which high rainfall (76.7mm) was received.
Potential evapotranspiration also dropped to 2.6mm/day during the second week. Maximum
temperature and minimum temperature rose towards the 10th week. After the third week,
maximum temperature remained above 340C.
After the fourth week, saturation VPD
remained more than 1.0 kPa. However, it did not reached 2.0kPa, which is found to be
critical for tea cultivation in the area (Wijeratne 1994).
78
Table 5.3 Meteorological condition during first 10 weeks of 2007(time period when physiological observations
were made). Except rainfall others daily average values; ET0- potential evapotranspiration, N- sunshine
hours/day, RF-Rainfall, VPD – Vapor pressure deficit
Temperature
Wk
Wind
0
( C)
Period
N
-1
Max
Min
(kmh )
RF
VPD
ET0
(mm)
(kPa)
(mm)
1
1-7 Jan
34.1
19.2
0.31
6.9
0.0
0.9
3.6
2
8-14 Jan
31.3
21.6
0.24
3.0
76.7
0.8
2.6
3
15-21 Jan
33.6
21.8
0.33
5.2
4.5
0.9
3.3
4
22-28 Jan
34.0
20.4
0.30
4.9
22.6
0.9
3.2
5
29 Jan-4 Feb
34.4
20.9
0.53
6.6
0.4
1.1
3.8
6
05-11 Feb
34.8
20.8
0.24
7.5
0.0
1.2
4.0
7
12-18 Feb
35.9
21.6
0.05
7.7
0.0
1.3
4.2
8
19-25 Feb
35.9
22.0
0.04
5.6
2.6
1.3
3.7
9
26F-4 Mar
34.6
22.6
0.35
5.0
57.9
1.0
3.6
10
05-11 Mar
35.6
22.2
0.18
6.9
0.0
1.2
3.5
25
rain
ET0
200
20
150
15
100
10
50
5
0
ET0 (mm/week)
Rain (mm/week)
250
0
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Month
Figure 5.1 Average rainfall (mm/week) and potential evapotranspiration–ET0 (mm/week) during 2007. (Standard
error bar indicates the fluctuation of the rain and ET0 in each of the week in a month)
Overall the year 2007 was a typical year for the low elevation tea growing areas showing the
bimodal pattern of rainfall. Heavy rains were not available in the first three months period.
Also wide spread plant deaths due to drought, such as what happened in1982 or 1992, were
79
not reported. Each month’s average weekly potential evapotranspiration and weekly rainfall
are shown in Figure 5.1. Weekly values of rainfall are presented here as it reflects more
accurately the effect of rain or drought on tea yield, which is harvested at 7 day intervals.
Rainfall was low during the initial 3 month period of the year and during February; the
maximum water use per week exceeds weekly rainfall. April and September typically
received the highest rains, and rain was lower again in December.
Maximum soil water deficit during the study period, calculated as the difference between
maximum potential water use and rainfall, is shown in Figure 5.2. From the second week,
there was a reduction in soil moisture continuously until the eighth week. Rainfall of 23mm
during fourth week was unable to check the decreasing soil moisture content, but there was a
replenishment of the soil moisture after the ninth week when 58mm rain was received.
Week
0
2
4
6
8
10
12
Soil water deficit (mm)
-20
-40
-60
-80
-100
irrigated
rainfed
-120
Figure 5.2 Maximum potential soil water deficit(mm/m) of irrigated and rain-fed treatments up to 1m depth of soil
from January to March, 2007
In summary for this section, it is clear that this was a typical rainfall season and that
unirrigated plants were exposed to significant water stress during the excessively hot dry
period.
5.2.3.2
Leaf water potential (Ψ)
Pre-dawn and mid-day water potential after week 5 is given in Figure 5.3. Differences in predawn water potential between irrigated and non irrigated TRI 2023 were not significant until
the week 6. After that it decreased rapidly for rain-fed plants of TRI 2023. Similarly for TRI
3025 too, it was not significantly different until week 7 for rain-fed and irrigated plants. For
both cultivars there was an increase in pre-dawn water potential of the irrigated plants by
week 10, but for irrigated TRI 2023, the increase was 54% higher than irrigated TRI 3025.
80
Mid-day water potential of TRI 2023 showed a distinct difference between irrigated and rainfed plants in many of the observed days. The widest gap was observed at week 10. There
was no significant difference in TRI 3025 mid-day water potential due to irrigation till 8th
week. But after that, rain-fed TRI 3025 showed the highest mid-day water potential of
10.1bar. Mid-day water potential of the irrigated plots of both cultivars did not increase with
the progress of the drought (until week 10). This may be due to the irrigation application of
the plants since morning.
0.0
(b)
(a)
-0.5
-1.0
Water potential (MPa)
-1.5
-2.0
-2.5
-3.0
0
(d)
(c)
-2
-4
-6
-8
-10
TRI 3025 irrigated
TRI 3025 rainfed
TRI 2023 irrigated
TRI 2023 rainfed
-12
-14
4
5
6
7
8
9
10
11 4
5
6
7
8
9
10
11
Week no
Figure 5.3 Leaf water potential of two irrigated and rain-fed cultivar (a & b) at 0530hrs, (c & d) at 1200hrs
In summary, rain-fed and irrigated TRI 3025 showed a higher Ψdawn than TRI 2023 most
weeks. In TRI 2023, Ψdawn started decrease only after 8th week. There was a difference in
Ψnoon of TRI 2023. Highest Ψnoon was reported in rain-fed TRI 3025 at 10th week.
5.2.3.3
Photosynthesis (Pn)
The time courses of variation of Pn in both cultivars (Figure 5.4) showed significant increases
of Pn in the irrigated treatment as compared to the respective rain-fed treatment. The response
to irrigation was clearer and greater in TRI 2023 than in TRI 3025. P n of the rain-fed
treatment of both cultivars showed a gradual decline during the study period, probably
because of increasing soil water deficits. The decline in the rain-fed TRI 3025 was higher
(r2=0.82, P=0.002) than rain-fed TRI 2023 (r2=0.61, P=0.02). Pn of the rain-fed TRI 2023 had
81
increased in week 5, probably in response to the rainfall that occurred during week 4(Table
5.4). In contrast, Pn of TRI 3025 did not respond to this rainfall. Moreover, Pn of both
cultivars was not able to respond to the rainfall that had occurred during week 9. Under
irrigation, TRI 2023 had a slightly greater Pn than TRI 3025. During the period from week 3
to week 6, irrigated treatments of both cultivars showed a gradual decline in Pn, with the
decline in TRI 3025 being more pronounced than that of TRI 2023. However P n recovered in
later weeks for irrigated plants.
16
Rain
14
TRI 2023 irigated
TRI 2023 rainfed
100
80
12
60
8
40
6
4
20
2
0
0
100
16
Rain
TRI 3025 irrigated
TRI 3025 rainfed
14
12
Rainfall (mm/week)
Photosynthesis rate (Pmol CO2 m-2s-1)
10
80
10
60
8
40
6
4
20
2
0
0
0
2
4
6
8
10
12
Week no
Figure 5.4 Mid-day photosynthetic rate of top most mature tea leaves of two cultivars (TRI 2023 above, TRI 3025
below) during drought affected 10 weeks. Vertical bar indicates the rain received during each week.
In summary, the photosynthetic rates of both cultivars were suppressed during the dry period,
even when under drip irrigation. Nevertheless, photosynthesis of the irrigated cultivars was
33% and 38% higher for TRI 2023 and TRI 3025 respectively than under the rain-fed
treatments. Under irrigation TRI 2023 was 32% stronger photosynthesis than TRI 3025.
82
5.2.3.4
Stomatal conductance (gs)
The instantaneous stomatal conductance (gs) showed a similar pattern between the two water
regimes and the cultivars (Figure 5.5). (Measurements were taken only for 7 weeks, because
of breakdown of the instruments). Particularly, gs of TRI 3025 did not differ significantly
between the two water regimes. Although the gs of TRI 2023 showed significant variation
between the two water regimes, the variation was not consistent. For example, the irrigated
treatment had significantly greater gs during weeks 1 and 7, whereas the rain-fed treatment
had significantly greater gs during week 3. Stomatal conductance of both cultivars and water
regimes were within the range of 0.1 – 0.3mmol H2O m-2 s-1 on most days of measurement,
with occasional increases up to 0.5mmol m-2 s-1. During week 5, there was a surge in the
stomatal conductance of the all treatments, irrespective of water application or cultivar.
0.6
100
Rain
TRI 2023 irrigated
TRI 2023 rainfed
0.5
80
0.4
0.3
40
0.2
20
0.1
0.0
0
0.6
100
Rain
TRI 3025 irrigated
TRI 3025 rainfed
0.5
Rainfall (mm/week)
Stomatal conductance (mmol H2O m-2s-1)
60
80
0.4
60
0.3
40
0.2
20
0.1
0
0.0
0
1
2
3
4
5
6
7
8
Week
Figure 5.5 Mid-day stomatal conductance of top most mature tea leaves of two tea cultivars during drought
affected 7 week period. Vertical bar shows the rain received during each week.
83
In summary, both cultivars showed a similar trend line and a surge in the stomatal
conductance in week 5. However, gs were 40% and 7% higher under irrigation for TRI 2023
and TRI 3025 respectively, while rain-fed gs for both cultivars showed similar values.
5.2.3.5
Leaf transpiration (El)
In TRI 2023, El under irrigation was significantly greater than that under rain-fed conditions
on a majority of measurement days (Figure 5.6). In contrast, there was no such significant
variation between El the two treatments in TRI 3025. In TRI 3025, there was a continuous
decline in the transpiration over the drought season. Irrigated TRI 3025 plants showed the
greatest reduction (r2 = 0.8, P = 0.003). On the other hand, with the exception of week 4
which received rainfall, El of rain-fed TRI 3025 showed a gradual decline during the
experimental period. In rain-fed TRI 2023, such a downward decline was evident only after
week 5. On average, TRI 3025 showed a lower El than TRI 2023 under both irrigated and
rain-fed conditions. Even though gs showed a surge at week 5, El was increased under rainfed TRI 2023 only. For the irrigated TRI 2023 it was the same rate as the previous week.
In summary, TRI 3025 showed a decline in transpiration in rain-fed and irrigated plants. The
average El irrigated plants were 36% higher for TRI 2023 and 6% lower for TRI 3025.
Nevertheless under rain-fed conditions, TRI 2023 transpiration rate was 17% higher than TRI
3025.
5.2.3.6
Diurnal variation of photosynthesis, transpiration and leaf
temperature
Diurnal variation of Pn showed clear differences between the two cultivars, especially under
irrigation (Figure 5.7). The irrigated TRI 2023 maintained higher Pn levels throughout the
morning and up to 1430 hours in the afternoon beyond which a significant decline was
observed. On the other hand, in TRI 3025, the significant decline of Pn started earlier around
1230 hours.
Furthermore, TRI 2023 achieved a greater maximum Pn than TRI 3025. In both cultivars, Pn
of the rain-fed treatment remained significantly lower than the respective values in the
irrigated treatment at all times of the day except 1800 hours. Moreover, there was no
significant diurnal variation of Pn in the rain-fed crops until after 1600 hours. The Pn of rainfed TRI 2023 was slightly higher than the corresponding values of TRI 3025.
84
Rain
TRI 2023 irrigated
TRI 2023 rainfed
8
100
80
6
60
40
2
20
0
0
8
100
Rain
TRI 3025 irrigated
TRI 3025 rainfed
6
Rainfall (mm)
Transpiration rate (mmol H2O m-2s-1)
4
80
60
4
40
2
20
0
0
0
2
4
6
8
10
12
Week
Figure 5.6 Leaf transpiration rate during mid-day of top most mature tea leaves of two tea cultivars during drought
affected 10 weeks of 2007. Vertical bar shows the rain received during each week.
The diurnal variation patterns of El revealed clearer differences between the two cultivars and
between the two water regimes. El of the irrigated treatment of both cultivars increased
during the morning, reached a peak around mid-day and decreased thereafter throughout the
afternoon. El of irrigated TRI 3025 reached its peak earlier and also began its afternoon
decline earlier than TRI 2023. The peak El was significantly greater in TRI 2023 as compared
to TRI 3025. El of the rain-fed TRI 2023 also showed a diurnal pattern of variation which
was similar to that of its irrigated treatment. However, the increase of El during the first half
of the day was much lower than that of the irrigated treatment. Except at 1800 hours, E l of
irrigated TRI 2023 was significantly greater than its corresponding rain-fed treatment. In
contrast, in TRI 3025, a significant increase of El in the irrigated treatment over the rain-fed
was shown only up to 1430 hours.
85
Transpiration rate (mmol H2O m-2s-1)
Photosynthesis rate (PmolCO2m-2s-1)
1000
800
30
600
20
400
10
air temperature
solar radiation
200
0
16
Solar radiation (Wm-2)
Air temperature (0C)
40
0
14
TRI 2023 irrigated
TRI 2023 rainfed
TRI 3025 irrigated
TRI 3025 rainfed
12
10
8
6
4
2
0
4
3
2
1
0
800
1000
1200
1400
1600
1800
2000
Time (hours)
Figure 5.7 Diurnal variation of air temperature, and solar radiation and physiological parameters (photosynthesis
and transpiration) from 800 to 1800 hrs at 29 Feb 2007
In summary whilst under irrigation TRI 3025 cultivar had a decline in both P n and El rate as a
result to the increasing temperature, during day time.
Irrigated TRI 2023 maintained
significantly higher Pn and El during day time.
The diurnal variation in leaf temperature (Tl) of the irrigated treatment of both cultivars
closely followed that of air temperature (Ta) during the morning up to noon (Figure 5.8).
Importantly, Tl of the irrigated crops was significantly lower than Ta throughout the afternoon,
with the difference reaching a maximum of around 5oC around 1600 hours. In contrast, rainfed treatments of both cultivars had Tl values which were significantly greater than Ta
throughout the morning and early afternoon. Tl of both rain-fed cultivars exceeded 40oC
86
during the period by 1200 hours and this represented an increase of about 8 oC over the
prevailing Ta. There was significant cultivar variation in Tl under both water regimes. In the
rain-fed treatments, TRI 3025 had significantly greater Tl than TRI 2023 during the morning,
but, this trend was reversed during the afternoon. For rain-fed plants of both cultivars, Tl
remained above the critical temperature threshold of 350C during most of the day (from nearly
1000 to 1600 hrs). Also the leaf temperature remained more than 50C above air temperature,
for rain-fed plants.
45
TRI 2023 irrigated
TRI 2023 rainfed
TRI 3025 irrigated
TRI 3025 rainfed
air temperature
0
Temperature ( C)
40
35
30
25
20
600
800
1000
1200
1400
1600
1800
2000
Time (hours)
Figure 5.8 Diurnal variation of leaf temperature of top most mature tea leaves and air temperature from 800 to
1800 hours on 29 Feb 2007
In summary leaf temperature of rain-fed, TRI 3025 rose rapidly up to 1200hrs showing
significantly higher values than rain-fed TRI 2023. The difference among rain-fed and
irrigated plants were as high as nearly 100C, at around 1400hours when all the treatments
showed the maximum leaf temperature increase.
5.2.3.7
Light response
Light response showed clear cultivar variation (Figure 5.9), with TRI 2023 showing
significantly greater response to increasing incident light intensity than TRI 3025
(i.e.0.0066µmol CO2 m-2s-1 [µmol PAR m-2s-1]-1 as compared to 0.0033µmol CO2 m-2s-1
[µmol PAR m-2s-1]-1). On the other hand, light response did not differ significantly between
the two cultivars under rain-fed conditions.
response of Pn in both cultivars.
Irrigation significantly increased the light
The figure shows the light saturated maximum
87
photosynthetic rate (Pmax), dark respiration (Rd) and quantum efficiency (Φ). Pmax increased
under the irrigation by 119% and 100% for cultivar TRI 2023 and TRI 3025 respectively.
The cultivar TRI 2023 had a 62% higher Pmax under irrigation than for the cultivar TRI 3025.
Higher quantum efficiency (Φ) was observed in the TRI 3025 than TRI 2023 under both
irrigated and rain-fed conditions, and the former showed the higher value. For the cultivar
TRI 2023, Φ was much smaller comparatively and the highest Φ was observed with rain-fed
plants. For the cultivar TRI 2023, dark respiration (Rd) was 52% higher. In contrast for TRI
3025, under irrigated plants, it was 51% higher than rain-fed plants.
In summary, cultivar TRI 2023 showed a higher efficiency in assimilation under irrigated
condition and rain-fed conditions as well. Higher photorespiration was observed in TRI 3025
showing reduced photosynthetic efficiency in its leaves.
88
20
TRI 2023 rain-fed (Pmax = 6.8, Rd = 0.47, ) = 0.003)
15
10
5
0
-5
-10
TRI 2023 irrigated (Pmax = 14.9, Rd= 0.31, ) = 0.002)
20
15
Photosynthetic rate (Pmol CO2 m-2s-1)
10
5
0
-5
-10
20
TRI 3025 rain-fed (Pmax = 4.6, Rd = 0.72, ) = 0.013)
15
10
5
0
-5
-10
20
TRI 3025 irrigated (Pmax = 9.2, Rd = 1.09, ) = 0.021)
15
10
5
0
0
500
1000
1500
2000
2500
-5
-10
Solar radiation(Wm-2)
Figure 5.9 Light response of TRI 2023 and TRI 3025 rain-fed and irrigated plants during the dry spell of JanuaryMarch 2009. Measurements were taken on 27 Mar 2007. Note:Pmax=light saturated maximum photosynthetic
rate(μmol CO2 m-2s-1), Rd=dark respiration, Φ=quantum efficiency(μmol(PAR)-1)
5.2.3.8
Response to ambient temperature
Figure 5.10 presents the relationship between photosynthetic rate and the daily maximum air
temperature of TRI 2023 and TRI 3025 under rain-fed and irrigated conditions during January
to March 2007.
A more negative relationship was observed in rain-fed plants of both
cultivars with TRI 3025 was more sensitive.
For each 10C increase in air temperature
between 33-360C, Pn of TRI 3025 was reduced by 2.75 µmol CO2 m-2s-1, compared with 2.44
µmolm-2s-1 in TRI 2023. Among the irrigated treatments, TRI 3025 showed a significantly
negative relationship (r2=0.57, P=0.03) with increasing ambient temperature. Decline in
89
photosynthesis with rising temperature in irrigated TRI 3025 was 1.83µmol CO2 m-2s-1 per
10C. Nevertheless, photosynthesis in the irrigated TRI 2023 was not significantly affected to
temperature.
14
TRI 2023
Photosynthetic rate (Pmol CO2 m-2s-1)
12
10
8
6
4
irrigated y = 36.9 - 0.76x (r2=0.11, P=0.4)
rain-fed y = 92.2 - 2.44x (r2=0.75, P=0.005)
2
0
14
TRI 3025
12
10
8
6
4
irrigated y = 71.8 - 1.83x (r2=0.57, P=0.03)
rain-fed y=101-2.75x (r2=0.75, P=0.005)
2
0
33
34
35
36
Maximum air temperature (0C)
Figure 5.10 Relationship between maximum air temperature and photosynthetic rate during January-March, 2007
in low elevation tea growing area. (Air temperature shows the maximum air temperature of each day)
In summary TRI 3025 was more sensitive to the increasing ambient air temperature, even
with irrigation in maintaining the mid-day Pn.
5.2.3.9
Annual yield variation
Average weekly yield in made tea kg/ha is given in Figure 5.11. According to the graph,
during months of January to April, there was a decline in the tea harvest in both cultivars,
even under irrigation. Lowest yields were recorded in months of February and March. For
the cultivar TRI 2023 average bi-monthly yield of February and March was 75 and 63%
lower than monthly average of the year for 2007, for rain-fed and irrigated plants respectively.
For the cultivar TRI 3025, yield reductions for the same moths were 84 and 71% respectively
for rain-fed and irrigated plants.
90
200
irrigation
150
Made tea yield (kg/ha/week)
100
50
TRI 2023 irrigated
TRI 2023 rainfed
0
200
irrigation
150
100
50
TRI 3025 irrigated
TRI 3025 rainfed
0
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Month
Figure 5.11 Average made tea yield (kg/ha/week) during 2007. Standard error shows the yield fluctuation within
month
Even though irrigation was applied only during first three months of the year, as shown in the
Figure 5.11, cultivar TRI 2023 produced significantly higher yield under the irrigation
treatment in each month except January and June. The rain-fed treatment never produced
higher yield than irrigated treatment for cultivar TRI 2023. Irrigated TRI 2023 produced
significantly higher yields (P=0.004) than rain-fed TRI 2023 in July, August and September.
Only in August, irrigated TRI 3025 produced significantly higher (P=0.03) yield than rain-fed
TRI 3025 during wet season. There were some months in the year where rain-fed treatment
produced higher yield than irrigated treatment for TRI 3025.
Total made tea yield during for the year 2007 is given in Table 5.4. Cultivar TRI 2023
produced the highest yield of made tea 5650 (±220) kg/ha. Irrigated TRI 2023 yield is 16%
higher than irrigated TRI 2023 cultivar. Both cultivar effect and treatment effect are highly
91
significant. However, there is no cultivar and irrigation interaction. The monthly variation of
yield within the year was higher for cultivar TRI 2023 than TRI 3025.
Table 5.4 Total tea yield for year 2007. Standard error is given in parenthesis.
Treatment
Yield (Made tea
kg/ha)
TRI 2023 rain-fed
4468(±327)
TRI 2023 irrigated
5650(±220)
TRI 3025 rain-fed
4176(±137)
TRI 3025 irrigated
4859(±160)
Significance
Cultivar
0.0009
Irrigation
0.0001
Cultivar X Irrigation
0.24
In summary, while the irrigation treatment did not rise the dry season yields to that of the wet
season yields it did significantly increase the annual made tea production. Irrigated TRI 2023
yielded 27% more than rain-fed TRI 2023. TRI 2023 also yielded 16% more than TRI 3025
under irrigation. There was relatively little effect of irrigation on TRI 3025.
5.2.3.10
Yield response to climatic factors:
The climate of 2007 followed a normal pattern for the WL1 agro-ecological zone of Sri Lanka
(Punyawardana 2008). The drought period of 2007 is normal for the area, which prevailed
during January to March of the year. The choice of 2007 as the reference year was valid
because it is the second year of the third pruning cycle and it justifies the presence of average
rainfall for the year. A stepwise regression (forward selection) was used to analyse best
predictive variable for expressing tea weekly tea yield. Weekly tea yield of 47 harvested
weeks and climate parameters of the previous week, temperature (min. and max.), sunshine
duration, rainfall, evaporation and vapour pressure deficit was used for the analysis. Table
5.5 presents the relationships between key micrometeorological parameters and made tea
yield under the four irrigation-cultivar treatments.
92
Table 5.5 Relationship between micrometeorological parameters and yield of irrigated and rain-fed two tea
cultivars during 2007 (Tea yield of 47 weeks were used for the calculation)
TRI 2023 rain-fed
TRI 2023 irrigate
TRI 3025 rain-fed
TRI 3025 irrigate
Variable
slope
P
slope.
P
slope.
P
slope.
P
Tmin
6.1
0.27
13.0
0.02
4.5
0.32
9.6
0.06
Tmax
-28.8
<0.001
-26.4
0.002
-27.1
<0.001
-22.9
0.003
1.2
0.58
1.0
0.64
-0.09
0.64
-0.9
0.66
-0.06
0.64
-0.04
0.78
0.03
0.75
0.03
0.83
Evaporation
1.4
0.42
0.9
0.59
0.15
0.91
-0.09
0.95
VPD
30.0
0.60
53.0
0.36
73.7
0.13
66.7
0.2
N
RF
This analysis shows that daily maximum air temperature is the best variable to express the
made tea yield in low elevation tea growing area for both cultivars, under irrigated or rain-fed
condition. Nevertheless, the lowest effect was shown by irrigated TRI 3025 cultivar with the
coefficient of -22.9(P=0.003). The irrigation treatment tended to reduce the effect with the
greater response being observed in TRI 3025.
5.2.3.11
Stem canker infection
Severity of the stem canker infection of the plant is illustrated in Figure 5.12, according to an
assessment on June 2008. Analysis revealed the level of disease infection after exposing the
plants to series of dry spells for nearly 9 years in the field. Cultivar TRI 2023 was more prone
to the infection even under irrigation.
Nevertheless, irrigation reduced the infection
significantly among both cultivars. Under the usual practise of rain-fed cultivation, 93% of
TRI 2023 was severely infested and almost all the plants were infested to a certain degree in
both cultivar. Cultivar difference in resistance to the disease was prominent even under
irrigation. The proportion of severely and moderately affected plants was more than double
in irrigated TRI 2023 than irrigated TRI 3025.
93
100
severe
moderate
less infection
no infection
Infected population (%)
80
60
40
20
0
I
TR
20
2
d
-fe
ain
r
3
I
TR
20
2
d
ate
rrig
i
3
I
TR
30
2
d
-fe
ain
r
5
I
TR
30
2
d
ate
rrig
i
5
Treatment
Figure 5.12 Incidence of stem canker among tea plants of two tea cultivar as assessed by June 2008.
In summary when grown as a rain-fed crop, all TRI 2023 and TRI 3025 plants were infected
with stem canker. Severe infestation level in TRI 2023 was more than double that of rain-fed
TRI 3025. Irrigation reduced the sever infestation nearly 80% in both cultivars.
5.2.4
Discussion
The physiological behavior of two drought-susceptible and drought-resistant tea cultivars was
evaluated during the dry spell of January to March, 2007. In addition to physiological
behavior, made tea yield and the incidence of major drought related fungal infection were
also evaluated as these two factors are very important in irrigation adaptation and cultivar
selection for low elevation tea growing area.
This section discusses mainly the above
characters of the two cultivars in relation to irrigation.
Leaf water potential of both cultivars at pre-dawn and mid-day are shown in figure 5.3.
Irrigation effect was observed mainly in TRI 2023. Till the week 10, Ψmidday was equal in
both irrigated and rain-fed TRI 3025. But the difference was seen in Ψmidday of irrigated and
rain-fed TRI 2023. For both cultivar under rain-fed condition Ψmidday had a close negative
relationship with Pn (r2=0.82 for TRI 2023 and 0.78 for TRI 3025). But the relationship is
poor for irrigated plants.
94
Initial decline in the soil moisture caused the Pn to reduce in the plants. Even for a small
reduction of soil moisture (nearly 20%) of the irrigated plants, coupled with high temperature
caused further decline of Pn in irrigated plants. The physiological response to irrigation varies
with tea cultivar. Similar cultivar difference as response to irrigation was reported under low
temperature growing areas as well (Smith, Burgess et al. 1993).
TRI 3025 cultivar showed a continuous decline in the Pn and El even under irrigated
conditions. This is a strong adaptation to survive the drought period. Due to the horizontal
nature of the TRI 3025 plants, the leaf surface receives more solar radiation and could excite
energy to build up heat within short time as shown in Figure 5.5. By 1200hours, rain-fed TRI
3025 leaves were showing 40C above rain-fed TRI 3025. On the other hand TRI 2023 leaves
are not horizontal and leaf temperature increase is slower within day time. Even though 340C increase in leaf temperature above the ambient temperature were recorded in other crops
(Tőkei and Dunkel 2004), such leaf temperature was not recorded for tea in other places with
cool dry periods.
As there was no major reduction in the gs of the tea plants, El reduction could more be
attributed to the less absorption of the water by roots. There is a slight difference in the rates
of gs for both treatments in both cultivars, however the rate of reduction is not as prominent as
it is for Pn and El. Stomatal closure of the leaves is related to increasing VPD (Ishida, Toma
et al. 1999) or low-light condition (Larcher 1995) and mainly in tea the response of gs is more
associated with high vapour pressure deficit in tea (Hajra and Kumar 1999). VPD however
did not exceed 1.3kPa during the trial period, which is much lower than the critical value of
2.0kPa for the tea plant (Squire 1985). Hence in the low elevation tea growing areas hence,
vapour pressure deficit is not the most influential factor controlling the stomatal activity.
The factors controlling the effects of temperature on the physiology of the plant are not very
clear in tea (Barbora 1994). However, both stomatal and mesophyll factors control the
temperature dependence of the photosynthesis of the plant (Joshi and Palni 1998). As there
was no significant variation in gs, the control of Pn is more likely attributed to mesophyll
factors (Lin 1998) in the low elevation tea growing area. But in other crops, mid-day
depression of the Pn is more attributed to the reduction in gs (Winkel and Rambal 1993;
Pathre, Sinha et al. 1998).
The two cultivars showed clear difference in their response to light and temperature. The
irrigated TRI 2023 was very high in its photosynthetic activity as a response to PAR. Lower
quantum efficiency (Φ) of rain-fed plants indicates the photo-inhibition caused by the heat
and drought stress (Pallardy 2008). Damage to the photosynthesis system may be the reason
95
for the rain-fed plants to produce lower yield even after the soil moisture condition
replenished with onset of rains.
In earlier studies of tea there were suggestions that Pn was more related to plant productivity
than other perennial crops as vegetative young shoots are harvested (Roberts and Keys 1978).
But later came the argument that Pn and shoot growth could be uncoupled during drought
(Squire and Callander 1981). Nevertheless in early studies, drought effect consisted of soil
moisture deficit, high saturation deficit and low temperature, factors that control the shoot
growth not only photosynthesis (Smith, Stephens et al. 1993). In the present study, high
temperature effect can be termed as the limiting factor of Pn.
Leaf temperature increase up to 80C than ambient temperature was observed among rain-fed
TRI 2023 and TRI 3025. Among the two cultivars showed a higher Tl increase during day
time (Figure 5.8). Leaf orientation of the TRI 3025 is horizontal and exposes to more solar
radiation. As shown in Figure 5.9, TRI 3025 receive higher PAR than it required for Pmax.
Receiving excess light beyond photosynthetic demand causes photo-inhibition of photosystem
II (Long, Humphries et al. 1994) and continuous photo-inhibition leads to photo-damage
(Asada 1999). Photo-damage of rain-fed plants then result in lower Pn and subsequently
lower yield in wet season months like April and May.
Irrigation increased the yield of TRI 2023 by 27% and TRI 3025 by 16%.
Cultivar
differences were clearly visible in response to irrigation. Similar differences have been
observed in other tea growing environment as well (Stephens and Carr 1991; Kigalu,
Kimamboa et al. 2008). Of particular note in the current study the irrigated plants produced
higher yield long after ceasing irrigation during wet period of the year. In addition to
damages to photosynthetic apparatus of rain-fed plants, the high leaf area usually associated
with irrigated plants could be another reason for higher yields. Production dips in irrigated
plants in dry months can be related to lower Pn and lower dry matter partition to shoot
production. As it is one irrigation regime was tested in this experiment, it is premature to
assume that irrigation deficiency caused the yield loss. However, it will be important to test
several water application efficiencies, with higher application rate than the tested one.
The incidence of stem canker was very high among rain-fed TRI 2023 (92% severely
infested) while cultivar TRI 3025 was more resistant to the disease. Irrigation has the ability
to reduce the severely affected percentage by nearly 80% in both cultivars. This experiment
confirms Carr’s (2010) proposal that plants grown under irrigation from field planting are
healthier and more resistant to drought-related stem canker. This fungal disease caused by
Phomopsis theae is encouraged by cavitations in the stem due to deficits in the plant water
balance (Kuhns, Garrett et al. 1985). The main reason to abandon the highly productive TRI
96
2023 in the cultivation program is the heavy infestation of stem canker in dry weather. On the
other hand TRI 3025 was more resistant to the disease in rain-fed condition and is the main
reason for introducing to the growers as drought tolerant cultivar. Under irrigation there is
good cause to re-consider TRI 2023’s role in the national cultivation program.
5.2.5
Summary of Results
x
Difference in Ψ was observed among two cultivars at dawn mostly.
x
TRI 2023, however, showed difference in Ψnoon according to irrigation treatment.
x
Rain-fed TRI 2023, showed the lowest Ψnoon by 10th week, closer to permanent wilting
point.
x
There was a decline in Pn rate with drought and it did not recover even after rain
occurrence under rain-fed conditions for both cultivars.
x
For cultivar TRI 3025, even though gs, increased in some weeks amidst drought
condition, due to favourable atmospheric conditions, El did not increase accordingly.
x
Cultivar TRI 2023 responded favourable to changes in gs. This factor further indicates
the increased water requirement for highly productive cultivar and the stomatal
activity of TRI 3025 was lower than TRI 2023, even under irrigated conditions.
x
Diurnal pattern of Tl showed that under rain-fed conditions in low elevation tea
growing areas, Tl increased above critical level of 350C for several hours during the
day even within short dry periods.
x
There is a dip in the average weekly made tea production even for irrigated plants
during dry periods.
x
The cultivar difference in the production gap is visible under rain-fed and irrigated
conditions. There is no interaction between irrigation and cultivar selection
x
Advantage of irrigation in yield is reflected prominently even during wet period for
this leafy crop.
x
Incidence of dry weather stem infection of canker was more related to cultivar
variation even under irrigation. However, irrigation was effective in controlling the
disease.
97
5.3
Experiment 2: Water Use of Tea under Rain-fed and Irrigated Conditions
5.3.1
Introduction
The previous experiment showed that TRI 2023 responds more favourably to irrigation than
TRI 3025. It seems that TRI 2023’s advantage lies, at least partially, in its ability to maintain
photosynthesis under higher temperature. A short period of 3 months irrigation during the dry
season resulted in 27% increased annual yield. The effect of irrigation in the dry period
carried over into increased yields during the unirrigated months of the wet season. This
ability to produce more even when the irrigation is turned off may lie in the fact that greater
branching and plucking points develop over time and also physiological reasons. However, to
follow on from our understanding of the influence of temperature and photosynthesis on
yield, this next experiment, evaluates the relationship between the water use of the TRI 2023
and dry matter production.
It was undertaken in 2008-2009, on the same site as in
Experiment 1, when the necessary sap flow measurement equipment became available.
Together, Experiments 1 and 2 satisfy the first aim of this chapter.
5.3.2
Materials and Method
Experiment 2 was conducted at Tea Research Institute, Ratnapura, Sri Lanka, at the same site
where experiment 1 was conducted. A site description is provided in Chapter 4.0.
5.3.2.1
General experiment details
Mature (8 years old) tea plants of cultivar TRI 2023 were used for the experiment. The plants
were established in the field in 1999 at a spacing of 60cm X 90cm X 150cm (double hedge
row planting). Shade trees were not planted in the field to avoid possible use of irrigation
water by the shade trees and to prevent uneven solar radiation availability to the field. The
plants were in the third pruning cycle and last pruning was done in 2006 to a height of 90cm.
Data collection was conducted from August 2008 to March 2009. This duration had a wet
period and a short inter-monsoon dry spell. Sensors were removed during prolonged rainy
days, as the stem flow and through fall was very high causing possible moisture leak to the
sensors. Netafim Ram17D integral drip lines were used to irrigate the plants. The spacing
between two drippers was each 60cm and each drip delivered 1.6L/hour. This spacing of the
drippers was selected to ensure irrigation water for each plant from a dripper. To measure the
dry matter production during non drought period and environmental effects, transpiration and
shoot development of rain-fed tea was monitored continuously during August–November,
2008 for nearly 3 months. This season is a wet period with a short inter-monsoon dry spell.
To monitor the water use of irrigated and rain-fed plants for a continuous dry period,
98
transpiration and shoot development of irrigated and rain-fed TRI 2023 plants were measured
during January–March 2009. This period was a dry season, and supplementary irrigation was
applied throughout the period as described earlier. To measure dry matter production, all new
shoots were removed at 7 day intervals. The shoots were then oven dried at a temperature of
1050C to reach a constant weight after 12 hours. Total dry matter production of the plant per
week was estimated using a harvest index of 0.10 (Magambo and Cannell 1981)
Figure 5.13 Sap flow sensor fixed to a drip irrigated TRI 2023 tea plant. Sensor is covered with insulating
material to prevent heat absorption. (Note: Data logger shown is Campbell CR10X datalogger)
5.3.2.2
Instrumentation
Though there are several methods available to estimate the transpiration, sap flow estimation
is the most popular method (Dragoni, Lakso et al. 2005) and this method provide direct
readings on the water use of plants. When compared to lysimeter technique, this is a less
expensive method as well. Sap flow is measured by heat pulse as a tracer (Cohen, Fuchs et al.
1981). The principle of the measurement is based on measuring the time required for a heat
pulse applied at a given point of the stem to be transferred and measured at a specific location
above the point where the heat pulse was generated (Ananthacumaraswamy, De Costa et al.
2000). The time taken is a measurement of the rate of sap flow in the xylem. By analysing
cross sectional area, flow geometry and flow velocity, flow density of sap can be calculated
(Swanson 1994).
Measurements were taken with East 30 sap flow sensors (www.east30sensors.com).
It
consists of a pair of 35mm long stainless steel needles, 6mm apart. One needle contains a
heater and other needle contains three precision thermistor sensors evenly placed at 5, 17.5
and 30mm distance from base. Before insertion, two holes were drilled precisely using a drill
99
guide template. The holes were drilled in such a manner that facilitated smooth insertion and
full contact of the two rods to the stem. The needles were inserted into the stem after
removing the bark slightly. A wax layer was applied to the two rods for easy penetration.
The heater was placed below the thermocouple. After installation, the sensor was covered
with a sponge material by wrapping over the stem and then it was covered with insulating
material to prevent radial heat loss and preventing unnecessary heat build up during very dry
day (Figure 5.13). The sensor was then fitted to a CR21X Campbell data logger (Campbell
Scientific Inc.,USA). The heater was powered by a 12V rechargeable battery. Data logger
control ports switch on/off the heat pulse every second. A current is applied for 8 seconds to
generate heat. Duration of the heat pulse was controlled by a programmed counter (Ischida,
Campbell et al. 1991). The rise in temperature of the thermocouple was recorded every
second and data were averaged and stored at 60 minute intervals. The time taken by pulse
peaks to reach the thermistor was related to sap flow velocity. Three thermistors gave the sap
flow at three different depths and by multiplying with sap wood area, transpiration rate was
calculated (Thermallogic 2002).
5.3.2.3
Meteorological data
Meteorological data were collected using an Automatic Weather Station (Measurement
Engineering Australia) established in the same field.
Data were collected to Starlog-
Prologger (www.unidata.com.au) data logger at 15 minute interval.
Temperature, solar
radiation, relative humidity and wind speed were recorded in the data logger. Daily rainfall
and sunshine hours of each day were recorded at a manual operated weather station, located in
the nearby tea estate (<200m), which supply weather data to Department of Meteorology, Sri
Lanka.
5.3.2.4
Soil moisture
Soil moisture content was measured gravimetrically using carefully driven undisturbed core
samples up to 60cm depth at weekly intervals. The fresh weight of each sample was recorded
soon after obtaining the samples in the field. Then the samples were oven dried for 24 hr
period at 1050C to reach a constant dry weight. Weight of oven dried samples was recorded
to calculate the moisture weight of the core sample.
5.3.3
5.3.3.1
Results
Transpiration (E) of rain-fed plants during wet season
The study period of August to November, 2008 was generally a wet period of the year, where
rainfall of more than 400mm was received. The average rainfall availability during the study
100
period was more than 100mm per month. Vapour pressure deficit remained below 1.2kPa.
Also maximum and minimum temperature did not show large increases.
Figure 5.14 presents a combination of daily rainfall events (mm/d), weekly gravimetric soil
moisture measurements at 3 depths, daily potential evapotranspiration (ET0) as calculated by
modified Penman-Monteith and daily transpiration (E) (mm/d) as measured by sapflow. This
can be used to indicate the boundaries of transpiration under non-irrigated conditions.
70
Rain
20cm
40cm
60cm
25
60
50
20
40
15
30
10
Rainfall (mm/day)
Volumetric moisture content (v/v)
30
20
5
10
0
0
6
E
ET0
Water use (mm/day)
5
4
3
2
1
0
4/8/08
18/8/08
1/9/08
15/9/08
29/9/08
13/10/08
27/10/08
10/11/08
Date
Figure 5.14 Daily rainfall and weekly volumetric soil moisture level of top 60cm soil (a) and daily plant
transpiration E and daily potential evapotranspiration-ET0 (b) from August to November, 2008
Initially, the moisture content of 60cm depth was approximately 10%, which is very low and
reaching the permanent wilting point. Low rainfall during the early part of the month may
have caused the reduction of the soil moisture in deeper layers.
Soil moisture levels
replenished with the onset of rain in early September 2008. Soil moisture dropped again with
the cessation of rain in the later part of the month. With the start of rain again in October, soil
101
moisture at both 20 and 40cm depth fluctuated, but at 40cm depth it increased with the rainy
season.
The average level of transpiration was 2.3(±0.3)mm/day. Although this was the wet season,
soil moisture was low and the transpiration was low. The general trend of the transpiration
pattern is an upward pattern, which was associated with the increase in rains receiving at the
end of the season. Between 19 September 2008 and 21 October 2008, E was over 3mm/day.
E was less than ET0 for the most of the period. The average crop coefficient was 0.848(±0.1).
This value is equal to the crop coefficient given in the FAO guideline for tea (Allen, Pereira et
al. 1998; Kigalu 2007). However, this is not necessarily a universal value as the Kenyan
cultivar AHP S15/10 showed a higher crop coefficient value of 0.98 (Kigalu 2007).
The transpiration exceeded the ET0 value when the solar radiation level became lower (data
not shown in figure) during period 16-27 October 2008. Though, ET0 showed a high daily
variation, actual plant transpiration was more consistent.
During the cloudless periods when it was not raining (e.g.15 Sept-10 Oct) and soil moistures
was low (<20% v/v), transpiration remained relatively high due to relatively increased
temperature and sunshine. The maximum temperature level during the study period did not
reach the critical level of 350C, unlike the prominent dry periods of January- March each year.
The plant was not under significant soil moisture stress during latter part of the study period,
nor did temperature reach the critical upper threshold, transpiration remained high.
In summary, transpiration can remain high even without recent rain as critical temperatures
are not reached as well as soil moisture is adequate. However, even during the wet season
soil moisture deficits in deeper soil layers can impact transpiration processes.
5.3.3.1.1 Dry matter production in the wet season
The average dry matter production of TRI 2023 during this 3 month wet season was
73.6(±8.5)g/plant/week (Figure 5.15). The minimum dry matter production per week was
29.3(±3.4)g/plant and the highest dry matter was recorded in the 110.5(±17.1)g/plant.
Generally dry matter production followed the transpiration pattern. The drop off in dry matter
production in the first two weeks of September is probably related to the heavy rainfall (see
Figure 5.14) and low light levels during this period. This will explain a similar drop off in
November. This is also reflected in a slowdown in cumulative transpiration during these
events.
102
4
140
120
3
100
2
80
60
1
40
0
20
200
(b)
1200
Cumulative plant water use (mm)
180
1000
160
140
800
120
600
100
80
400
60
40
200
20
0
Cumulative plant dry matter production (g)
Plant water use (mm/day)
E
dry matter
Total plant dry matter production (g/week)
(a)
0
4/8/08
18/8/08
1/9/08
15/9/08
29/9/08
13/10/08
27/10/08
10/11/08
Date
Figure 5.15 Daily transpiration-E and weekly average total dry matter production (a) and cumulative transpiration
and dry matter production from August to November, 2008 (b)
5.3.3.1.2 Response to environmental variables:
Figure 5.16 shows the transpiration response to different environmental variables.
Transpiration showed strong positive relationships with solar radiation, vapour pressure
deficit, maximum temperature and vapour pressure deficit.
Among above parameters, maximum temperature level of the day had a strongest relationship
(r2=0.62) with transpiration. According to the forward regression analysis of the parameters,
both solar radiation and maximum temperature could be used to predict the transpiration of
the tea plant (P=0.001) during wet season. However, maximum temperature level had the
highest correlation coefficient. This appears to be the characteristic limiting environmental
parameter for tea grown in low elevation areas. Earlier studies on the tea plant transpiration
103
found that transpiration is mainly controlled by solar radiation (Ananthacumaraswamy, De
Costa et al. 2000) in low temperature highland areas of Sri Lanka.
Transpiration (mm/day)
4
4
r2 = 0.44, P = <0.0001
3
3
2
2
1
1
0
r2 = 0.52, P = <0.0001
0
0.2
0.4
0.6
0.8
1.0
1.2
8
1.4
10
12
14
16
18
20
22
24
26
Solar radiation (MJ/m-2)
Vapor pressure deficit (kPa)
4
4
r2 = 0.49, P = <0.0001
r2 = 0.62, P = <0.0001
3
3
2
2
1
1
0
0
30
32
34
36
Maximum temperature (0C)
0
1
2
3
4
5
6
Potential evapotranspiration (mm/day)
Figure 5.16 Relationships between daily transpiration and VPD, solar radiation, maximum temperature and
potential evapotranspiration of TRI 2023, during August - November, 2008 period
In summary, during the wet season transpiration is positively related to increasing VPD, solar
radiation, air temperature and ET0. However the closest relationship (r2=0.62, P=<0.0001)
was observed with increasing temperature.
5.3.3.2
5.3.3.2.1
Transpiration of Rain-fed and Irrigated Tea Plants during Dry Season
Climate during the study period
Starting from 12 Feb 2009, the climate of the study area was dry. During the initial period
there was no rain up to 22 Feb 2009. A rainfall event of 13mm occurred in day 23 February
ending the dry spell (Figure 5.17). There was a rainfall of 76mm in 06 Mar 2009. Solar
radiation level was above 20MJm-2day-1, during the initial 3 day period and lowest solar
radiation level was observed on 08 Mar 2009, amounting to 8.8MJm-2day-1. Vapour pressure
deficit remained around 1.0kPa. Volumetric moisture content of the rain-fed area felt to
104
12.9% during the second week of the experiment. However, later the soil volume moisture
content increased with the availability of rain. A gradual decline in the soil moisture content
of the irrigated field can be observed with the study period. In overall, the moisture content
of the irrigated plots was above 20%.
35
100
Rain
moisture (irrigated)
moisture (rainfed)
80
25
60
20
40
15
20
10
Rainfall (mm/day)
Soil moisture (v/v)
30
0
E(irrigated)
E(rainfed)
ET0
8
7
Water use (mm/day)
6
5
4
3
2
1
0
09/02/09
16/02/09
23/02/09
02/03/09
09/03/09
16/03/09
Date
Figure 5.17 Daily rainfall and weekly volumetric moisture content of irrigated and rain-fed plots up to top 60cm
(above) and transpiration of irrigated and rain-fed plants and potential evapotranspiration from February 09 to
March 16, 2009 (below)
5.3.3.2.2
Transpiration rate of irrigated and rain-fed tea
The average estimated ET0 of the period was 3.5(±0.3)mm/day.
The potential
evapotranspiration level fluctuated and was low at the end of study period. Reduction in
VPD, (Table 5.6), mainly caused the reduction of estimated ET0 (r2=0.87, P <0.0001). The
average E of rain-fed and irrigated plants, remained at 1.3(±0.2) and 2.7(±0.5)mm/day
respectively for the season.
Difference in E, between irrigated and rain-fed plant was
105
significant (P=<0.001). After the occurrence of high rainfall (06 March 2009), E of the both
treatments increased more than 100%..
Table 5.6 Climate, soil moisture content, transpiration and crop coefficient of irrigated and rain-fed TRI 2023
cultivar during February to March, 2009
Soil
moisture(V/V)
Period
Rain-fed Irrigated
Rn
-2
(MJm )
Tmax
0
( C)
VPD
(kPa)
ET0
(mm/day)
E
(mm/day)
Kc
Rain-fed Irrigated Rain-fed Irrigated
12-18 Feb
12.9
27.7
20.9
36.4
1.6
5.4
0.76
1.54
0.14
0.29
20-26 Feb
15.2
18.7
16.4
35.5
0.8
2.8
0.59
1.11
0.23
0.43
27Feb-5Mar
14.9
22.5
20.4
36.5
1.2
4.2
0.56
1.34
0.13
0.31
6 -12 Mar
16.4
21.3
15.1
34.9
0.7
2.3
1.8
3.6
0.84
1.6
In summary, irrigated plants showed a 113% higher transpiration rate than rain-fed plants.
Until the major rain event occurred on 6 March 2009, Kc remained below 1.0 for both
treatments. With commencement of the rains, Kc value increased in both treatments. During
the entire experiment period, Kc value of the irrigated plants was around 100% higher than
that of rain-fed tea. For the irrigated tea after the rain event, Kc value increased more than 1,
showing much higher water use than that of the ET0, estimated by the Penman Monteith
equation.
5.3.3.2.3
Dry matter production during dry season
Plant dry matter production of rain-fed and irrigated plants differed significantly (P=0.08)
during the dry spell. The average dry matter production per plant during this period of the
rain-fed plant was 21 (±6.1)g/wk, whereas irrigated plants production was 37(±5.3)g/week.
Dry matter production during the dry period is significantly lower than average dry matter
production during wet period. Dry matter production of both cultivars increased with the
commencement of rain.
5.3.3.2.4
Diurnal variation in transpiration
The patterns of hourly transpiration of tea plants in two days representing wet season and dry
season are shown in Figure 5.18. In the wet season, the transpiration rate of measured two
plants showed almost identical diurnal pattern. Air temperature reached never exceeded 350C
between 1400-1500 hours. There was a sharp drop in solar radiation after 1200 hrs from 778
to 538 W/m-2. Both plants showed highest E between 1100-1200 hours.
In the dry season day, air temperature reached above 340C by 1400 hours, and by 1500 hours
it reached 350C. Plants experienced above 300C, temperature for nearly 4 hours from 1300
106
hours. Solar radiation was higher than during the wet season. It reached 811 Wm-2 by 1100
hours and remained above 800 Wm-2 up to 1300 hours. Both rain-fed and irrigated plants
showed lower peak transpiration rate than dry season day. However in both plants, peak E
remained stable from 1100 to 1400 hours. Rain-fed plants showed a higher transpiration rate
from 900 to 1500 hours. During rest of the day, irrigated plants showed a higher transpiration
rate. During night time also, irrigated plants showed a significant transpiration, unlike rainfed plants and the cumulative E was 34% higher in irrigated plant than rain-fed plant in the
day.
1.0
(a)
0.8
(b)
0.4
0.2
solar radiation
temperature
800
(c)
(d) 36
solar radiation
temperature
34
32
0
-2
Dry Season
0.6
0.0
1000
Solar radiation (Wm )
irrigated
rain-fed
Temperature ( C)
Transpiration (mm/hr)
Wet Season
plant 2
plant 1
30
600
28
400
26
24
200
22
0
20
0
500
1000
1500
Hours
2000
0
500
1000
1500
2000
Hours
Figure 5.18 Diurnal variation of tea plant transpiration as changed with air temperature and solar radiation during
wet period and dry period in Ratnapura Sri Lanka (a) diurnal transpiration in wet season in rain-fed plants (b)
diurnal transpiration on dry season under rain-fed (continuous line) and irrigated (dotted line) plants. (c & d)
diurnal variation of solar radiation (dash line) and air temperature (continuous line)
5.3.4
Discussion
Transpiration of the tea in hot humid low elevation area of Sri Lanka was measured in wet
and dry seasons. To establish the relationship between E and dry matter production, dry
matter production was also monitored during the period. Factors determining transpiration
and the relationship between the estimated potential evapotranspiration and plant transpiration
were established for the dry season.
107
5.3.4.1
Transpiration and dry matter production
Based on the dry and wet season daily transpiration and dry matter production, transpiration
efficiency (TE) for cultivar TRI 2023 is given in Table 5.7. During dry season, tea plant
transpiration ratio was >50% less than wet season. The difference between irrigated and rainfed plants was also only 4%.
Even though irrigated tea produced higher dry matter
production than rain-fed, transpiration efficiency remained almost the same. During the dry
period, transpiration was not mainly meant for dry matter production, but for minimizing the
drought/heat stress through higher transpiration.
Table 5.7 Transpiration efficiency (TE) of rain-fed and irrigated TRI 2023, as the ration between daily dry matter
production and transpiration per plant.
Season
TE
(g/mm)
Wet season
6.0(±0.9)
Dry season, rain-fed
2.9(±0.6)
Dry season, irrigated
3.0(±0.7)
The experiment revealed transpiration of the plants is controlled by not only soil moisture but
by other environmental factors like air temperature, solar radiation and vapour pressure
deficit. With the provision of soil moisture during dry periods, E occurs at a higher rate than
rain-fed plants. However, irrigated plants too reduced its E in dry season than wet season.
Decrease in E of irrigated plants can be due to increased canopy resistance or hydraulic
resistance (Passioura 1996). It will be productive to find ways to minimize the canopy
resistance of irrigated plants, e.g. such as methods like increased water application or high
shade planting (minimize ambient temperature). Due to lower soil moisture in lower soil
depths, transpiration was lower when the measurements were commenced in 2008. This
finding is important in designing irrigation systems, ensuring the irrigation for entire root
system. . According to other findings, partial drying of deep root layers may have resulted in
the limiting plant transpiration (Sinclair, Holbrook et al. 2005; Duursma, Kolari et al. 2007).
Partial drying of the roots is not favourable for the tea plant which produce a leafy product as
the final yield, unlike a fruit crop where harvested yield is either fruit or nut (Zhao-JunYing,
Wang-LiJun et al. 2005)
Estimated potential evapotranspiration value showed a reflection of the water use of the plant
with reference to climatic variables. It is a physical model based or surface and aerodynamic
resistance (Kabir, Alam et al. 2007). The tea plant transpiration has however controlled
mainly not only by surface and aerodynamic factors, but by plant and soil factors itself
108
according to the experiment.
The transpiration of the water mainly controlled by the
environmental factors as can be seen in the reduction of transpiration of both irrigated and
rain-fed tea. Despite the seasonal changes, transpiration rate of the both plants have been
increased, while ET0 decreased at the end of dry season. This suggests that transpiration of
the plant is not only governed by the seasonal factors. In a similar experiment it has been
found that transpiration from well-irrigated sugar cane has been found to be independent of
diurnal variation of stomatal conductance (Meinzer, Goldstein et al. 1993).
The crop coefficient(Kc) of tea is 0.85 (Allen, Pereira et al. 1998). However, variation of Kc
was found evident within dry period specially (Table 5.6). With the resumption of rainfall
after dry spell, Kc increased >1.0. Not only in tea (Kigalu 2007), but in other crops like
grapevine (Dragoni, Lakso et al. 2006) and Chrysothamnus sp (an arid zone species called
rabbit brush) (Steinwand, Harrington et al. 2001), there were instances where Kc >1.0 was
reported, mainly in hot humid conditions. However, as with irrigated tea in low elevation tea
growing areas, increase in Kc is seasonal and depends on cultivar performance as well within
a selected crop. For the seasonal increase in tea Kc value, there could be broadly two reasons.
One is related to plant performance factors and other related to the calculation procedure of
potential Et0.
For the tea plant, during drought under humid conditions, soil moisture stress and temperature
effect have negative effect on plant physiological activity. With the onset of rainfall season,
maximum air temperature level drops, as well the duration of tea plant exposing to high
temperature reduces as a result of afternoon showers. During the drought period, even though
tea is irrigated with drip, dry and hot weather reduces transpiration by partially closing
stomata (Dragoni, Lakso et al. 2006). (In the glass house experiments, Experiments 4 and 5,
transpiration was increased with increased air temperature, as there was no high radiation
effect, unlike the humid field condition in Sri Lanka). The start of the rain again rejuvenates
the physiological activities. The irrigated plants had a high leaf area index and a higher shoot
rate than unirrigated plants. During the drought period also, it maintained twice higher
transpiration rate than unirrigated plants. As the transpiration process is more with new leaves
than old leaves, irrigated plants which contain more fresh leaves even during drought season,
as reflected by higher yields, achieve higher transpiration rates. With the onset of rainy
season, is the time when tea plants start producing more leaves and subsequently increased
the transpiration rate of both irrigated and non irrigated plants.
109
5.3.5
x
Summary of Results
Plant transpiration (E) largely followed the calculated ET0 during wet season.
However, daily changes in evaporative demand are not reflected immediately on daily
E. However the relationship was poor in dry season.
x
Dry matter production during wet season is closely followed the transpiration pattern
of the plants.
x
During wet season of the yearly (nearly 9 months), increase in ambient temperature
mainly drives the transpiration demand.
x
During the dry period, irrigated plants transpire more water, as high as twice the water
use of rain-fed plants. Still with irrigated plants, there is a reduction in water use in
dry periods compared with the wet season, perhaps due to stress from atmospheric
conditions.
110
5.4
Experiment 3: Physiological Response of New Cultivars to Drought
5.4.1
Introduction
The first experiment of this chapter showed the higher response to irrigation by the bench
mark high yielding cultivar TRI 2023 through high assimilation rates and tea yield. In
contrast cultivar TRI 3025 showed physiological response of lower stomatal conductance and
reduced transpiration. These characters are helpful for the plant to survive the adverse
drought period in the field. But the lower yields of the cultivar made it unattractive for the tea
growers.
In the second experiment in this chapter, the water use of high yielding cultivar TRI 2023 was
evaluated under wet and dry season. In the dry season, average transpiration of the irrigated
plants was more than double than that of rain-fed plants, even with lower water productivity.
Irrigating the entire plantations in many tea fields at the rate of nearly 2mm/plant/day is a
difficult task with the given high density of plant population (12,500 plants/ha). Suitable
water sources are not available especially in mountainous areas. For the majority of the tea
fields, cultivar selection has to be considered as the alternative to irrigation for drought
mitigation. In contrast to TRI 2023, TRI 3025 had shown drought mitigation effects like
lower Pn and El . TRI 3025 is resistant to stem canker infection, which is related to drought in
low elevation area. As a result, TRI 3025 is advised to be used as a drought tolerant cultivar.
Yet with this cultivar, there are some characters that affect productivity and sometimes
survival in major drought event, such as high temperature increase in the leaf. Also if the P n
is increased without significant effect to survival, it would be a welcome change by the
growers in new cultivar during drought.
Therefore an investigation was carried out to
evaluate the physiology under rain-fed conditions during dry season together with benchmark
drought-resistant TRI 3025 cultivar.
5.4.2
Method
The experiment was conducted at the Field no 04, St. Joachim Estate, Ratnapura. Details of
the soil and climate of the area are given in a Chapter 4.
The trial was planted in 2004 at a spacing of 60cm X 120cm for evaluating new cultivar
performance and to supply vegetative cuttings to plant nurseries. The experimental design
was a complete randomized design, with three plots for each cultivar. There were no shade
plants in the area.
Gas exchange measurements were made using an ADC LCA4 (ADC Bio Scientific, UK)
during the period 13 February to 13 March 2009 at weekly intervals. All measurements were
111
collected between 1200 – 1300 hrs in each day. This was the time, that plants receive
maximum water stress in the field. The top-most mature tea leaves, exposed to fully sunlight,
were selected for measurement. At least three plants were selected for measurement in each
plot. Volumetric soil moisture measurements were taken using core samples driven to 30cm
and 60cm depths at weekly interval. After obtaining samples, the fresh weight of the soil
samples were taken in the field. Samples were then oven dried at 1050C for 24 hours to
measure the dry weight of the soil.
When transporting the soil samples, additional
precautions were taken to prevent any soil loss.
All weather measurements were recorded at the automatic weather station at Field no 01 of
the estate (described in experiment 1 of the Chapter 5). Rainfall measurements were recorded
from the manual rain gauge of the estate weather observatory.
Plant potential
evapotranspiration was calculated based on FAO modified Penman-Monteith calculation
(Allen, Pereira et al. 1998), using Instat climate analysis software, version 3.036 (Statistical
Service Centre, University of Reading, UK)
5.4.3
5.4.3.1
Results
Weather during the study period
The experiment field experienced a usual dry period during January-March period. During the
study period, dry period was observed in January and February months. The experiment was
started after the plants experiencing a dry month of January (20.1mm rainfall, over 6 days).
The climate during the study period is shown in Table 5.8. Maximum temperature reached
350C in almost all weeks, except the second day of the measurements (20th Feb). There was
no rain during the first two weeks; however rain started from week 3 onwards. The vapour
pressure deficit was higher in the first two weeks, however dropped it below 1.0kPa with the
onset of rain.
5.4.3.2
Soil moisture
Volumetric moisture content of the soil was around 12% at the beginning of the experiment
(Figure 5.19). Initially, the top 30cm layer had slightly higher moisture content. In the first
week, soil moisture content fell to 11% as there was no rain. However, the soil moisture
content was recharged and had reached the level of around 20% during the latter stage of the
experiment. Even though soil moisture level reaches to saturated level, the maximum
temperature levels reached more than 350C and ET0 level reached more than 3.0 mm a day.
112
Table 5.8 Weather during February – March, 2009. Except rainfall, others are daily values. Rainfall is given as
weekly total
Date
13 Feb
20 Feb
27 Feb
06 Mar
13 Mar
21.1
15.5
20.5
17.4
17.2
18.7
19.9
22.5
22.9
22.5
Max. temperature( C)
35.7
34.0
35.7
35.3
35.1
Sunshine hours
11.4
10.8
11.5
10.0
10.8
Vapour pressure deficit (kPa)
1.3
1.26
0.9
0.9
0.7
Potential evapotranspiration (mm)
3.8
2.8
3.8
3.7
3.3
0
0
19
79
24
-2
Solar radiation(MJm )
0
Min. temperature( C)
0
Precipitation (last 7 day)
In summary, even though this was a dry season, with the resumption of the rain in the end of
experiment period, soil moisture content rose to 20%.
22
100
Rain
0-30cm
30-60cm
20
80
60
16
14
40
Rainfall (mm)
Soil moisture (V/V)
18
12
20
10
8
2/9
2/16
2/23
3/2
3/9
0
3/16
Date
Figure 5.19 Volumetric moisture content and total weekly rainfall from February 09 to March 16, 2009
5.4.3.3
Leaf Transpiration (El)
Figure 5.20 shows the El of tea leaves during the study period. With the depletion of the soil
moisture, El decreased for many plants in the second week, except cultivar TRI 4049. The
lowest transpiration rate of 0.54(±0.1)mmol H2O m-2s-1 was recorded from the variety TRI
4047 during the second week of observation, under the lowest soil moisture condition.
However, before the onset of rains, there was no significant difference in the El in all varieties
and generally it was lower. With the start of the rains, in the third week of assessment, E l
113
increased in all plants. The cultivar TRI 3025 was the highest El after the initial rains on 27
February. In all varieties El fell in 6th March (week 4 of assessing, even though there were
rains and soil moisture level was improved.
In summary, with the increment of rain and soil moisture El increased for all plants.
Transpiration (mmol H2O m-2s-1)
5
TRI 3014
TRI 3025
TRI 4053
TRI 4047
TRI 4049
4
3
2
1
0
2/9
2/16
2/23
3/2
3/9
3/16
Date
Figure 5.20 Transpiration rate of top most mature tea leaves exposed to fully sunlight in mid-day during drought
months of 2009
5.4.3.4
Photosynthesis (Pn)
The plant photosynthesis rate (Pn), which can be considered as one of the most sensitive
physiological activity for environmental variations, showed a greater difference among the
varieties, except the second week of experiment (Figure 5.21). Interestingly, the highest Pn
was observed in all varieties in the second week (20 February) of the assessment.
Pn
increased in all varieties during this particular day and there was no significant difference
among different cultivars. This increase in Pn was so significant that it even exceeded the
level during the favourable soil moisture periods of later measurement days. Overall, cultivar
TRI 4047, showed a lower Pn during the measurement period, except at week 4. Even though,
not significantly different, Pn of cultivar TRI 4047 was only 3.37(±1.9) μmol CO2 m-2s-1.
114
Photosynthetic rate (mmol CO2 m-2s-1)
14
12
10
TRI 3014
TRI 3025
TRI 4053
TRI 4047
TRI 4049
8
6
4
2
0
2/16
2/23
3/2
3/9
3/16
Date
Figure 5.21 Photosynthetic rate of topmost mature tea leaves exposed to fully sunlight at mid-day during drought
months of 2009
In summary, cultivar TRI 4049 showed higher Pn (18%) and El (4%) higher than TRI 3025
during the period.
5.4.3.5
Leaf temperature (Tl)
Leaf temperature levels (Table 5.9) reached above 430C, during the first observation day.
There was no significant difference in cultivars when reaching the highest temperature levels.
Highest recorded Tl of 45.9(±0.5)0C was recorded on cultivar TRI 4053 in the second week.
There was a significant difference in the leaf temperature at the 20th February and 6th March.
In summary all the cultivar showed a higher leaf temperature during the dry days, average leaf
temperature for all cultivars was 39.5(±0.3)0C, a high value, potentially damaging the
photosynthetic mechanism of leaves.
115
Table 5.9 Leaf temperature at mid-day. (Top most mature tea leaves exposed to direct sunlight were measured,
standard error in parenthesis)
Cultivar
13 Feb
20 Feb
06 Mar
TRI 3014
43.8(±0.6)
36.1(±0.2)
36.8(±0.2)
TRI 3025
43.2(±1.2)
37.3(±0.4)
38.4(±0.2)
TRI 4053
45.9(±0.5)
39.4(±0.9)
36.1(±0.3)
TRI 4047
44.9(±0.06)
38.9(±0.4)
34.7(±0.05)
TRI 4049
44.6(±0.4)
38.6(±0.3)
34.4(±0.3)
ns
0.002
0.0001
P<
5.4.3.6
Water use efficiency (Wi)
The instantaneous water use efficiency (Wi) of each cultivar was calculated as the ratio
between transpiration and photosynthesis (Field, Merino et al. 1983; Jones 2004). Daily Wi
and seasonal average is shown (Figure 5.22). Second day of observation showed a larger Wi
from two varieties. However, when calculating the average this date was omitted as it was a
gloomy (solar radiation=15.5MJm-2) and cool day (Tmax = 340C). In average, TRI 3014 and
TRI 4049 showed Wi of above 2.0 during dry season.
Among the tested cultivars, TRI 3014 and TRI 4053 showed significant negative relationship
with maximum temperature (Table 5.10). Among the above two cultivars, TRI 4053 showed
the most sensitivity (r2=0.88, P=0.02) than TRI 3014.
Nevertheless, one positive note
regarding the new cultivars is that 3 out of 5 tested cultivars showed no significant negative
relationship with maximum air temperature.
116
16
TRI 3014
TRI 3025
TRI 4053
TRI 4047
TRI 4049
(a)
14
12
10
8
Instantaneous water use efficiency (Wi)
6
4
2
0
2/12
3.0
(b)
2/19
2/26
Date
3/4
3/11
3/18
2.5
2.0
1.5
1.0
0.5
0.0
TRI 3014
TRI 3025
TRI 4053
TRI 4047
TRI 4049
Cultivar
Figure 5.22 Instantaneous water use of efficiency (Wi) of tea cultivars (a) during the measuring period (b) average
Wi for the season
Table 5.10 Relationship between maximum temperature and photosynthetic rate
Cultivar
y-intercept
slope
r
2
P<
TRI 3014
137
-3.7
0.73
0.06
TRI 3025
85
-2.3
0.51
ns
TRI 4053
118
-3.2
0.88
0.02
TRI 4047
34
-0.9
0.17
ns
TRI 4049
109
-2.9
0.53
ns
117
5.4.4
Discussion
The experiment tested four new cultivars with the benchmark TRI 3025 for the physiological
performance in the dry season. Three of the tested cultivars showed significant increases in
assimilation rates as compared to the benchmark cultivar. However, only TRI 4049, showed
a higher Pn and El rate. High Pn and El were physiological traits found in the productive
cultivar TRI 2023 in Experiment 1. Anecdotal evidence from the early growers who have
already experienced yield performance of these cultivar in the fields, too identify TRI 4049 as
a high yielding cultivar. The characters shown by the cultivars like lower P n and El rate could
be important for the drought mitigation (Monclus, Dreyer et al. 2006).
However their
productivity remained. In the experiment 1, high productive TRI 2023 cultivar, under rainfed, showed a 38% higher Pn rate than rain-fed TRI 3025.
In terms of net carbon assimilation rate also TRI 4049 showed 27% higher W i than TRI 3025
and it is also insensitive in carbon assimilation to increasing ambient temperature (Table
5.10). Nevertheless, critical temperature increase, observed in all cultivars tested during dry
and hot period is a matter of concern. This shows the vulnerability of the cultivar selection as
a drought mitigation method in hot water stress periods. Water deficit, coupled with high
temperature stress, cause photochemical damages to C-3 type plant leaves (Joshi and Palni
1998; van Bel, Offler et al. 2003). As the air temperature increase is a one key result of the
climate change (Sokona 2009), looking for other drought mitigation options, like growing
high shade plants in tea fields is important in the long run. High shade plants control the
penetration of solar radiation and prevent increase in the air temperature in tea plant micro
climate.
5.4.4
Summary of Results
x
TRI 4049 showed 17% higher Pn and 4% higher El than TRI 3025.
x
TRI 3014 and TRI 4049 showed significantly higher Wi than TRI 3025..
x
TRI 4049 showed higher Wi and Pn was resilient to increasing temperature.
x
Maintenance of favourable leaf temperature during dry days is not significantly
different among cultivars
5.5
Conclusion
The aim of this chapter was to describe the effect of irrigation on the physiology, water use
and yield of tea, and to assess the performance of some new tea cultivars under rain-fed
conditions against a bench mark drought resistant cultivar. Experiment 1 examined the yield
118
and physiological aspects, and Experiment 2 evaluated the water use of tea. In Experiment 3,
physiological performance of new tea cultivars was evaluated under rain-fed condition.
In summary, it can be concluded that different tea cultivars respond quite differently to
irrigation. Temperature has two contrasting effects on plant production in this environment.
During the short dry season (3 months) high ambient temperature (>350C) decreases
photosynthesis and yield; meanwhile, during the 9 month wet season moderately high
ambient temperature drives transpiration, which increases dry matter production and yield.
As high ambient temperature is a critical factor associated with water stress in low elevation
growing areas, cultivar selection as a means of drought mitigation seems to be not successful,
unless some additional measures like high shade planting are carried out.
119
120
Chapter 6
Experiment 4: Evaluation of Irrigation Technology
6.1
Introduction
The preceding chapters concerned the various aspects of the effect of drip irrigation on the
growth and yield of two contrasting tea cultivars. Drip irrigation is considered to be the most
appropriate irrigation technology for tea cultivation in Sri Lanka because of the concern for
low water availability for irrigation (Eriyagama, Smakhtin et al. 2010) and energy demand.
In irrigation system design wise too, drip irrigation is preferred over sprinkler irrigation in hill
terrain areas, where wind can cause low uniformity of application. In tea and other crops,
there are attempts now to replace the sprinkler irrigated fields with drip irrigation (Möller and
Weatherhead 2006; Comis 2011). Yet for the ease of installation, low capital cost and
maintenance, some growers may prefer to adopt sprinkler instead of drip irrigation
technology. For this reason a parallel, but smaller evaluation trial was established on the St.
Joachim Estate.
As the second step in the process of understanding the feasibility of irrigation application in
low elevation areas of Sri Lanka, this chapter fulfils the following aims:
1. to describe the effect of irrigation method on key physiological parameters affecting
yield, final yield and total plant growth; and
2. to assess the ability of different irrigation methods to alleviate the productivity related
environmental limitations during drought.
This evaluation will lead to the following objectives of:
a) understanding the best irrigation system according to the location which mainly
defines the drought severity; and
b) suggesting technological improvements for drip and sprinkler irrigation.
In summary, this chapter presents the results of a trial which compares the performance the
drought-susceptible cultivar but highly productive TRI 2023 under drip and sprinkler
irrigation as against the rain-fed cultivation.
6.2
Materials and Method
The experiment was conducted at Field No 01, St. Joachim Estate, Tea Research Institute,
Ratnapura, Sri Lanka, during January 2008 to March 2009. A detailed description of the site
is available in Chapter 4. High yielding TRI 2023 cultivar was planted in May 2000. When
121
the experiment was commenced in January 2009 plants were in the second year of their third
pruning cycle (i.e. the plants were 8 years old). Planting, fertilization, harvesting and other
cultural practices were according to the standards specified for low elevation tea fields by Tea
Research Institute of Sri Lanka (Zoysa 2008). Plants belonging to the irrigation treatment
were irrigated during the January- March drought period in each year by the field staff of St.
Joachim Estate. The following treatments were included in the experiment:
1. Rain-fed cultivation
2. Drip irrigation
3. Sprinkler irrigation
The experimental design was a complete randomized block design, with three blocks and each
treatment having two replicates in a block. Each plot consisted of 30 tea plants (6 X 5 rows).
Two additional plant rows were kept as guard rows at either side of drip and rain-fed plots
and 5 additional rows at either side of sprinkler plots to prevent possible moisture seepage.
There was a drain of 30cm deep and 30cm width in between two blocks. There were two tea
rows were planted as guard rows surrounding the blocks.
6.2.1
Irrigation application
Drip irrigation was provided with Netafim RAM17D integral pressure compensated drip lines
which has a dripper discharge of 1.6L/hr. Sprinkler irrigation was provided with Rainbird
SW 2000 Plastic impact sprinklers with a water application rate of 4.4mm/hr (Stamps and
McColley 1996)
Sprinklers were placed at 5m apart, creating a more uniform water
application. The trajectory angle of sprinkler nozzles was placed at lowest angle (15-200) to
have a minimum operating radius of approximately 5-6m. Irrigation scheduling was based on
the previous day pan evaporation value and based on crop coefficient value of 0.85 (Laycock
1964). When operating the irrigation systems, the drip system was assumed to have 0.8
system efficiency and for the sprinkler system it was 0.7 system efficiency (Burman, Cuenca
et al. 1983).
During the experiment, irrigation was applied during January – February 2008 and January –
March 2009 period.
To apply the irrigation in each dry season, it was waiting for 5
consecutive rainless days. Five consecutive rainless days are considered to create water stress
in the wet zone of Sri Lanka for perennial crops (Sumanasena 2008). Irrigation was applied
continuously, unless it received at least 10mm rain in a day. The number of irrigation days
and amount of water applied for each treatment are shown in Table 6.1.
122
Volumetric moisture content was measured at weekly interval, by carefully taken soil cores
(~137cm3) to depths of 20, 40 and 60cm. Soil moisture samples were taken at a distance of
15-30cm from plant base. After obtaining the samples, holes were filled with soil.
Table 6.1 Number of irrigation days, amount of water applied and rainfall in Jan-Feb, 2008 and Jan-Mar, 2009
Year
6.2.2
2008
2009
Rain (mm)
99
310
Irrigation (days)
21
58
Drip (mm)
85
264
Sprinkler (mm)
96
302
Harvesting and yield factors
Green leaf was harvested at 7 day interval using manual pluckers. The standard of the
harvesting was 2 leaves and a bud. However, single leaf plus bud and third leaf was also
harvested depending on the tenderness of leaves and season of the year. Total harvest is
usually a collection of the above three shoot types with the majority being 2 leaves plus bud
type. Yield weight was recorded at the field for separate plots. To convert the green leaf
weight into made tea equivalent, a standard conversion ratio of 22.2% was used (Stephens and
Carr 1991).
In the first week of February 2008, after harvesting shoots were separated to three categories
of 1 leaf + bud, 2 leaves + bud and 3 leaves + bud. Average fresh weight of each shoot type
was weighed then. Shoot extension rate of the plants were measured in between January and
February, 2008 after commencement of the irrigation treatments. To measure shoot extension
rate, very young shoot from a plant was tagged and initial shoot length was measured using a
vernier calliper. Then the growth of the same shoot was measured over two week period at a
weekly interval. Harvestable shoot count per plants was measured selecting a plant from each
plot during the months of January and February 2008. Measurements were taken prior to
harvesting.
6.2.3
Physiological measurements
Physiological measurements, moisture measurement and climate measurements were as
according to the description in Chapter 5.0. Soil moisture measurements were taken at
weekly interval using carefully obtained gravimetric soil core samples.
Samples were
0
measured at the field and then oven dried at 105 C to obtain dry weight of the soil. Average
volumetric moisture content of each month is shown in Figure 6.3.
123
6.2.4
Growth measurements
After the experimental period, in April 2009, one plant from each plot was excavated
carefully and separated the root and stem parts. When excavating, care was taken to remove
all roots of the plants including fine roots. After removing the soil by washing, root samples
were air dried for one day. Then root and stem samples were oven dried at 850C till they
reached a constant weight. Dry weight of the total root and stem weight was then measured.
6.2.5
Root measurements
To measure the root density, metal cores with a volume of 136cm3 was inserted to depths of
20, 40 and 60cm at each plots. Fine (<2mm) and coarse/structural (>2mm) root samples were
separated after washing by using set of sieves. Air dried separated samples were then oven
dried at 850C till they reached a constant weight to get the dry weight of the samples. To
measure the root density distribution in the soil profile, soil cores were obtained at 20, 40 and
60cm depth using a brass core (height – 6cm, inner diameter 5.4cm). One plant was sampled
from each plot for root distribution analysis. Root samples were washed and separated for
fine (<2mm) and coarse (>2mm) samples before oven dried at 850C.
6.3
Results
6.3.1
Weather during trial period
The year of 2008 was very wet year, receiving 4257mm of annual rainfall. This value is
approximately 10% more than the 10 year average rainfall, in what is already a very high
rainfall area. Initial one month period is low rainfall weeks in generally and in the year 2008,
rainfall during initial 5 week period, fell below average. Also week 3 and 4 were recorded as
completely dry weeks, with no rainfall. High annual rainfall is mainly caused by very high
rainfall received during April (week 17), May/June (week 22) and July (week 29) weeks
exceeding 300mm. Nevertheless the rainfall pattern still followed the normal bimodal rainfall
pattern of the two monsoons (Figure 6.1).
In 2009, the initial months were dry compared to an average year, till the rain start in early
March 2009.
Monthly rainfall for January and February 2009 was 21.2 and 23.2mm
respectively. Irrigation was applied from January to first week of March, 2009. During this
period, there were several weeks where maximum temperature exceeded 350C.
124
SW monsoon
250
Rain (mm/wk)
200
150
NE monsoon
rain
average
irrigation
irrigation
100
50
0
Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr
Month (Jan 2008 - Mar 2009)
Figure 6.1 Weekly rain during January, 2008 to March, 2009. 10 year average weekly rainfall is given in dotted
line (SW – South West, NE – North East). Error bars =standard error
During the physiological measurement period of January – February, 2008, there was a
rainless period from 16 to 28 January (Figure 6.2). Again major rain events started in 4th
February and continued the rain for rest of the month. Potential evapotranspiration varied
from 0.85 to 4.1mm/day.
Rainy days showed a less water demand, average being
3(±0.16)mm/day. The average maximum temperature during the period was 34(±0.3) 0C.
Average solar radiation during the period was 977(±30)W/m-2. However, there are some days
which recorded high solar radiation of more than 1300 W/m-2.
125
40
4.5
Rain
ET0
4.0
Rain (mm/day)
3.0
2.5
20
2.0
ET0(mm/day)
3.5
30
1.5
10
1.0
0
7/01/08
0.5
14/01/08
21/01/08
28/01/08
4/02/08
11/02/08
Date
Figure 6.2 Daily rainfall and potential evapotranspiration (ET0) during Janury 12 – February12, 2008. This period
was used for evaluation in dry weather physiology
6.3.2
Soil moisture
Monthly volumetric soil moisture variation of the 20, 40 and 60cm depth is displayed on
Figure 6.3. Field capacity and volumetric moisture content of the soil is 27% and 14% for the
soil in this field. The point at which the tea plant begin to experience water stress was
considered to be 50% of available soil moisture (Anandacumaraswamy 2008). Hence, below
20% moisture content can be considered as a water stress level for tea plant. During January
and February 2008 moisture content of the sprinkler plots was 2-5% higher than drip plots.
Moisture content at the 60cm layer depth was similar or sometimes lower than rain-fed plots.
It is only in May 2008 where soil moisture levels became equal in all three treatments.
Moisture content was higher in sprinkler plots even during some rainy months (e.g. April and
August 2008). However, during the rainy months, where irrigation was not practiced,
moisture content in all plots remained above 50% of the available moisture content.
126
Feb 08
Jan 08
Volumetric moisture (%)
-10
15
20
25
Mar 08
Volumetric moisture (%)
30
15
20
25
Volumetric moisture (%)
30
15
20
25
30
Depth (cm)
-20
-30
-40
-50
-60
rainfed
drip
sprinkler
-70
PWP
50%
FC
PWP
50%
FC
PWP
50%
Apr 08
May 08
Jun 08
Jul 08
Aug 08
Sep08
Oct 08
Nov 08
Dec 08
Jan 09
Feb 09
Mar 09
FC
-10
Depth (cm)
-20
-30
-40
-50
-60
-70
-10
Depth (cm)
-20
-30
-40
-50
-60
-70
-10
Depth (cm)
-20
-30
-40
-50
-60
-70
-10
Depth (cm)
-20
-30
-40
-50
-60
-70
PWP
50%
FC
PWP
50%
FC
PWP
50%
FC
Figure 6.3 Average monthly volumetric soil moisture content up to 70cm depth. Continuous line shows the rainfed plots, dotted line shows drip irrigated plots and sprinkler plots are shown by dash line. Vertical bar
represent permanent wilting point (PWP), 50% of available moisture and field capacity (UNFCCC)
6.3.3
Photosynthesis (Pn)
Pn of the mature, top-most leaves, measured during January and February 2008, are shown in
Figure 6.4. During the measurement period, mid-day Pn of sprinkler irrigated plots showed
127
the highest activity and lowest by the rain-fed plants.
Low solar radiation of 809 and
948W/m-2 caused a reduction in all three treatments on 28th and 30th January. This type of low
light conditions is not common usually in the inter-monsoonal dry spell of January-March.
The average Pn rates for the rain-fed, drip irrigated and sprinkler irrigated plants were
7.7(±0.8), 11.1(±0.7) and 12.8(±0/9) µmolm-2s-1 respectively.
Photosynthesis rate (Pmol CO2 m-2s-1)
16
14
12
10
8
rainfed
drip
sprinkler
6
4
22/01/08
25/01/08
28/01/08
31/01/08
3/02/08
6/02/08
Date
Figure 6.4 Photosynthetic rate of topmost mature tea leaves in 3 irrigation treatments during mid-day in January February 2008
In summary Drip irrigated plants had a 46% higher Pn rate than rain-fed plants. Sprinkler
irrigated plants Pn rate was 15% higher than drip irrigated plants.
6.3.4
Stomatal conductance (gs)
Stomatal conductance of the rain-fed plants was the lowest values for study period (Figure
6.5). Initially the difference among irrigated and rain-fed plants was lower. The difference
widened with the progress of the drought, perhaps related to depletion of soil moisture as
well. Drip irrigated plants showed slightly higher gs value of 0.18(±0.01)mol H2O m-2s-1 as
compared to 0.17(±0.01) mol H2O m-2s-1 of sprinkler. Rain-fed plants showed a significantly
lower value (0.14(±0.008) mol H2O m-2s-1), than other two treatments.
128
Stomatala conductance (molm-2s-1)
0.20
0.15
0.10
0.05
control
drip
sprinkler
0.00
22/1/2008
25/1/2008
28/1/2008
31/1/2008
3/2/2008
6/2/2008
Date
Figure 6.5 Stomatal conductance of mature tea leaves during mid-day for three irrigation treatments
In summary rain-fed plants showed the lowest gs and for irrigated plants it was 26-30% higher
than rain-fed plants.
6.3.5
Transpiration (El)
On average, both drip irrigated plants showed a higher El than other two treatments (Figure
6.6), except for the 23rd day. On the 23rd day, highest El was shown by control or rain-fed
plants, but without significant difference among three treatments. On the 28 th day, drip
irrigated plants showed a significantly higher El value of 5.1(±0.3)mmol H2O m-2s-1. On the
28th day, sprinkler and rain-fed plot showed a lower El than drip irrigated plants, which was a
cloudy day. Transpiration increased for both irrigated treatments towards the last day of
observation, while the rain-fed plants remained at the same level. Lower soil moisture
condition in the rain-fed plots could be the reason for lower transpiration rate.
Transpiration rate (mmol H2O m-2s-1)
6.0
5.5
5.0
4.5
4.0
control
drip
sprinkler
3.5
3.0
22/01/08
25/01/08
28/01/08
31/01/08
3/02/08
6/02/08
Date
Figure 6.6 Instantaneous leaf transpiration of top most mature tea leaves during mid-day
129
In summary drip irrigated plants had a 20% higher transpiration rate than sprinkler irrigated
plants.
6.3.6
Diurnal variation of leaf physiology
Diurnal variation of Pn, gs, El and Tleaf was measured on 30 January 2008, from 0800 to 1700
hours as shown in Figure 6.7.
Also the air temperature (Tair) and the incidence of
photosynthetically active radiation (PAR) were recorded at 15 minute interval. This day was
comparatively cooler (maximum temperature <350C) and gloomy (PAR <900Wm-2) day.
Nevertheless, within day time, Tair remained above 250C, observed lowest being at 0800hrs
and increasing gradually at an average rate of 10C/hr. It rose to 340C by 1400 and then
increased to 350C, in between 1400 and 1500 hrs. Highest PAR level was also recorded at
1400hrs and it declined steeply after.
130
air temperature
solar radiation
Air temperature (0C)
34
(a)
800
600
32
30
400
28
200
26
Stomatal conductance (mol H2O m-2s-1)Photosynthetic rate (Pmol CO m-2s-1)
2
Transpiration (mol H2O m-2s-1)
24
Solar radiation (Wm -2)
36
0
14
(b)
12
10
8
6
4
rainfed
drip
sprinkler
2
0
0.4
(c)
0.3
0.2
0.1
0.0
6
(d)
5
4
3
2
1
0
600
800
1000
1200
1400
1600
1800
Time (hours)
Figure 6.7 Diurnal variation of: (a) air temperature and solar radiation; (b) photosynthesis rate; (c) stomatal
conductance; and (d) transpiration of tea leaves during day time 30 January 2008
Rain-fed plants showed the highest Pn rate of of 8.1 (±0.6) µmol CO2 m-2s-1 at 0800hrs, which
is 30% higher than both irrigated treatments. At 1000hrs, both rain-fed and sprinkler irrigated
plants had a Pn rate of over 12µmol CO2 m-2s-1, At the time Pn of drip plants only rose
9.8µmolm-2s-1 and remained >9.0 for up to 1400hrs. Rain-fed plants then showed a rapid
decline in Pn rate. Sprinkler irrigated plants too showed a decline after 1000 hours, but at a
gradual pace. By 1600 hrs, Pn was between 4.5 -5.0µmolm-2s-1 for the all treatments and it
showed no significant difference among treatments, and reached towards 1.0-1.5µmolm-2s-1
131
by 1700hrs. The steep reduction in the PAR could be the reason for the decline of P n in the
rain-fed plants as well.
Morning 0800hours was the time highest gs shown by rain-fed and sprinkler irrigated plants.
Drip irrigated plants had a 17% lower gs rate of 0.25mol H2O m-2s-1. Stomatal conductance of
rain-fed plants decrease rapidly than sprinkler and drip irrigated plants. By 1600hrs, all three
treatments showed similar gs values
Variation of El was more symmetrical throughout the day than gs. Highest El was observed
from drip plants throughout the day. It had a peak value of 4.7mmol H2O m-2s-1, which is
more than 15% higher than rain-fed and sprinkler plants. Nevertheless, all three showed the
highest El at 1200 hours. Rain-fed plants showed a steep decline in El after 1200 hours than
other two treatments. By 1500hrs, El was below 1mmol H2O m-2s-1 for all three treatments,
without significant difference among each other.
In summary Pn of the sprinkler irrigated plants were 10-25% higher than drip irrigated plants,
while El drip irrigated plants were 16-45% higher than sprinkler irrigated plants in mid-day.
Nevertheless gs of irrigated treatments remained almost equal. .
45
0
Leaf temperature ( C)
40
35
30
rainfed
drip
sprinkler
air temperature
25
20
600
800
1000
1200
1400
1600
1800
Time (hours)
Figure 6.8 Diurnal variation of leaf temperature and air temperature in Experiment 4 (30 January 2008)
On this day, drip irrigated plants showed almost similar temperature levels as ran-fed plants
apart from morning 1000hrs (Figure 6.8). But sprinkler irrigated plants showed a 2-40C lower
leaf temperature level than the other two treatments between 1000-1400 hours. However,
132
form 0800hrs to 1400 hrs, sprinkler plants too showed a higher leaf temperature than air
temperature.
At 1200hrs, difference was as high as 70C.
During that time, difference
between rain-fed/drip irrigated plants and air temperature was more than 10 0C. Even though,
air temperature rose to 340C by 1400hrs, leaf temperature fell in all three treatments.
In summary, sprinkler irrigated plants Tleaf was 2-40C lower than drip treatment in mid-day.
In 1200-1400 hours, Tleaf of drip and rain-fed plants were 6-100C higher than ambient
temperature.
6.3.7
Irrigation effect on shoot weight
Size of the harvestable tea shoots during dry period of January 2008 is given in Figure 6.9.
Shoot sizes were selected as one leaf + bud, two leaves + bud and three leaves + bud. Shoot
weight increased with the size of the shoot. On average, the largest portion of harvested tea
consists of two leaves and a bud category. Such shoots showed no distinct difference in shoot
weight among the three treatments. The single leaf bud category showed a difference among
three treatments, sprinkler showing highest weight.
Highest shoot weight of
0.47(±0.1)g/shoot, was recorded by sprinkler treatment, which is 87% higher than rain-fed
single leaf+bud shoot weight. In the sprinkler treatment, shoots with a leaf + bud and 2 leaves
+ bud showed similar weight. But there was a high variation in the weight of leaf + bud shoot.
The largest shoot weight was observed from 3 leaves+bud category in all three treatments.
However, both irrigated treatments showed a significantly higher shoot weight in this
category. Drip irrigated plants produced an average shoot weight of 0.85(±0.06)g/shoot.
In summary, Both single leaf+bud and 3 leaves+bud shoots showed increased weight with
drip irrigation. There was a significant difference in the 2 leaves+bud category, with drip
irrigated plants showing 18% increase in shoot weight than rain-fed plants.
133
1.0
Shoot weight(g)
0.8
Control
Drip
Sprinkler
0.6
0.4
0.2
0.0
leaf+bud
2lvs+bud
3lvs+bud
Shoot size
Figure 6.9 Fresh weight of different shoots according to three treatments during the dry period of January 2008
6.3.8
Shoot extension rate and shoot count
TRI 2023 cultivar shoot extension rate in January/February 2008 under 3 different irrigation
treatments are shown in Figure 6.10. Sprinkler irrigation showed a higher shoot extension
rate of 4.35(±0.5)mm/day than drip irrigation, showing 31% increase among irrigation
treatments. Though statistically not significant, drip irrigation too had 11% average shoot
growth than rain-fed cultivation.
Figure 5.5.10 illustrates the variation of harvestable total shoot count per plant as measured in
January, and February, 2008 (). In January, both sprinkler irrigated plots and rain-fed plots
produced similar shoot counts of 58.0(±5.2) and 57.3(±8.9) shoots per plant. Even though
there was no significant increase in the number of shoots in the drip irrigation treatment in
February, in the sprinkler irrigation plots it rose to 78.2(±7.0) shoots per plant, 57% higher
than rain-fed plants. Meanwhile for the rain-fed plots, harvestable shoot count reduced by
13% to 50(±7) shoots per plant.
134
6
(a)
Shoot extension (mm/day)
5
4
3
2
1
0
rain fed
100
drip
sprinkler
(b)
Control
Drip
Sprinkler
Harvestable shoots (no/plant)
80
60
40
20
0
January
February
Month
Figure 6.10 Yield factors in January-February, 2008. Average shoot extension rate according to irrigation
treatment (a) and harvestable shoot count per plant (b)
In summary, sprinkler irrigation increased the shoot extension rate by 31% than drip
irrigation. Effect of sprinkler irrigation was more significant in increasing harvestable shoot
count by 57% in February than control plants.
6.3.9
Tea yield
The average weekly yield of TRI 2023 cultivar from January 2008 to March 2009 is shown in
Figure 6.11. Unlike an average year, in 2008 average weekly tea yield has shown a less
variation since March, probably due to high rainfall received throughout 2008. In January
2008 sprinkler plots gave a significantly higher yield than both drip and control plots.
Sprinkler treatment produced made tea equivalent of 176(±16) kg/ha/wk as against the
135
140(±16) and 136(±14) respectively from drip and sprinkler plots. But in January, 2009
difference between drip irrigation and control treatments were much less.
In February 2008, yield of all treatments reduced compared to previous month, though there
was a high rain from week 5 onwards. Sprinkler irrigated plots produced made tea at
100(±14)kg/ha/wk. For drip and control plots, yield results were 72(±8) and 53(±6)kg/ha/wk
respectively. In the month of February 2008, there was a significant difference among three
treatments. Since March 2008, tea yield of all three treatments were increased. Also the
difference among three treatments became less significant, with a slight higher yield from
irrigated treatments. Again in July 2008, sprinkler irrigation plots produced a significantly
higher yield of 127(±10)kg/ha/wk. In this month, rain-fed treatment produced marginally
higher yield increase of (6%) than drip irrigation treatment. In the months of September,
October and December, 2008 also sprinkler treatment gave significantly higher yield than
other two.
220
rainfed
drip
sprinkler
200
Made tea (kg/ha/wk)
180
160
140
120
100
80
60
40
irrigation
irrigation
Mar 09
Feb 09
Jan 09
Dec 08
Nov 08
Oct 08
Sep 08
Aug 08
Jul 08
Jun 08
May 08
Apr 08
Mar 08
Feb 08
Jan 08
20
Month
Figure 6.11 Average weekly made tea during each month according to irrigation treatment. Continuous line
shows the average rainfall received in each week.
With the onset of the usual dry season in 2009, there was a drastic reduction in the
productivity of all three treatments. During the January 2009, there was a less significant
difference between sprinkler and drip treatment, but in February 2009, there was a significant
difference among all three treatments. Sprinkler irrigated plants produced 20% and 57%
136
higher yield than drip and rain-fed plots. In March 2009, with the onset of rains, a yield
increase of 24% was observed only from rain-fed plants.
Table 6.2 Total made tea production according to irrigation treatment from January 2008 to March 2009
Treatment
Yield (made tea kg/ha)
Rain-fed
5119(±174)
Drip
5341(±226)
Sprinkler
5688(±336)
LSD (5%)
865.6
Table 6.2 shows the total made tea yield in kilogram per hectare obtained from each irrigation
treatment during January 2008 to March 2009 period (total of 15 months). Though treatments
are significantly not different from each other in yield terms, higher yields were obtained by
both drip and sprinkler irrigated treatments. As compared to rain-fed cultivation, sprinkler
and drip irrigation result in 11% and 4% increase respectively. Large plot variation in yield
was observed in both sprinkler and drip irrigated fields as compared to rain-fed treatment.
In summary, throughout the study period sprinkler irrigation provided the highest yield in dry
and wet season of the year. The final yield increase by sprinkler irrigation is 6 and 11%
higher than drip and rain-fed plants.
6.3.10 Water use productivity (WUP)
Irrigation water use productivity was calculated as the made tea yield per 1mm water applied
as irrigation or received through dry season for all three treatments (Table 6.3). In 2008 only
tea yield of January and February months was considered for calculation and for 2009,
January to March monthly yield was considered. In the wet year of 2008, all three treatments
showed a nearly 100% higher productivity due to a short dry spell. But in both years,
sprinkler treatments water use productivity was 7-9% higher than drip irrigation, being
highest in the dry year of 2009. Even though water currently is considered to be freely
available in Sri Lankan conditions, this information would be helpful in the event if there is
any price allocated for water.
In summary, WUP of rain-fed plants were 57% higher than sprinkler irrigated plants in 2008,
but it decreased to 38% in 2009, with longer drought.
137
Table 6.3 Water use productivity (made tea kg per water(mm) applied or received as rain)as a reflection on the
yield during the irrigated months of 2008 (Jan-Feb) and 2009 (Jan-Mar)
Year
6.3.11
2008
2009
Rain-fed
3.51
1.74
Drip
2.09
1.16
Sprinkler
2.23
1.26
Irrigation water use efficiency (IWUE)
Irrigation water use efficiency of the drip and sprinkler irrigated plots were calculated as the
ratio between increased yield due to irrigation and the amount of water applied as irrigation.
‫ ܧܷܹܫ‬ൌ
ሺܻ݅ െ ܻ‫ ݎ‬ሻ
‫ܴܴ݅ܫ‬
Where, Yi is the irrigated made tea yield; Yr is the rain-fed made tea yield during dry season.
IRRi is the amount of water applied as either drip or sprinkler irrigation. Table 6. 4 shows the
IWUE of drip and sprinkler irrigated plants for 2008 and 2009 dry season. IWUE was higher
in both years for the sprinkler irrigation. But when the extended dry period was active in
2009, IWUE dropped by 16%. However, for drip irrigation it increased by 12%.
Table 6. 4 Irrigation water use efficiency (as made tea kg/ha per water (mm) applied as irrigation) during irrigated
months 2008(Jan-Feb) and 2009(Jan-Mar)
Year
2008
2009
Drip
0.43
0.48
Sprinkler
0.91
0.76
In summary, IWUE was 112% higher for sprinkler irrigated plants in 2008 and only 58% in
2009, showing a decrease with increase of drought duration.
6.3.12
Total plant growth
The total growth of the main stem sections and roots are given in Figure 6.12. Sprinkler
irrigation canopy growth was 2880(±269)g/plant, which is 43% higher than the drip irrigated
plants. Drip irrigated plants only had a 6% increase in canopy growth compared with rain-fed
plants. But the variation among drip irrigated plant was much smaller than other two
treatments.
In the root development, highest root growth per plant was observed from
sprinkler irrigated plants, with an average value of 708(±96)g/plant. This is respectively 17
and 34% higher than root development observed in drip and rain-fed plants.
138
In summary, both root and canopy growth was significantly higher in sprinkler irrigated plots.
3500
(a)
Stem weight (g/plant)
3000
2500
2000
1500
1000
500
0
2500
(b)
Root weight (g/plant)
2000
1500
1000
500
0
rainfed
drip
sprinkler
Irrigation Treatment
Figure 6.12 Dry mass weight of plants (a) stem weight, (b) root weight, after 9 years growth in the field according
to irrigation treatment. (Error bar shows the standard error)
6.3.14
Root Density
Root depth distributions according to soil depth are given in Figure 6.13. Fine root density in
top 20cm soil layer was 24% higher in rain-fed plants than sprinkler plants. In the 40cm soil
depth also lowest fine root density was observed from drip irrigated plants. But in the bottom
60cm depth there was an increase in root density (65% of drip and 47% of sprinkler) of
irrigated plants. As similar to fine root density, coarse root (>2mm) density was highest in
top 20cm layer of soil for all treatments, without significant treatment differences. But in
bottom 40cm layer, irrigated treatments had a significantly higher root density than rain-fed
139
plants and similar observation was observed in 40cm layer as well. Unlike with fine roots,
there was no increase in density in 60cm layer soil, as against 40cm layer.
Density (kg/m-3)
0
1
2
3
4
5
(a)
Depth (cm)
-20
-40
rainfed
drip
sprinkler
-60
0
10
20
30
40
50
(b)
Depth (cm)
-20
-40
-60
Figure 6.13 Root density of rain-fed, drip and sprinkler irrigated plants according to soil depth up to 60cm depth.
(a) fine (<2mm) roots (b) coarse (>2mm) roots, (Note weight scales are not equal in both graphs) (Error bar
represent the standard error)
In summary, fine root density is 24% higher in rain-fed plants than sprinkler irrigated plants
only in top 20cm soil layer.
In the structural root development, irrigated treatments,
outperformed the rain-fed plants in all depths.
6.4
Discussion
Irrigation of mature tea plants by drip and sprinkler irrigation was evaluated in this chapter.
Cultivar TRI 2023, a drought susceptible, but highly responsive to drip irrigation, (Chapter
5.0), was used. The main findings which are discussed in details in this section are:
140
(1)
physiological activities of the tea plant respond more favourably to sprinkler than
drip irrigation;
(2)
similar effects were found in the yield and plant growth as well;
(3)
however, drip irrigation too had advantages over rain-fed cultivation in the
productivity;
(4)
moisture variation in the treatment plots over the assessment period had some
important revelations about monthly moisture movement which would be important
for identifying correct moisture management practices.
Sprinkler irrigation was the most efficient in stimulating Pn during drought. During the
drought Pn rate of the sprinkler irrigated plants were 15% higher than drip irrigation. Not
only higher Pn rate, but also sprinkler irrigated plants were able to maintain higher Pn rate
during most of the day time (Figure 6.7). The increased Pn rate of sprinkler treatment over
drip can be caused by either low soil moisture in the drip treatments (Figure 6.3) or increased
leaf temperature (Figure 6.8). But in this instance main the reason can be considered as the
increased leaf temperature. In this instance, gs was not significantly different among sprinkler
and drip treatments (Figure 6.5).
Stomatal conductance has a close association with
photosynthesis with tea under irrigation (Smith, Stephens et al. 1993).
In contrast leaf
0
temperature was 2-4 C, higher in drip irrigated plants during day time. However in other
crops, mainly with C-4 plants, similar Pn rates were obtained with drip and sprinkler irrigation
(Antony and Singandhupe 2004).
Even though stomatal conductance was similar for drip and sprinkler irrigated plants, the
transpiration rate of the drip irrigated tea leaves were 20% higher than sprinkler irrigated
plants (Figure 6.6).
More transpiration
sustained the lower leaf temperatures and for
increased photosynthesis than rain-fed plants (Remorini and Massai 2003).
However,
lowering leaf temperature was not achieved in drip irrigation plants as shown in Figure 6.8,
where leaf temperature is almost similar to rain-fed plants during day time. Perhaps less
water availability in drip irrigated plots than sprinkler irrigated plots could be one reason for
lower transpiration rate. One notable feature in the diurnal leaf temperature pattern of this
experiment is the leaf temperature levels were 8-120C higher in all treatments than ambient
temperature, unlike in the drip irrigation experiment described in Chapter 5.0. Lower ambient
temperature during the observation period could be assumed as the reason for this.
Sprinkler irrigated plants had a 43% higher canopy growth than drip irrigated plant. In a
densely populated crop of tea, less canopy structure means leaves and could get exposed to
more sunlight thereby increasing the leaf temperature (Annandale and Stockle 1994) as well
141
the soil evaporation (Lane, Morris et al. 2004). Increased canopy and root growth in sprinkler
irrigation is more related to better wetting pattern of the sprinkler irrigation than drip
irrigation, mainly during young stages of the growth (Sivanappan 1987).
Shoot development and the shoot count in the plant were affected with irrigation method.
Both newly developed (single leaf+bud) and the oldest (3 leaves+bud) shoot weight was
enhanced with irrigation. Low rain and soil moisture in December and January has resulted in
loss of shoot weight. The fresh mass of individual shoot linearly increases with the shoot
number in shoot (Carr 2010). But under sprinkler irrigated plants, the weight difference
between single leaf+bud and 2 leaves+bud was only 15%. In contrast for the rain-fed plants,
the difference was 108%. Higher Pn rate in sprinkler irrigated plants has caused more shoot
development.
6.4.1
Relationship between physiology and irrigation method
The measurement period for physiological activities was not a prolonged drought period
because of the commencement of unexpected rains. Yet the period showed clear drought
stress characters on the tea plant with a few rainless weeks and with the high ambient
temperature. During the measured two week period, there are days where evapotranspiration
varied significantly. Physiological performance of the plants too followed as a response to
environmental factors. The high Pn rate observed in the 23 January in all treatments fell when
measured in 28 January.
Continuous rainless days caused soil moisture depletion and
temperature stress can be considered as the causes for reduction. There were rainfall events
on 4th and 5th February at a magnitude of 9 and 10mm respectively. Since the rainfall event of
10mm occurred after the measurement in mid-day and it cannot be considered as having
effect on moisture replenishment of the field. But due to the previous day rainfall and
reduction in atmospheric stress, Pn rate increased in all treatments. Sprinkler irrigation was
able to maintain a higher Pn rate than drip irrigation, due to the fact that it maintains a lower
leaf temperature level, optimal for Pn. As a result, as shown in Figure 6.14, Sprinkler irrigated
plants were able to maintain a higher productivity through maintaining a higher Pn rate during
hot humid-days.
Water use of the plant as shown by instantaneous leaf transpiration rate showed a higher
water use for drip than sprinkler for three days out of four measurement days. The reason for
higher water use by the drip plant can be due to more water use for maintaining a favourable
leaf temperature level during the day. For the sprinkler irrigated plants, plant leaves are
cooled by irrigation itself. The sudden drop in the leaf temperature level in the 30th January
can be considered as a reduction caused by sudden cloudiness over the measuring period,
rather than a drought related effect.
142
Photosynthetic rate (Pmol CO2 m-2s-1)
16
rain fed
drip
sprinkler
14
12
10
8
6
4
32
33
34
35
Maximum temperature (0C)
36
1.6
2.0
2.4
2.8
3.2
3.6
Potential evapotranspiration (mm/day)
Figure 6.14 Relationship between photosynthesis rate and maximum temperature and Potential
evapotranspiration for period 23-30 January, 2008
In considering the productivity in utilizing the water for irrigation, it can be noted that there is
a higher advantage of using sprinkler for irrigation specially during short dry periods, like in
2008. But with the severity of drought increases, drip irrigation increases the IWUE while
decreasing for sprinkler treatment.
6.4.2
Yield response to environmental variables during dry season 2009
The relationship between treatment yield and environmental variables of maximum
temperature, solar radiation, vapour pressure deficit and potential evapotranspiration was
analysed for the period January – March 2009 (Figure 6.15). This period was a typical
drought period and there were significant yield reductions in all three treatments. In this
analysis, it was evaluated whether environmental parameters can be used to explain the yield
reduction.
With all environmental parameters tested, sprinkler treatment did not show any significant
relationship with any of the parameters. Drip irrigation showed a negative relationship with
maximum temperature (r2=0.2, P=0.1) and to a lesser extent with vapor pressure deficit
(r2=0.11, P=0.2). Both increasing temperature (r2=0.42, P=0.03) and vapor pressure deficit
(r2=0.36, P=0.05) had strong negative relationship with rain-fed yield.
143
80
sprinkler
70
60
drip
control
drip
sprinkler
40
Made tea (kg/ha/wk)
r2=0.17, p=0.2
r2=0.42, p=0.03
50
control
30
33
34
35
36
37
3.2
3.4
Maximum temperature (0C)
3.6
3.8
4.0
4.2
4.4
Solar radiation (MJm-2s-1)
80
70
60
r2=0.36, p=0.05
r2=0.17, p=0.2
50
40
30
0.8
0.9
1.0
1.1
1.2
Vapor pressure deficit (kPa)
1.3
2.6
2.8
3.0
3.2
3.4
3.6
3.8
Potential evapotranspiration (mm/day)
Figure 6.15 Relationship between treatment yield and climate variables during January - March, 2009 (Only
relationship between control yield and environmental parameters are given in equation)
Sprinkler irrigation produced highest yields during the drought as well during wet period of
the year. However, during the wet period of the year, there were some months where rain-fed
treatment produced higher yields than drip treatment. The hypothesis that sprinkler irrigation
technology produce higher yield than drip irrigation can be supported by the findings
presented in this experiment. Even though with a small drought period in 2008, there was a
marked difference among key physiological parameters, like photosynthesis, among irrigation
treatments. Sprinkler irrigation produced enhanced performance over drip irrigation.
6.4.3
Yield response to irrigation
The results here indicate that irrigation application can be used in either as sprinkler or drip
system to maximize the tea production in low elevation tea growing area. Yield increases can
be expected in even very wet years like 2008, where total rainfall is 20% higher than average
annual rainfall. Even though irrigation was applied only during first two months of the year,
yield increase was obvious even throughout the wet season as well. The reason for higher
productivity of the irrigated plants can be attributed to better overall growth of the plant
including root system. In irrigated plants root, system was more developed in the lower soil
depths as well, specially the structural (>2mm) roots (Figure 6.13). Soil moisture level in top
144
soil layers, can be depleted during even a short dry spell as shown in April and December
2008 (Figure 6.3). But the deep soil layers may contain higher moisture content for the plant
to use. Better growth in plants, perhaps devoid of drought related diseases as well, may cause
the irrigated plants to perform better during the optimum climate occur for the high growth.
In this regards, however, it can be considered that irrigating the plant from field planting
itself, is the best way to achieve higher productivity in later years, than irrigation for the
mature plants established without irrigation.
6.4.4
Soil moisture variation and plant growth
The variation of the soil moisture up to 60cm depth of the soil was illustrated in the Figure
6.3. There was a lower soil moisture concentration on the drip irrigated plots during irrigated
months, even though both treatments are applied with same amount of water. For the low
moisture content in the drip irrigation plots can be described as follows. Irrigation water is
applied at a lower application rate (2.2mm/hr) than compared to sprinkler irrigation
(4.4mm/hr). Application rate of irrigation has an impact on the infiltration of irrigation water
to the soil (Clothier and Green 1994). At this rate, much of the applied water may leave the
soil as soil evaporation in hot dry conditions. Soil evaporation is a critical factor in losing soil
moisture during dry days. In a parallel experiment measuring soil evaporation in a tea field, it
was found that average soil evaporation in January–February, 2009 period was 1.2(±0.1)
mm/day in a rainless day. Usually drip irrigation was applied mostly during day time, as it
needed more application hours than sprinkler irrigation. As a result, the moisture penetration
to lower soil depth was limited under drip irrigation.
Sprinkler irrigation according to this experiment can be interpreted as beneficial for the
maintaining higher physiological activities and yield during drought conditions in low
elevation tea growing area. But attention should be given to energy consumption and total
water requirement during the drought for this intensely grown crop.
6.5
Summary of results
x
Sprinkler irrigation has a higher ability to maintain 2-40C lower leaf temperature level
than drip irrigation.
x
Pn was 15% higher in sprinkler irrigated plants than drip irrigated plants, gs remained
same in both treatments, though El 20% higher in drip irrigated plants than sprinkler.
x
Sprinkler irrigation yielded 6% higher than drip irrigation and 11% higher than rainfed plants.
145
x
During wet season as well irrigated plants has shown 15-27% increase in yield in
some months. However yield increase was consisted only with sprinkler treatment.
x
Yield drop in 2009 dry months were 53% for rain-fed plants and 43 and 44% for drip
and sprinkler irrigated plants compared to wet months. Shoot extension rate was 11
and 45% higher in drip and sprinkler irrigation, than rain-fed plants.
However,
increase of 57% in harvestable shoot count only observed in sprinkler irrigated plots.
6.6 Conclusion
Experiment 5 was conducted with the aim of describing the effect of irrigation method on key
physiological parameters, yield and plant growth and assessing the ability of different
irrigation methods to alleviate the environmental limitations.
It can be concluded that
sprinkler irrigation resulted in higher yields than under drip irrigation, due to enhanced
physiological activities.
The reason for higher physiological activity is the lower leaf
temperature under sprinkler irrigation during day time. Since these results were based on an
experiment conducted in a very wet year, it is suggested that longer term evaluation under a
range of annual rainfalls be undertaken.
146
Chapter 7
Effect of Short Term Water Stress and Raised Beds on Young Tea Plants
7.1
Introduction:
The importance of alleviating the moisture stress on tea by means of irrigation was discussed
in earlier chapters.
It was shown that irrigation maintains high growth rate, increased
physiological activity during drought periods which result in enhanced annual yield (Chapter
5.0 and 6.0). All the experiments were conducted with mature tea, which were grown as
irrigated plants from the field planting itself.
Other preliminary trials at the St Joachim Estate, Ratnapura, which are not reported as part of
this thesis, applied drip irrigation to an established mature tea crop (>6 years old) during dry
months of January to March. Under this situation it was not possible to deliver more than 2030% yield increase with irrigation (TRISL 2001).
To get the maximum benefit from
irrigation it is proposed that it must occur from the point of field planting. There could be two
reasons for the poor response to irrigation by mature tea plants: plant characteristics and site
characteristics. Repeated exposure to water stress during early growth stages can weaken the
bush formation resulting in infection by various pathogens such as Phomopsis thea, which
cause stem canker (Shanmuganathan and Rodrigo 1967; Carr 2010). Investigation already
showed that rain-fed tea plants had a higher chance of infection with stem canker than
irrigated plants in low elevation tea growing area (Liyanage and Bandara 2008). Drought
stress will also lead to a poorly developed canopy which consist of relatively fewer shoot
buds or plucking points (Wijeratne 1994).
Plant establishment in the field is done in accordance with the availability of monsoonal rains
(Wadasinghe and Peiris 1987). Yet even within the monsoonal heavy rain periods, shorter
duration droughts causes moisture stress to plant, as enhanced by low moisture retention in
the soil (Ananda and Herath 2001). As described in the monthly soil moisture variation
during January, 2008 – March, 2009 in Chapter 6.0, soil moisture storage does not rise
immediately after the rainfall. So there is a chance for the delicate young plants being
subjected to the moisture stress at any time of the year, in addition to the expected intense dry
season of January – March each year.
Site characteristics may also explain the relatively poor performance to irrigation when it is
applied to an established mature tea crop. The soil bulk density of low-grown tea areas are
higher than that of high and mid elevation tea (Mapa, Somasiri et al. 1999). The natural
higher bulk density accompanied with higher rainfall, floating stones, erosion of topsoil and
147
regular traffic can make low-grown tea area soils highly compacted and poorly aerated. Under
such conditions in the tropics hard pans readily build up, reducing the availability of
connected macro pores (Shougrakpam, Sarkar et al. 2010) and impeding infiltration of
irrigation water to root zone (Kim, Chon et al. 2004). Conventional practice is to establish tea
on levelled ground without any deep ripping of the soil profile (see Chapter 2). Raised beds
have shown to significantly increase infiltration and yield in many and varied crops from
wheat and maize (Verhulst, Kienle et al. 2011) to cactus pear (Labib 1998; Verhulst, Kienle et
al. 2011). However, raised bed cultivation has not been evaluated for tea.
Three experiments are discussed in this chapter with the aims to:
1. quantify the effect of short term water stress on young tea growth,
2. assess the effect on partial irrigation on young tea growth, and
3. evaluate the interaction between raised beds and irrigation.
These aims are to achieve the objectives of:
a) understanding the limitations to young tea growth especially during the wet season,
and
b) developing better land preparation techniques to achieve higher returns to irrigation.
This chapter describes two naturally lit, glasshouse experiments, one field experiment
conducted at St. Joachim Estate, Ratnapura, and a desk-top analysis of historical rainfall data.
Glasshouse experiments were conducted during 2007, using cultivar TRI 4042, while the field
experiment was conducted during May 2008 to March 2009 period.
Experiment 5 is a glasshouse experiment evaluating the growth response of young tea plants
subjected to range of irrigation intervals, from daily irrigation to bi-weekly irrigation.
Experiment 6 is a glasshouse experiment examining the effect of partial irrigation for 20 days
on young plant growth.
Experiment 7 is a field experiment evaluating the irrigation effect on young plant growth
when was exposed to short duration drought and examined the interaction between irrigation
and raised-bed cultivation.
Ideally, Experiments 5 and 6 should have been conducted in the field. However, isolated
rains can disturb drought treatments, thereby necessitating the experiments to be conducted in
a glasshouse.
148
The desk-top analysis of historical rainfall data (section 7.5) serves to establish the frequency
and duration of rain-free periods, and by applying a water stress coefficient, establish the fact
that these periods of water stress can also occur even in the wet season.
149
7.2
Experiment 5: Effect of Water Stress Duration on Young Tea Plant
Growth
7.2.1 Introduction
Short duration droughts of 5 days or more causes poor establishment and low productivity in
perennial crops in wet zone of Sri Lanka including the low elevation tea growing area.
(Sumanasena 2008), Such periods of short term water stress are considered to be a major
impediment to the establishment of young tea in the field resulting in the industry being
unable to achieve the required replanting rate of 2% (Illukpitiya, Shanmugaratnam et al.
2004). Similar drought casualties in replanting shrubs and trees in reforestation programs
have been overcome through irrigation (Siles, Rey et al. 2010). While tea researchers in Sri
Lanka are aware of this issue, the effect has not been quantified and thus no scientific basis on
which to recommend short-term irrigation of young tea. The hypothesis for this experiment is
that even very short term water stress negatively impacts on the growth and development of
young tea plants.
7.2.2 Method
7.2.2.1
Plant material and water application
The experiment was conducted at a naturally lit glasshouse of Tea Research Institute, St.
Joachim Estate, Ratnapura, from July 2007 to November 2007. Cultivar TRI 4042 was used
for the trial. Selection of TRI 4042 was based on the readily availability of the plants and
morphological characters (leaf size, leaf angle etc.,) are closer to previously tested cultivar
TRI 2023. (Since the TRI 2023 is not advised for planting, young plants were not available in
plant nurseries). The cultivar, TRI 4042 is also a broad leaf cultivar with flat horizontal leaf
surface, producing high yield in the low elevation area and resist drought (TRISL 2002).
Table 7.1 shows the soil chemical constituents of the soil in the top 20cm layer of the soil in
field where other field experiments were carried out. The same soil was used as the potting
media in this experiment. The soil pH content is within the prescribed range of 4.5 -5.5 for
tea (Zoysa 2008).
Ten month old tea plants of the cultivar TRI 4042 were grown in the estate nursery. They
were first transferred to 12L plastic pots from nursery bags and kept inside the glasshouse for
4 weeks for acclimatisation. A black polythene layer covered the soil surface to minimize soil
evaporation.
150
Table 7.1 Major soil chemical constituents of top 20cm soil layer in field no 01. Same soil is used as potting media
for pot experiments
Parameter
Amount
pH
4.6
Organic carbon %
1.8
Nitrogen %
0.3
Potassium(ppm)
90.0
Plants were divided into 5 blocks and 3 plants were assigned for each treatment within a
block. Irrigation treatments were:
T1 -
daily watering
T2 -
watering at 4 day intervals
T3 -
watering at 7 day intervals, and
T4 -
watering at 14 day intervals.
The total amount of water applied to each treatment, during 112 days experiment period, is
described in the Table 7.2. The watering regime was calculated to provide more than 3 mm
of water per day for T1 plant (non water stressed). This is the average depth of water required
for a field plant based on the long term weather data. Though, the actual water requirement
for a potted plant inside the glasshouse could be lower, it ensured that the plants were not
subjected to any water stress. To supply approximately 3mm water requirement, T1 plants
were watered with 200ml/plant daily. There was no water drainage from the pots after
watering with 200ml. For the other treatments, drainage was observed after watering. The
amount of drained water was collected one hour after watering to calculate the actual amount
of water retained in a pot. (Seepage ceased on all treatments after one hour).
151
Table 7.2 Water application rate and total amount of water applied in Experiment 5
Net water
Total
Application
/application
Water
rate
(ml)
(ml)
(mm/day)
0
200
22400
3.4
28
143
657
18410
11.2
1400
16
426
974
15584
16.6
2800
8
842
1958
15664
33.2
Rate
Watering
Seepage
(ml)
events
(ml)
T1 – daily
200
112
T2 – 4 day
800
T3 – 7 day
T4 – 14 day
Treatment
7.2.2.2
Measurements
Volumetric water content of the soil was measured using a single soil moisture probe (Delta-t
MLX2 Theta probe – www.delta-t.co.uk) at the end of each treatment, prior to the next
watering. One probe was used for all measurements. This is the time where all treatments
were subjected to the maximum possible water stress. Base stem girth, plant height, total leaf
number and number of branches were measured after the treatment period. Root weight, leaf
weight and stem weight were measured after drying the samples in the oven at 850C for 24hrs.
Results were analysed using SAS (Version 9.0) statistical software (SAS Institute Inc,).
7.2.3 Results
7.2.3.1
Soil moisture
Table 7.3 shows the moisture level maintained by each treatment at the end of the relevant
irrigation interval. Volumetric moisture content was decreased by 10% between first 4 days.
However, moisture depletion within next 3 days was 0.9%. From day 7 to day 14, (7 day)
moisture depletion was only 2.2%.
This suggests a reduction in transpiration with the
increasing limitations of soil moisture.
7.2.3.2
Plant growth response
Plant girth at the base, showed a significance difference between treatments (Table 7.4). The
pattern shows a significantly higher growth among well-watered plants. Girth of the daily
watered plant is 26% higher than T4 plants. There is no significance difference among
different treatments in plant height, though T1 plants showed a little higher growth. Plant
height cannot be considered as a very good indicator for a bush type of plant like tea.
152
Table 7.3 Average volumetric moisture content at the end of Experiment 5 (T1-after 1 day, T2-after 4 day, T3after 7 day, T4-after 14 day)
Treatment
Volumetric moisture (%)
T1 – daily
18.7(±1.1)
T2 – 4 day interval
8.7(±0.4)
T3 – 7 day interval
7.8(±0.6)
T4 – 14 day interval
5.6(±0.4)
Both leaf number and branch number showed a significance difference among different
treatments, which is important for bush formation. Leaf number of T1 plants was 86% higher
than T4 plants at the time of sampling. T2 and T3 plants leaf number was almost similar.
Leaf shedding was seen in the most stressed (T4) plants and it could be the reason for the high
significant (P=0.002) difference in leaf number at the time of sampling. Branching of the
plants was affected with the water stress of both availability and the duration. In the branch
formation, both T3 and T4 plants showed no significance difference. T1 plants produced 53%
higher branch number than T4 plants.
Table 7.4 Plant base girth, height, leaf number and branches at the end of Experiment 5
Treatment
Girth (mm)
Height
Leaf no
(cm)
(plant)
Branches
T1 - daily
7.7(±0.2)
a
69.0(±2.9)
a
63(±6.2)
T2 - 4 day
7.1(±0.2)
a
63.9(±2.2)
a
46(±4.4)
T3 - 7 day
6.2(±0.3)
b
64.2(±3.8)
a
49(±5.1)
T4 - 14 day
6.1(±0.2)
b
65.7(±2.1)
a
34(±4.4)
LSD (0.05%)
7.2.3.3
0.69
7.9
14.2
a
8.1(±0.9)(
a
bc
6.4(±0.5)
ab
5.1(±5.1)
b
c
5.3(±0.7)
b
ab
2.0
Stem and root growth
Table 7.5 shows the effect of watering frequency on stem and root growth. There is 65%
increase in the stem growth when watering daily compared with the 14 day interval. The
advantage of daily irrigation was more visible in the root growth, where root growth of T1
plants showing significant root growth (17±2.5g/plant) than other 3 treatments. By increasing
the watering interval to 4 days from daily watering, root growth was lowered by 46%. The
root: shoot ratio of 1.48 was observed in T1 plants.
153
Table 7.5 Plant stem and root weight and root:shoot ratio of Experiment 5
Treatment
Stem
Root
(g/plant)
Root: shoot
ratio
(g/plant)
T1 - daily
25.3(±2.0)
a
T2 - 4 day
20.3(±1.8)
b
9.1(1.0)
T3 - 7 day
17.5(±2.1)
b
7.5(±1.1)
b
2.3
T4 - 14 day
15.3(±1.8)
b
6.9(±0.8)
b
2.2
LSD (0.05%)
5.5
17.0(±2.5)
a
b
1.5
2.2
3.7
7.2.4 Discussion
Experiment 4 simulated the results of subjecting the young plants to various irrigation
intervals. In other words, it is like simulating the young plants in the field receiving 4 to 14
day duration of water stress. This is a common situation in low-grown tea areas. Plants were
irrigated with regular intervals to provide necessary water requirement. There is a difference
in total water applied among the treatments as the water seeps through the pots. Similar
conditions are visible in the field itself, where sometimes heavy rain events are not effective
because of runoff and deep drainage (Burman, Cuenca et al. 1983). For the mountain crops,
soil moisture storage in the root zone is limited for most rain storms (Harden 2001; Pieri,
Bittelli et al. 2007). The majority of the tea fields in Sri Lanka also lay in mountainous or
hilly regions. So the results in this experiment can be described as reflecting the field
condition itself.
The girth of the plants was affected with the water application interval. Trunk growth of
woody plants indicates the moisture stress experienced during the growth sage of the plant
(Kozlowski and Pallardy 2002). Once the plants subjected to moisture stress even for few
days, like 7 days, the plant stem growth reduces significantly. As a tree crop, avoidance of
rainless days would be important for tea. There was no significant difference between the
treatments of T3 and T4, though they were irrigated on two different frequencies with the
same amount of water.
Daily irrigated plants showed a higher number of branches per plant.
Maintaining a
favourable moisture level for the plant is important to increase the branch number as well the
growth of the branches (Fordham 1969). Branching is key factor in formation of proper
canopy. Low leaf number in water-stressed treatments, were due to the shedding of leaves.
Leaf shedding is a common situation among tea plants during sever dry spells (Fuchs 1989).
One of the clear observations in this trial is the significant difference in the leaf number
154
between, T1 and T2 treatments, where number of leaves observed in T1 was 46% higher than
T2. Even a 4 day interval of watering could not maintain a high leaf number.
The total stem and root growth of T1 plants exceeded other treatments significantly. Increase
in the branch thickness and root growth could be the reason for higher dry matter production.
The ratio between shoot and root has a significant difference among treatments.
The
healthiest ratio of 1.5 (Bannerjee 1993) was maintained by T1 plants.
In conclusion, the results of this experiment support the hypothesis that even very short term
periods without watering, as little as 4 days, has a significant negative impact on the growth
and development of young tea plants. This information provides some confidence behind the
advice for growers to maintain a frequent, even daily, watering regime when establishing a
new tea stand in the low elevation growing areas.
7.2.5 Summary of Results
x
Daily watering produced 25% higher stem growth and 87% higher root growth than
the plants with 4 day irrigation interval.
x
However, stem growth, root growth, and branch formation did not have significance
difference among T3 (7 day) and T4 (14 day) irrigation treatments. The water stress
caused by keeping plants without watering for 7 days, cause significant growth affect
similar to a two week dry period.
x
Most sensitive for the irrigation interval is the leaf number. Even for the short
irrigation interval like 4 day, plant lost 27% of its foliage.
155
7.3
Experiment 6: Effect of Partial Irrigation on Young Tea Growth
7.3.1 Introduction
The effect of exposing young tea plants to short periods of water stress, from 4 to 14 days,
over a 3 month period was analysed in the previous experiment. This represents the field
situation in low elevation tea growing areas well even in the wet season where there are
similar rainless periods and many days with rainfall less than 3mm. Rainfall less than 3 mm
is found to be ineffective in this region as much of it is intercepted by the canopy and
evaporated. Nevertheless, growers usually identify such low-rainfall days as rainy days and
assume that the crop receives enough soil moisture.
Following the recommendation from Experiment 5 for frequent, even daily, watering of
young tea plants may exhaust local water resources and incur an unacceptably high energy
cost. So a grower may consider frequent, but partial, irrigation as an option. This practice of
partial irrigation to save water has been used successfully in other tree crops, e.g. almonds,
without significant economic loss (Romero, Pablo et al. 2004). Similarly, if the growth or
yield is not affected significantly partial irrigation would provide an alternative for tea
plantations, where water is scarce for irrigation.
Specifically this would provide an
alternative way to protect young tea plants from short-term water stress during early stages in
the field. To evaluate the effect of partial irrigation on the growth of young tea plant another
experiment was conducted at a naturally lit glasshouse irrigating at different rates based on
the total plant water requirement.
The objective of this was to simulate the partial
rain/irrigation sometimes experienced in the area and to evaluate the effect on young tea
growth. The hypothesis for the experiment was that it is possible to maintain optimum young
plant growth and development with frequent but partial irrigation. By ‘partial irrigation’ is
meant irrigation that does not completely replace the plant’s water use in the period
immediately before irrigation.
7.3.2 Method
The experimental design was a Randomised Complete Block with 4 treatments in 4 blocks.
Each treatment consists of 3 plants. Ten month old plants of cultivar TRI 4042 were used.
The prior treatment and potting media was similar to the experiment 5. The experiment was
conducted for 20 days from 14th June to 4th July, 2007.
Plants were watered daily according to the following treatments. Treatments were applied as:
T1 – 100% water requirement,
T2 – 75% water requirement,
156
T3 – 50% water requirement and
T4 - 25% water requirement of the plant.
To calculate the daily water requirement 3 plants of T1 treatment were weighed at the same
time each day. The loss of the weight in container was assumed to be the previous day’s
water loss of the plant and irrigated accordingly.
At the end of the experiment period plant growth samples were taken from all plants. Base
stem girth, plant height, total leaf number and number of branches were measured prior to
getting plant destructive samples. As for the growth indicators, stem and root weight of the
plants were measured after oven drying the samples for 24 hours at 850C.
7.3.3 Results
7.3.3.1
Plant growth response
Plant conditions, at the end of experiment period are shown in Figure 7.1. Plant growth
characters according to each treatment are shown in Table 7.6. There is no significant
difference in the plant height and branch number of a plant of the plant among treatments.
However, for those parameters the highest value for each was recorded from T1 plants. The
highest girth of the plant of 0.6(±0.04)cm was observed from T1 plants, which is 50% higher
than T4 plants. Leaf number was affected significantly with 50% deficit irrigation. T1 plants
produced 83% higher leaf number than T4 plants.
Figure 7.1 Picture of potted plants after treatment period in Experiment 6(T1 to T4 from left to right). T3 and T4
plants showed wilting and defoliation, while T4 plants showed most defoliation
157
Table 7.6 Plant growth characters of Experiment 6 (as affected by 4 different watering intensities)
Treatment
Height(cm)
Girth(mm)
Leaf no.
T1 – 100%
45.1(±3.3)
5.8(±0.4)
a
33(±3.3)
T2 – 75%
41.3(±2.1)
5.6(±0.5)
a
28(±2.5)
T3 – 50%
41.2(±2.8)
4.8(±0.3)
ab
21(±1.7)
T4 – 25%
39.5(±2.2)
4.5(±0.2)
b
18(±2.3)
LSD (0.05)
ns
1.1
7.3.3.2
Branch no.
a
4.4(±0.6)
ab
3.5(±0.4)
bc
3.8(±0.6)
c
4.0(±0.3)
7.1
ns
Dry matter production and partition
Both stem and root growth was affected with decreasing irrigation intensity (Table 7.7). The
highest stem weight of 8.6(±0.9)g/plant as well root weight 8.3(±1.7)g/plant were observed
from T1 treatment. Increase in stem and root growth of T1 plants against T4 plants were
respectively 110 and 124%. Plant root shoot ratio, which represent the vigour of the plant
(Hajra 2001) was not significantly affected with water application rate.
Table 7.7 Plant root and stem weight at the end of Experiment 6 (under 4 different watering intensities)
Treatment
Stem
Root
Root : shoot
(g/plant)
(g/plant)
ratio
a
8.3(±1.7)
ab
5.6(±0.7)
5.5(±0.3)
b
T4 – 25%
4.1(±0.4)
b
LSD (0.05)
1.8
T1 – 100%
8.6(±0.9)
T2 – 75%
7.4 (±0.7)
T3 – 50%
a
1.0(±0.3)
ab
1.3(±0.2)
4.5(±0.5)
b
1.2(±0.2)
3.7(±0.4)
b
1.1(±0.2)
2.8
7.3.4 Discussion
Partial irrigation during drought period to save water and for better economic returns is a
common practice with many tree crop cultivations (Romero, Pablo et al. 2004; Testi,
Goldhamer et al. 2008) and in tea in Iran (Salimi and Latif 2008) as well. The experiment
evaluated mainly the growth impact of deficit or partial irrigation during a short period under
sun lit glasshouse condition. Even though the experiment was conducted for a short duration,
the effect on the young plant growth is significantly high among treatments.
According to the results, 50% irrigation for a short period does not have a significant effect on
growth. However, T3 or 25% irrigation results in significant reduction in the plant growth.
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Both stem (r2=0.99, P=0.003) and root growth (r2=0.92, P=0.04) showed more linear
relationships with water application rate.
These results support the hypothesis that it is possible to maintain optimum young plant
growth and development with frequent but partial irrigation. Specifically, it is possible to
maintain optimum growth when daily applying 75% of the water use of the previous 24 hour
period. Daily irrigation under larger deficit regimes still has a negative impact on growth.
7.3.5 Summary of Results
x
Even though experiment was conducted for short time, there was a significant
difference in leaf growth in plants.
x
Young plants were sensitive for even 25% reduction in crop water requirement in
canopy growth. Root growth was the most sensitive for the full requirement of crop
water. Even with 75% partial irrigation root growth was affected by 32%.
x
Branching, or height did not change with water application rate, perhaps due to shorter
period of experiment.
159
7.4
Experiment 7: Effect of Irrigation and Raised Bed on Young Tea Growth
7.4.1 Introduction
The effect of short duration water stress and partial irrigation were examined in Experiments
5 and 6 respectively. These experiments established that: young tea plants grown under the
hot humid conditions of low elevation areas required daily watering for optimum growth; and
that a partial irrigation regime that replaces only 75% of the previous day’s water use is
adequate to maintain optimal growth. These are important findings as poor establishment and
growth in the initial stages of tea establishment can later cause considerable effect on the
productivity and longevity of cultivation. In Experiment 2 (Chapter 5) it was reasoned that
irrigation of young plants at field establishment underpinned the ability of irrigated plots to
yield better than non-irrigated plots even in the wet season when irrigation water was not
being applied. The stronger and healthier bush developed through irrigation at establishment
could make more productive use of wet season rainfall.
Experiments 5 and 6 were by practical necessity conducted in a glasshouse and we need to be
conservative in translating glasshouse results to the field. The main reason for this is that,
even though the glasshouse pots used soil derived directly from the field, the bulk density of
the glasshouse soil will be quite different from the field.
A key limiting characteristic of soils in the low elevation growing areas is their high bulk
density. Such conditions inevitably restrict root growth and limit the potential of the tea bush
to develop a canopy and root structure to make the most of available water. This was the
interpretation for the relative low response to drip irrigation in a mature tea field, but had not
been irrigated since establishment, in early years leading up to the current research project
(TRISL 2005). In recognition that bulk density, and by extension infiltration and run-off,
may be limiting the establishment, a field experiment examined the effects and interactions of
irrigation and raised beds on young tea plant growth. The hypothesis of this experiment was
that raised beds enhance the ability of young tea plants to productively use irrigation water.
7.4.2 Method
The experiment was conducted at St Joachim Tea Estate, Tea Research Institute Ratnapura
during May 2007 to March 2007. Healthy ten month old plants of cultivar TRI 2023 were
planted in the field in May 2007. May is the month recommended for field planting with the
onset of south west monsoonal rains. It ensures young plants are not exposed to a severe dry
season drought at least for next 8 months (Wadasinghe and Peiris 1987; Zoysa 2008). Also
the relatively lower ambient temperature and absent of higher solar radiation facilitates young
160
plant growth. The experimental design was a Randomized Complete Block. Following are
the treatments of the experiment.
T1 - 30cm raised beds with no irrigation
T2 - normal ground with irrigation
T3 - 30cm raised beds with irrigation
T4 - normal ground with no irrigation
There were 4 blocks and each plot consisted of 16 plants in two 8-plant rows spaced 1.2m
between rows X 0.6m along rows. In between blocks and plots, 1.2m space (1 row) was kept
vacant to separate the treatments. In the irrigated plots of treatment 2 and 3, a thick polythene
layer was inserted into the soil, 60cm apart from the plant row, to a depth of 60cm. The
polythene layer was inserted to prevent possible soil moisture seepage from irrigated plots to
rain-fed plots. In the treatments 1 and 3, plants were established in the middle of soil beds
with 60 cm wide, 5.6 cm long and 30cm in height. When constructing the beds, all large
stones and boulders were removed manually. In the initial months, beds were reconstructed
manually following heavy rains. However, once the soil got established, reconstruction was
not necessary after rain.
After planting, all plots were thatched with Mana (Imperata
cylindrica) mulch to minimize any soil moisture loss. After nearly two months in the field, all
plants were cut to 30cm height to encourage lateral branching in week 4 of July 2007. This is
the recommended practice to encourage the lateral branch growth.
Drip irrigation system was established in June 2007, using Jain Turbo Tape, an in-line drip
tape, with a emitter spacing of 60cm and discharge rate of 1.5L/hr/emitter at 10psi. The
system was run by water from a deep tube well fitted with a submerged pump. Irrigation was
applied in the first week of December, first and third weeks of January and the first two weeks
of February, 2008. Unexpected heavy rains continued in following weeks until April, 2008.
Irrigation was applied as the difference between plant water requirement and the rainfall of
the previous day. Irrigation was begun for 5 days after the rain event following the current
industry recommendation (Sumanasena 2008). The plant water requirement was calculated
based on the Class A pan evaporation of the day. A crop coefficient of 0.85 was used to
calculate the plant water requirement (Laycock 1964). Rainfall and pan evaporation data was
collected from the Estate weather station.
Plant height, number of branches and leaf area index per plant were measured using 4 plants
per plot, prior to drought (January, 2008) and after drought (February 2008). After the
drought period, destructive samples were taken to measure the total growth, selecting one
plant from each plot. Plants, including the roots, were excavated carefully from the soil.
161
Additional care was taken to collect the all roots of the plant in the excavation. The plant
sample was separated to stem and root compartments. Dead twigs were separated from the
stem, as it was prominently available in some plants. Root samples were washed carefully to
remove the soil. Root samples were further separated into fine (<2mm) and coarse (>2mm)
roots. Roots samples were air dried before oven dried at 850C for 24 hours.
7.4.3 Results
7.4.3.1
Rainfall and soil moisture during the study
As the plants were grown during the onset of the rainy season (Figure 7.2), they received rain
each week until the 30weeks after planting (third week of November, 2007) after the third
week of May. This was followed by a completely rainless two weeks during January 2008
during which time irrigation was applied. February is normally a dry month too, but after
February 2008, there was rain in each week. Plant potential evapotranspiration was around
20mm/week on average. It dropped to nearly 15mm/week during some weeks. The reduction
in potential water use was observed in heavy rain months and in some comparatively dry
months like January.
25
rain
ET0
Rain (mm/week)
250
20
200
15
150
10
ET0 (mm/week)
300
100
5
50
0
May
0
Jul
Sep
Nov
Jan
Mar
Week (2007 May -2008 Feb)
Figure 7.2 Weekly rainfall and maximum potential evapotranspiration (ET0) since field planting
162
Week (2007 June - 2008 Feb)
May
0
Jul
Sep
Nov
Jan
Mar
Water deficit (mm)
-20
50% available water
-40
-60
rainfed
irrigated
Figure 7.3 Maximum potential soil water deficit in rain-fed and irrigated plots. Soil water retention in 60cm soil
depth was 75mm
The change in the maximum soil water deficit (SWD) is given in Figure 7.3. SWD was
calculated as the difference between rainfall and ET. The figure shows the soil water deficit
up to 60cm depth of soil. Initially also there was around 20cm deficit of soil water due to
absence of rainfall in the third week of May. However, there was no significant reduction in
soil moisture until December.
There were deficit and replenishment of soil water in
December and from January 2008 followed rapid decline in soil moisture. During the last
week of January soil moisture level plummeted to more than 50% of available soil moisture in
the soil. This is a serious shortage in soil water availability.
7.4.3.2
Soil bulk density
Figure 7.4 shows the average dry bulk density of the soil up to 60cm depth in g/cm-3. The
total depth was measured based on the average depth of 3 soil layers at 20cm interval. The
bulk density of soil under the conventional flat land was 20% denser than raised beds.
Among the two raised-bed treatments the irrigated bed had a slightly higher bulk density.
This may be due to the irrigation water settling the soil particles more firmly in the loosely
constructed earth bed.
163
2.0
1.8
-3
Dry bulk density (g/cm )
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Raised, rainfed
Flat, irrigated
Raised, irrigated
Flat, rainfed
Treatment
Figure 7.4 Average plot soil bulk density (dry basis) up to 60cm depth (error bars shows standard error)
7.4.3.2
Plant height
All plants were cut to a height of 30cm after two months in field planting, and resultant
growth during the non water stress period was measured at early January, 2008, prior to onset
of drought.
As shown in Figure 7.5, there was no significant difference between the
treatments. Plants in all 4 treatments grew at a similar pace with respect to the height. It must
be made clear that the two irrigated treatments were not actually irrigated at this early stage.
Although the growth of plants in the raised bed treatments (Treatments 1 and 3) are nominally
higher than the normal ground treatments the treatment effect was not significantly apparent
at this early stage.
164
120
prior to drought
after drought
Plant height (cm)
100
80
60
40
20
0
Raised, rainfed
Flat, irrigated
Raised, irrigated
Flat, rainfed
Treatment
Figure 7.5 Plant height of the young tea plants before and after the drought. (error bars = standard error)
However, when height was measured again after one month drought period of 2008, and after
irrigation applied to Treatments 2 and 3, the treatment effects became readily apparent (Figure
7.5). From this figure, highest increase in plant height was shown by Treatments 3 and 4,
values of 11.4(±1.2) and 11.5(±2.0)cm/month. Both these treatments involved irrigation
during the dry spell. The lowest plant height increase of 5.2(±1.1)cm/month was observed
from Treatment 4, which involved neither land preparation nor irrigation during the dry spell.
Both irrigation treatments produced similar height increases during this dry period; i.e. there
is no interaction with the raised bed treatments (p=0.05).
7.4.3.3
Growth of branch shoots
Development of new branches to have a spread canopy for higher productivity and healthier
canopy is very important in formation of the tea bush. The growth of new branches during the
dry spell is presented in Table 7.8. Growth of new branches in the control plots (Treatment 4)
was 1.6(±0.5) no/plant, which is significantly the lowest among the 4 treatments. The other 3
treatments cannot be statistically separated although the highest branch increase of 2.9(±0.6)
no/plant was found in the treatment 3.
165
Table 7.8 New branch growth during short dry spell 2008
Treatment
(no/plant)
1 Raised Bed No Irrigation
2.2(±0.3)
ab
2 Normal Ground With Irrigation
2.6(±0.2)
ab
3Raised Bed With Irrigation
2.9(±0.6)
a
4 Normal Ground No Irrigation
1.6(±0.5)
b
LSD(0.15)
7.4.3.4
Branch increase
1.28
Leaf area index (LAI)
Addition of new leaf to the plants is important to maintain healthier leaf area per plant. Leaf
fall can be observed when the plants are subjected to drought conditions. During the study
period leaf addition and leaf fall due to abscission from the plants were counted for each plot.
Some plants showed a gain in leaf number meanwhile some other plants showed leaf loss in
all 4 treatments. Change in leaf area index based on the addition of new leaf or loss (i.e. leaf
abscission) for each treatment was calculated and presented in Figure 7.6.
Irrigated
treatments under both methods of land preparation had greater LAI than their respective rainfed treatments. However, this difference was significant (P=0.05) in raised beds only. Both
irrigated treatments showed higher LAI gains per plot, increasing more than one. Lowest LAI
was observed from the raised bed, rain-fed plants.
166
0.10
LAI change
0.08
0.06
0.04
0.02
0.00
Raised, rainfed
Flat, irrigated
Raised, irrigated
Flat, rainfed
Treatment
Figure 7.6 Increase in leaf area index during dry spell (Error bar = standard error)
7.4.3.5
Development of stem parts and roots
The weight of the main stem and dead twigs are shown in Table 7.9 in dry weight g/plant.
Stem growth was higher in Treatment 1 and 2, though significantly not different (p =0.05).
Stem growth was similar in Treatment 3 and 4.
Table 7.9 Dry weight of main stem parts and twigs
Treatment
Stem
Dead Twigs
(g/plant)
(g/plant)
1 Raised Bed No Irrigation
408(±27.1)
a
10.5(±3.9)
2 Normal Ground With Irrigation
411(±71.3)
a
4.0(±1.0)
3 Raised Bed With Irrigation
353(±104.1)
4 Normal Ground No Irrigation
352(±66.5)
LSD (0.05)
ns
a
a
ab
2.3(±1.4)
4.3(±0.5)
a
b
ab
7.3
In the normal flat land plots irrigation facilitated 17% more stem growth than rain-fed normal
flat land. The amount of dead twigs in the Treatment 1 was significantly higher than other
three treatments (P=0.05). Drying of the soil and plant during dry days is likely to have
caused the die back of plant parts in drought.
Development of the different size roots are shown in the Table 7.10. Soil bed preparation and
irrigation facilitated more fine and coarse root growth. Irrigation alone resulted in more than
167
100% increase in the fine and coarse root growth. However, a combined effect of raised bed
and irrigation was not observed in root growth.
7.4.4
Discussion
Land preparation technique, i.e. constructing raised beds of 30cm to plant young tea was able
to reduce the soil compaction by around 10%.
This is an alternative to remove soil
constraints to improve productivity in this highly compacted soil in the region. Planting in the
raised beds lowered the soil bulk density and increased the more feeder root development
(<2mm) than flat non-irrigated treatment. Easy penetration of the roots in less compacted soil
may cause for more smaller root development, which is vital for water and nutrient uptake.
Similar results of root development also achieved with irrigation the plants in flat, normal
lands as well. Drought usually impaired the growth of roots in woody plants, with less
thickening of roots (Pace, Cralle et al. 1999). A similar increase was seen with the branch
increase in the month long drought period.
Table 7.10 Fine and coarse root weight (g/plant) of Experiment 7
Treatment
Fine (<2mm)
(g/plant)
Coarse
(>2mm)
(g/plant)
1 Raised Bed No Irrigation
40.3(±6.1)
a
185(±29.9)
a
2 Normal Ground With Irrigation
36.7(±4.8)
a
180(±27.7)
a
3Raised Bed With Irrigation
41.0(±1.7)
a
195(±5.0)
a
4 Normal Ground No Irrigation
18.3(±3.3)
b
94(±19.4)
b
LSD (0.05)
14.2
83.3
However, one of the main limitations for applying the raised beds for cultivation under rainfed condition is the very low leaf development during the dry period, as indicated by the LAI
increase. The reason for this can be attributed to the increased temperature conditions in an
around the micro- climate of the young tea plant. This also confirmed with more dead twigs
in the young tea plant in raised beds without irrigation.
Findings of this experiment showed that raised beds lower soil compaction. However, the
observed effects of decreased soil compaction in raised beds, under irrigation were not
conclusive as to whether it had a favorable affect on the growth of tea. Construction of raised
beds is a technique that has to be applied very cautiously. There are many factors to be
considered in constructing the raised beds for irrigation like, cost of establishment, long term
maintenance of raised beds, weed growth and feasibility to apply in sloppy lands etc.,
168
Nevertheless, lowering soil compaction is a recommendation for the tea growers, using
techniques like rehabilitation grass cultivation prior to replanting, burial of pruning and
envelope forking. So for maximizing the response to irrigation, it is advisable to pay attention
the soil compactness of the field.
7.4.5 Summary of results
x
This experiment showed that Irrigation supports better growth of young tea regardless
of ground preparation.
Raised Beds may further improve some measures of
performance of young irrigated tea, but it is worse than normal ground preparation
under rain-fed conditions.
x
There was no significant treatment difference in height growth in the very early
growth when plants were well watered by rainfall;
x
But during a short dry period irrigated plants grew on average 10% more than rainfed. There was no interaction between irrigation and land preparation method;
x
During this dry period there was a statistical difference between treatments in the
growth of branch shoots (P=0.15),
x
There was a strong treatment effect on leaf area index. The Raised Bed with Irrigation
had highest LAI and retained most leaves, whereas the Raised Bed with No Irrigation
had the lowest LAI. It lost even more leaves than the Normal Ground No Irrigation;
x
While there was no significant treatment effect on main stem weight and live twig
weight, the Raised Bed with Irrigation had significantly less dead twig material. The
Raised Bed No Irrigation had the most dead twig material;
x
Irrigated treatments both produced significantly more finer roots (<2mm) but there
was no interaction with raised beds.
7.5
Conclusion
The aims of this chapter were to quantify the water stress effect on young tea growth, assess
the effect on partial irrigation and evaluate the interaction between raised beds and irrigation.
Experiment 5 evaluated the water stress effect and Experiment 6 evaluated the partial
irrigation. In Experiment 7, interaction of raised beds and irrigation on young tea growth was
evaluated in field. In summary, it can be concluded that plant growth is highest under daily
rainfall or irrigation at young stage. However, 75% partial irrigation can be practiced without
significant growth reduction. In the field, construction of raised beds can be helpful in
169
improving the plant response to irrigation. Raised bed construction for young tea plant is not
suitable under rain-fed conditions.
170
Chapter 8
Financial Feasibility of Drip Irrigation in Low Elevation Tea Growing
Area
8.1
Introduction
The preceding chapters have all dealt with the physiological and agronomic aspects of
irrigating tea cultivation in low-elevation tea growing areas of Sri Lanka.
A summary
discussion of what the results from these studies mean for the practical establishment of
irrigation will be given in Chapter 9.
However, regardless of physiological and yield
responses to irrigation, and cultivar differences in these responses, the final determinant of the
feasibility of any innovation will be financial. This chapter presents such an evaluation.
The low-elevation tea growing areas produce around 60% of total tea production in Sri Lanka
and as such are critically important for the generation of export income and provision of local
employment (MPISL 2008). As described in Chapter 2 the demography and land-ownership
pattern of this tea producing region is quite distinct from the mid and high elevation tea
growing areas of Sri Lanka. The majority of the tea produced in this region is supplied from
small-holder enterprises, where more than 95% of farmers own 1 ha or less as shown in Table
8.1 (TSHDA 2005). For reasons also elaborated in Chapter 2, this sector of the industry is
also the most likely to adopt irrigation.
Table 8.1 Land extend distribution among tea small holder farmers (TSHDA 2005)
Extent (ha)
A
NOTE:
This figure/table/image has been removed
to comply with copyright regulations.
It is included in the print copy of the thesis
held by the University of Adelaide Library.
The drought management tools currently practiced among growers are: selection of suitable
cultivars, planting shade trees, and various agronomic practices like planting grasses to
improve the moisture holding capacity of the soil (Ananthacumaraswamy and Amarasekera
1986; TRISL 2002; Zoysa 2008). Drought tolerant cultivars are sometimes less productive
171
than drought susceptible cultivars. Also during prolonged rainless periods, even highly
drought tolerant cultivars are vulnerable. For this reason, small to medium tea growers are
interested to adapt irrigation as a drought mitigation technique.
Generally, the choice of the irrigation system depends on many factors such as water source,
area of land to be irrigated, plant canopy and root structure, topography and soil type (Çetin,
Yazgan et al. 2004). However, drip irrigation is emerging as the most popular method in
perennial crops because it confers better plant survival, greater yields, more efficient water
use, more efficient distribution of nutrients and less plant stress (Çetin, Yazgan et al. 2004;
Kigalu, Kimamboa et al. 2008). Accordingly, drip irrigation has become the only option
currently considered in Sri Lanka despite earlier consideration of overhead sprinkler systems
(Rogers 1959; Ananthacumaraswamy 1995).
Net Present Value and Internal Rate of Return analyses are the standard method to establish
financial feasibility of investments. These methods are used to evaluate the financial
feasibility of drip-irrigation technology in low-elevation areas using yield data for the two
cultivars, TRI 3035 and TRI 2023, from the St Joachim Estate irrigation trial. In the previous
chapters it became clear that there are significant differences in the responses of these two
cultivars to irrigation. This chapter asks whether these differences have an impact on the
relative feasibility of irrigating the two cultivars.
8.2
Methodology
The financial evaluation of implementing drip irrigation to the rain-fed tea cultivation in low
elevation tea growing areas of Sri Lanka, based on the yield response irrigation trial at
Ratnapura, is analysed here. Among the low elevation tea growing areas of Sri Lanka, the
tested location of Ratnapura receives a higher annual rainfall. So the results can be applied
validly to other tea growing areas like Galle and Matara, where annual rainfall is less. A drip
irrigation system was selected for irrigating the tea fields as it is more suitable for the
landscape of tea fields, which are mostly hilly and undulating.
8.2.1
Analytical methods
In the financial analysis net present value (NPV) and Internal Rate of Return (IRR) methods
were used. The NPV method examines the cash flows of a project over a given time period
and resolves them to one equivalent present date cash flow (Remer and Nieto 1995). NPV
value is calculated based on following equation.
Net Present Value = PV value of benefits – PV value of costs (Turner and Taylor 1989).
This equation can be written as
172
Where, NPV is the net present value, R the net cash flows, f the interest rate (rate of return)
and t the year (from zero to n). To calculate the NPV for 2006/07 period, interest rate of 10%
was used. This is the minimum attractive rate of return (MARR) used largely evaluating
agriculture projects in Sri Lanka (Liyanage 2009). The NPV method is suitable for analysing
the drip irrigation investment, as the project inputs and outputs are fixed (Remer and Nieto
1995). Generally, if the NPV is positive, project is accepted and if NPV is negative, project is
rejected (Beattie, Taylor et al. 1985).
Internal Rate of Return is a measure of investment worth which calculates the interest rate for
which the present worth of a project equals zero (Park and Sharp-Bette 1990). This method
does not represent any external factors like interest rate. However, accepting or rejecting the
project depends on the available minimum attractive rate of return. If IRR is greater than
MARR, then project is accepted, otherwise project is rejected.
As there is a chance to reduce the irrigation system cost with the expansion of the irrigation
extent.
Basically, using the same irrigation pump for higher irrigation extent leads to
reduction in the investment cost. Similarly, irrigation companies provide discounted price for
the growers who irrigate large extent (Herath 2011). This is specially applicable for the
growers with low yielding cultivar.
At the end of the chapter, sensitive analysis was done for the two different cultivars based on
different green leaf price and wage rate for discount rates of 0, 5, 10 and 15% respectively.
These economic factors do have variation over the tea economics. Price of green tea depends
on largely on the processing factory to which harvest is sold. However, external factors like
break down in international demand, due to either economic or sociological reasons can have
a negative effect on the price. Though wage rate is fixed on large company estates, some
small growers pay above the nominal rate for the skilled workers to retain them in the
property. Different discount rate would provide foresight at which rate the money should be
borrowed, for the capital investment.
8.2.2
Irrigation system and cost estimation
All the data for yield, prices received, establishment and maintenance costs used in this
analysis were gathered from the experience of growing irrigated tea on Field no 01, St.
Joachim Estate in Ratnapura, Sri Lanka, during 1999-2009. The soil and topography of the
site, generally flat with a 10m height difference and local well available, is indicative of the
173
majority of smallholders in the low-elevation areas. The establishment and maintenance costs
were also no different than what a private smallholder would pay.
The installed irrigation system was a Netafim RAM 17D, dripperlines system from Israel.
The irrigation system was installed during the field planting of 10 month old cultivar TRI
2023 and 3025 plants in May 1999. The young vegetatively propagated plants from the Tea
Research Institute breeder Nursery of St. Joachim Estate, and hence they were according to
standards laid by Tea Research Institute of Sri Lanka for young plants for the field cultivation.
Commencement of the plucking of the irrigated field commenced September, 2001. Hence,
harvesting year was calculated from September to August next year. In each year drought
prevailed during January–March period. The irrigation system was operated for 64 total
hours during the dry spell of 2006/07 (during January – March) Total cost for operating the
system 2006/2007 drought period was calculated from the field records of St. Joachim Estate.
Based on the hourly operating cost of the system, operating cost of the previous years was
calculated from irrigation records. Unit cost of electricity of electricity was Rs 30/kwh, and
accordingly cost for operating the system for one hour is Rs 121. To calculate additional cost
for the plucking, it is assumed that an average worker harvests 20 kg of green lead per day
and worker wage was Rs. 405 per day (there was no change in worker wage rate from 2007 to
2009).
8.2.3
Green leaf price and wage rate
As the price of green tea, low elevation average green leaf price of 2009 was used (Rs
51.70/green leaf kg). For the sensitive analysis of irrigation investment, Net present value of
irrigation investment under potential increase and decrease of the capital investment by 50%
was calculated over long term green leaf average of Rs 51.10/kg and current wage rate of Rs
540/day. Possible increase or decrease in the investment of the drip system was to facilitate
the investor’s ability to reduce the cost of investment through possible large area investment.
Also there is a risk of increasing the cost of drip irrigation system as it depends on the import
of materials.
Price variation of green leaf price (from Rs 40-70/kg) was then calculated for the 3 different
wage rate scenario of Rs 405, 500 and 700 a day. This would facilitate the grower to select
best price for the green leaf, under irrigation investment. Since, the green leaf price usually
vary with tea factory, choice is there to select a best available price.
174
8.3
Results and Discussion
8.3.1
Drip irrigation system cost
Total cost for the irrigation system including the pump was Rs 515,840. Main items of the
irrigation system include power source, filter, valves, main pipes, drip lines and other fittings
(Table 8.2). The highest cost of the system was for the drip tube and accessories, which is
63% of drip irrigation system cost (excluding pump cost).
Table 8.2 Investment cost of drip irrigation system for 1 ha tea field (Jinasena Ltd 2010; Herath 2011)
Equipment
Filter system 20m
3
Pressure gauge and air release valve
Drip tube, connectors and end caps
Cost(Rs)
15,000
8,100
284,564
PVC pipes and fittings
63,240
Installation charge
20,400
Sub total
391,304
Value Added Tax (15%)
Total Irrigation cost
3.0HP electric pump with accessories
Total System cost
58,696
450,000
65,841
515,840
Most of the growers looking for irrigation system are the ones with permanent water supply
source either like a river or well. The cost for irrigation well is not considered in this
investment, as for the irrigation, an existing well is used. However, the cost of unit irrigation
system can be brought down by installing more area than 1 ha at once.
Annual operating cost of the system was Rs 11776/ha, including the 10% of pump running
cost allocated for annual maintenance (Table 8.3). The operating cost was significantly
lower, when compared to capital investment cost. It was nearly a 3% of total investment cost.
Table 8.3 Total annual operational cost during 2006/07 dry season
Description
Cost(Rs)
Operational cost for 64 hours
7,755
Labour cost
3,245
Repair and maintenance cost (10%)
Total
776
11,776
175
8.3.2
Green leaf yield
Table 8.4 shows the yield response for drip irrigation for two contrasting cultivars, from year
2001 to 2009. Highest yield response to irrigation is from the cultivar TRI 2023 throughout
the period, which is a drought susceptible, fast growing cultivar, except 2009. Highest
percentage yield increase was recorded in the 2001/02 period (first year in harvesting) Yield
increases were 127 and 93% respectively for TRI 3025 and TRI 2023 cultivars in that year.
Yield increase during the pruning years (2005/06 and 2008/09) was lower for both cultivars.
Second year of the first pruning cycle (2002/03) recorded the highest yield increase in
quantity wise for both cultivars. Though there is a difference in cultivar, there is no price
difference for difference cultivar.
8.3.4
Net present value of installing a drip irrigation system
Net Present Value of installing a drip irrigation system for two cultivars is shown in (Table
8.5). According to the table, net return from the operating the irrigation system depends on
cultivar and age of the pruning cycle (apart from climatic factors). For the cultivar TRI 3025,
NPV at the end of 10 year operation was Rs -57898. The financial return after 10 years is a
negative value and hence this investment cannot be considered as financially sound project
for the cultivar on present green leaf price, wage rate and at a discount rate of 10% for a 10
year period.
Table 8.4 Yield response of two tea cultivars to drip irrigation
TRI 3025 - Green Leaf
TRI 2023 - Green Leaf
(kg/ha)
(kg/ha)
Year
Control
Irrigated
Control
Irrigated
2001/02
6318
14389
11232
21730
2002/03
16952
22824
24504
37306
2003/04*
16091
18000
19564
23467
2004/05
20061
23315
26716
32163
2005/06*
20357
20558
27966
29114
2006/07
15224
17830
20119
25437
2007/08
16543
17663
11634
21627
2008/09*
1451
4140
1756
3806
(* - plants were pruned during April in these years)
176
203
2606
1121
2691
2005/06
2006/07
2007/08
2008/09
Net Present Value (Rs)
3254
2004/05
2052
9993
5318
1148
5448
3902
139125
57956
134730
10495
168232
98644
106088
516623
274923
59349
281646
201754
65490
28546
64547
7284
72349
47381
125192
1908
661843
2003/04
303634
5873
2002/03
12802
170027
8071
TRI 3025
2001/02
542726
TRI 2023
5011
417271
TRI 3025
52550
208198
119458
26419
116771
87767
265496
219165
5011
7433
TRI 2023
Cost (add.)
2000/01
10498
TRI 2023
Income (add.)
7433
TRI 3025
Increased yield
1999/00
Investment cost
Year
Table 8.5 Net Present Value of installing drip irrigation system for two cultivars (price in Rs)
73635
29410
70184
3211
95883
51263
178442
247244
-5011
-7433
TRI 3025
53539
308425
155465
32930
164875
113987
396347
323560
-5011
-7433
TRI 2023
Net revenue
0.3855
0.4241
0.4665
0.5132
0.5645
0.6209
0.6830
0.7513
0.8264
0.9091
1
Discount
Factor
(10%)
-57898
28390
12473
32741
1648
54123
31830
121879
185758
-4141
-6757
-515,841
TRI 3025
391779
20641
130802
72526
16898
93067
70777
270710
243096
-4141
-6757
-515,841
TRI 2023
NPV
177
As shown in the table, except for 2009, net return was higher with cultivar TRI 2023 and the
NPV value of cultivar TRI 2023 after the study period was Rs 391779. Higher positive value of
the cultivar TRI 2023 suggests that installing a drip irrigation system for this cultivar is
worthwhile. Higher yield production of this cultivar is the reason for positive NPV of this
cultivar.
8.3.5
Internal rate of return (IRR)
As the investment of drip irrigation, only resulted positively with TRI 2023, Internal Rate of
return (IRR), was to evaluate further economic return on the investment. IRR value for cultivar
TRI 3025 is 7% and for cultivar TRI 2023, it is 23%. 7% discount rate is a lower value in the
investment, and at present market condition need some kind of concessionary discount rate for
the farmers to apply drip irrigation for low yielding cultivars like TRI 3025
8.3.6
Variation in capital cost
Cost of the drip irrigation system decrease with the increase of extent irrigating at once. Hence
there is a higher chance to reduce the investment cost per hectare by irrigating more area. Same
water pump can be used to irrigate more than 1 ha field. Sensitive analysis of irrigating two
cultivars based on potential decrease in the cost of system installation. Green leaf price of Rs
51.10 was selected based on the long term average of the green leaf price in the area
(Munasinghe 2010). The past trend of the green leaf price in the area is upward. However, there
is a risk of sudden drop of the price in some years, due to the facts like global recession. Hence
long term average green leaf value was used for this calculation. Present wage rate of Rs 540/day
was selected as the wage rate.
Table 8.6 Net Present Value (Rs) of installing a drip system at different investment costs (Green leaf Rs 51.10/kg,
wage rate Rs 540/day) (Herath 2011)
Extent
(ha)
System
cost
TRI 3025
0%
5%
TRI 2023
10%
15%
0%
5%
10%
15%
1
515841
25255
-93529
-247476
-369983
638180
360259
13706
-244545
2
452921
88175
-30609
-184555
-307063
701101
423180
76626
-181624
5
413168
127927
9144
-144803
-267310
740853
462932
116379
-141872
10
363168
177927
59144
-94803
-217310
790853
512932
166379
-91872
20
343168
197927
79144
-74803
-197310
810853
532932
186379
-71872
178
According to the sensitive analysis (Table 8.6), irrigating cultivar TRI 3025 is not economical up
to 20ha investment under the discount rate of 10 and 15%. If the grower is to irrigate more than
5 ha, it is economical at 5% discount rate.
If the discount rate is 15%, irrigation is not
economical even with cultivar TRI 2023. But at the discount rate of 105 or lower, irrigation is
economical even for 1 ha field.
8.3.7
Sensitivity to variation in green leaf price and wage rate
Unlike capital cost of investment, there is a higher chance of variation in green leaf price, due to
the constraints in export market and for the wage rate too. Even though, large plantation estates
have the fixed wage rate for workers, with employer contract, small tea growers may face mostly
increasing wage rate depend on the demand for the local labor. For the analysis Wage rates were
selected as Rs 405, 500 and 700 day. Rs 405/day was the wage rate during 2007 to 2011. In
2011, wage rate was revised to Rs 540/day. However, there is a likely hood increase in the next
wage revision to Rs 700/day
There is complete difference in the sensitivity of the two cultivars for different wage rates and
green leaf price. Installing a drip system at the wage rate of Rs 405/day was economical at
discount rates (0-15%). for cultivar TRI 3025, only if the price increases to Rs 65 per kg (Figure
8.1). To make the investment successful for the same cultivar at the 10% discount rate, green
leaf price has to be keep at a rate of Rs 55 or higher. But in contrast for the cultivar, TRI 2023,
keeping the wage rate above Rs 45 is sufficient to make the investment successful for discount
rates up to 15% at wage rate of Rs 405.
At the present wage rate of Rs 540, financial feasibility is very weak for cultivar TRI 3025, even
the price of green leaf reaches Rs 70, at higher discount rates of 10 and 15%. Financial
feasibility further sinks for the same cultivar, with potential increase in wage rate to Rs 700.
In contrast, the other cultivar TRI 2023 shows a higher resilient to fluctuation in the wage rate
and green leaf price in a wider range, thanks to its higher productivity. At the higher discount
rate of 15%, it need however to reach a green leaf price of Rs 60 or higher to reach the positive
NPV value.
The financial feasibility of installing a drip irrigation system from field planting upto 10 year
period, was evaluated. It analysed the financial feasibility of mitigating the short term dry spells,
tea plants are experiencing, specially during January – March period. Drip irrigation resulted in
positive yield increase from both cultivars tested. Unlike in the case of introducing irrigation to
mature cultivation, the yield response from young plant is significantly higher (Stephens and Carr
179
1991). As shown in Table 8.4, response to irrigation was lower during pruning years. However,
cultivar selection is a crucial factor for the financial viability.
Rs 405/day
2000
TRI 3025
TRI 2023
0%
5%
10%
15%
1500
1000
500
0
-500
Rs 500/day
Net Presen Value (Rs. 000)
2000
TRI 2023
TRI 3025
1500
1000
500
0
-500
Rs 700/day
2000
TRI 3025
TRI 2023
1500
1000
500
0
-500
35
40
45
50
55
60
65
70
75 35
40
45
50
55
60
65
70
75
Green Leaf Price (Rs/kg)
Figure 8.1 Variation of Net Present Value at different discount rates according to variation in green leaf price at
different wage rates
Though there are significantly higher yield increases, only cultivar TRI 2023 was economically
feasible for the irrigation investment, recording positive NPV value of Rs 391406. But for
cultivar TRI 3025, investment was not economically successful, as the NPV value was Rs 180
57898. Negative NPV means, it is not a financially acceptable project for installing a drip
irrigation system for cultivar TRI 3025.
The yield gap between TRI 2023 and TRI 3025 can be used as a guide line for the growers to
understand the selection for either suitable cost effective irrigation system.
Similarly high
yielding cultivar should be selected, if efficient irrigation system like drip is preferred. Still
growers has a chance to run financially viable drip irrigation project with TRI 3025 cultivar, if he
could install large extent of his field or if he could secure lower interest rate. (Even though
irrigation of large extent is not feasible with small growers, these findings would be important for
investment strategies of large plantation companies).
8.4
Summary of Results
x
Internal Rate of Return is 7% for TRI 3025 and 23% for TRI 2023. Only investing drip
irrigation for TRI 2023 is justifiable.
x
Net Present Value for TRI 3025 is Rs – 57898 and for TRI 2023 is Rs 391779
x
At present economic conditions, drip irrigation investment is not justifiable for even for
20 ha field of TRI 3025
x
If interest rate rose to 15%, drip investment becomes uneconomical for cultivar TRI 2023
as well.
x
If the wage rate increases to Rs 700/day, to make investment economical at present
interest rate (10%), for TRI 3025, green leaf price should be increased to Rs 70.00 and of
for TRI 2023, green leaf price should be increased to Rs 54.00
8.5
Conclusion
When considering the above economic analysis, it can be concluded that though drip irrigation is
associated with high capital cost with selection of high yielding cultivars like TRI 2023,
significantly higher returns can be achieved for the investment. The irrigation can be used to
increase the productivity of low-grown tea to a considerably higher level, if the capital
investment cost can be subsidised.
181
182
Chapter 9
Discussion
9.1
Introduction
The low elevation tea growing areas of Sri Lanka have, over the last two decades, changed from
being a minor to the major producer of the country’s tea. These areas are largely occupied by
smallholder growers. The growing environment in these areas are quite different from the mid
and high elevation growing areas in that crops are exposed to significant short-term water stress
even under relatively humid conditions, and this regularly impacts on productivity. This study
was conducted to assist the industry to mitigate the effects of this short term water stress through
irrigation. Even though there were earlier detailed studies to understand the agronomy of rain-fed
low elevation tea (Wadasinghe 1989; Wijeratne 1994), this is the first attempt to understand the
agronomy of low elevation irrigated tea. This chapter discuss the key findings of this research
program with interpretations and directions for future studies.
The aim of this thesis has been to evaluate the effect of short-term water stress on the agronomic
and physiological characteristics of low-grown tea, the responses to irrigation, and the financial
practicality of introducing irrigation in the low elevation tea growing areas of Sri Lanka. The
strategy to achieve this aim was to research the 5 objectives listed in Table 9.1. This table also
shows how the 7 experiments and financial analysis are linked to the objectives. The
experimental hypotheses are re-stated and the final column of the table indicates in which
sections of the chapter this work is discussed.
Following this Section 9.6 more generally discusses the seasonal variation in soil moisture in the
field trials. This section serves to explain an important finding; that soil moisture build up in
irrigated plots even in times of the year when irrigation was not being applied. As this is the first
detailed program of tea irrigation research in Sri Lanka, many questions emerged that call for
further study. Special attention is given to these new study areas in Section 9.7.
A lot of information has been generated in the course of this study and Table 9.2 gathers all the
summary points for Chapters 5 to 8.
183
E. To undertake a simple valuation of the
practical financial feasibility of irrigating tea
D. To quantify the effect of soil moisture
limitation on young tea plant growth
C. To evaluate plant performance in response
to different micro-irrigation methods
B. To evaluate the water use of tea and
environmental parameters that govern the
water use in low elevation tea growing
areas
A. To quantify the changes in physiology and
yield as affected by the water stress and
recovery by irrigation
Objectives
Table 9.1 Study objectives and relevant hypothesis
8
7
7.4
8.0
6
5
7.2
7.3
4
6.0
2
3
5.4
5.3
1
1
5.2
5.2
Ex
Chap
H8.Effect of irrigation on plant growth can be
enhanced by lowering the soil compaction in
growth bed.
Under what financial conditions is the irrigation of
two contrasting tea cultivars feasible in the low
elevation growing areas of Sri Lanka?
Only TRI 2023 profitable
under current conditions
accepted
Only up to 75% can be
allowed
Accepted
H6.For young tea, even short duration water stress
retard the plant growth
H7.Optimal growth of young tea can be maintained
under partial irrigation
Accepted
H5.Different micro-irrigation methods differ in their
effect on tea physiology and productivity
Accepted
Accepted
H4.Cultivar selection is, by itself, an inadequate
strategy to cope with water stress
H3.Transpiration is closely related to the plant
productivity and air temperature is the key
environmental factor controlling transpiration
Accepted,
Accepted,
Supported/Not Supported
H2.Air temperature is the main environmental factor
determining yield
H1.There is a cultivar difference in physiological and
yield response to irrigation
Hypothesis
184
Summary of Results
Sprinkler irrigation has a higher ability to maintain 2-4 C lower leaf temperature level than drip irrigation.
Pn was 15% higher in sprinkler than drip. gs remained same in both treatments, though El , was 20% higher in drip than sprinkler.
Sprinkler yielded 6% higher than drip and 11% higher than rain-fed.
During wet season, irrigation increased yield by 15-27% in some months. However it was consisted only with sprinkler.
Yield drop in 2009 dry months were 53% for rain-fed and 43 and 44% for drip and sprinkler, compared to wet months.
Shoot extension rate was 11 and 45% higher in drip and sprinkler, than rain-fed plants.
57% increase in harvestable shoot count only observed in sprinkler.
0
TRI 4049 showed 17% higher Pn and 4% higher El than TRI 3025.
TRI 3014 and TRI 4049 showed significantly higher W i than TRI 3025.
TRI 4049 showed a higher W i and Pn was resilient to increasing temperature.
Maintenance of favourable leaf temperature during hot dry days is not significantly different among cultivars.
E followed ET0 during wet season. Daily changes in evaporative demand were not reflected immediately.
Dry matter production in wet season closely followed the transpiration pattern.
During wet season of the yearly (nearly 9 months), increase in ambient temperature drives the transpiration.
During dry period, water use of irrigated plants was double than that of rain-fed plants. But it was lower in dry periods, than wet periods.
185
Difference in Ψdawn was observed among two cultivars, irrespective of irrigation.
TRI 2023, showed difference in Ψnoon according to irrigation treatment.
th
Rain-fed TRI 2023, showed the lowest Ψnoon by 10 week, closer to permanent wilting point.
Decline in Pn rate with drought, did not recover even after rain occurrence for rain-fed plants for both cultivars.
For TRI 3025, gs, increase did not increase El
TRI 2023 responded favourable to changes in gs. This factor further indicates the need for more water usage for highly productive cultivar and the stomatal activity of
TRI 3025 was lower than TRI 2023, even under irrigated conditions.
0
Tl increased above 35 C for couple of hours during the day even within short dry periods for rain-fed plants.
Irrigated plants too showed drop in monthly yield, in dry months, as compared to wet months.
The cultivar difference in the production gap is visible and there is no interaction between irrigation and cultivar selection
Advantage of irrigation in yield is reflected prominently even during wet period for this leafy crop.
Stem canker infection was higher in TRI 2023 even under irrigation. However, irrigation was effective in controlling the disease.
Table 9.2 Experimental summary of results
Chapter 5.2
Experiment 1
Chapter 5.3
Experiment 2
Chapter 5.4
Experiment 3
Chapter 6.0
Experiment 4
Chapter 7.2
Experiment 5
Chapter 7.3
Experiment 6
Chapter 7.4
Experiment 7
Chapter 8.0
Experiment8
Internal Rate of Return is 7% for TRI 3025 and 23% for TRI 2023. Only investing drip irrigation for TRI 2023 is justifiable.
Net Present Value for TRI 3025 is Rs – 57898 and for TRI 2023 is Rs 391779
At present economic conditions, drip irrigation investment is not justifiable for even for 20 ha field of TRI 3025
If interest rate rose to 15%, drip investment becomes uneconomical for cultivar TRI 2023 as well.
If the wage rate increases to Rs 700/day, to make investment economical at present interest rate (10%), for TRI 3025, green leaf price should be increased to Rs
70.00 and of for TRI 2023, green leaf price should be increased to Rs 54.00
This experiment showed that Irrigation supports better growth of young tea regardless of ground preparation. Raised Beds may further improve some measures of
performance of young irrigated tea, but it is worse than normal ground preparation under rain-fed conditions. Specifically it showed that
There was no significant treatment difference in height growth in the very early growth when plants were well watered by rainfall.
But during a short dry period irrigated plants grew on average 10% more than rain-fed. There was no interaction between irrigation and land preparation method.
During this dry period there was statistical difference between treatments in the growth of branch shoots.
There was a strong treatment effect on leaf area index. The Raised Bed with Irrigation had highest LAI and retained most leaves, whereas the Raised Bed with No
Irrigation had the lowest LAI. It lost even more leaves than the Normal Ground No Irrigation.
While there was no significant treatment effect on main stem weight and live twig weight, the Raised Bed with Irrigation had significantly less dead twig material. The
Raised Bed No Irrigation had the most dead twig material.
Irrigated treatments both produced significantly more finer roots (<2mm) but there was no interaction with raised beds
Even though experiment was conducted for short time, there was a significant difference in leaf growth in plants.
Young plants were sensitive for even 25% reduction in crop water requirement in canopy growth. Root growth was the most sensitive for the full requirement of crop
water.
Even with 75% partial irrigation, root growth was affected by 32%, though not significantly different with full irrigation.
Branching, or height did not alter with water application rate, perhaps due to shorter period of experiment
Daily watering produced 25% higher stem growth and 87% higher root growth than the plants with 4 day irrigation interval.
However, stem growth, root growth, and branch formation did not have significance difference among 7 and 14 day irrigation treatments.
Most sensitive for the irrigation interval is the leaf number. Even for the short irrigation interval like 4 day, plant lost 27% of its foliage
186
9.2
Tea Plant Response to Water Stress and Irrigation
Objective A of this study is concerned with the quantification the changes in physiology and
yield as affected by water stress and recovery by irrigation. The hypotheses raised in experiment
1 were supported by the evidence and it is now time to discuss this evidence and explain the
processes involved. This discussion covers physiological responses, yield responses and cultivar
differences.
Experimental evidences to support or reject the hypothesis H1- H3 (Table 9.1) are discussed in
this section. In summary it can be stated that above three hypotheses can be supported according
to the outcomes. Superior physiological and yield response to irrigation by TRI 2023 over TRI
3025 proved the fact that there is a cultivar difference in response to irrigation. In the yield
terms, maximum temperature was found to be the most influential environmental factor for
irrigated and rain-fed plants. The third hypothesis, about the evaluating the effectiveness of
cultivar selection as a drought mitigation strategy can be accepted based on the fact that there is
no significant difference in leaf temperature among tested cultivars in dry days.
9.2.1
Physiological response
As plants enter periods of water stress, diminished activity is observed in the leaves as a
defensive mechanism before large water deficits occur in the root zone (Chaves, Pereira et al.
2002). It is therefore important to examine the physiological response to the rapidly imposed
water stress at leaf level when needing to understand the causal relationships between water
stress and its impact on yield.
The water potential of the two cultivars tested in Experiment 1 showed clear differences at dawn
as well mid-day (Figure 5.3), with the greatest difference was observed in Ψdawn. The effective
water status of a plant is mostly reflected in its Ψdawn (Barros, da Se Mota et al. 1997). One of the
most notable observations was that there was no difference among Ψmidday in irrigated and rainfed plants of the TRI 3025 whereas there was a difference in TRI 2023. As the water stress
period progressed Ψdawn continued to decline so-called drought tolerant cultivar TRI 3025 even
when it was under irrigation. In contrast the Ψdawn or irrigated TRI 2023, a drought susceptible
cultivar, remained steady.
TRI 3025, which had been bred for constrained rain-fed conditions, could not take advantage of
the applied water; whereas TRI 2023, which had been developed for unconstrained rain-fed
conditions, appeared to take advantage of applied water well. In response leaf shedding was
187
observed in TRI 3025 in the latter part of the water stress period due to its very low water
potential. In contrast, TRI 2023 maintained a higher transpiration rate (Figure 5.6) under both
rain-fed and irrigated regimes and the leaves showed relatively higher leaf water potential.
Photosynthesis is the most sensitive physiological activity to water stress in tea (Jeyaramraja, Raj
kumar et al. 2003). Water stress caused decrease in the Pn of both cultivars in Experiment 1. At
the end of the 10 week long study period, Pn dropped by 45% and 75% respectively for rain-fed
TRI 2023 and TRI 3025, despite some scattered rain during the period. Reduction of Pn with
increasing ambient temperature (Figure 5.10) was more significantly related in rain-fed plants
(P=0.005) than irrigated plants (P=0.03).
Favourable, relatively low, leaf temperatures for
photosynthesis were found to be maintained by drip and sprinkler irrigation in Experiments 1 and
4 (Figure 5.8 and 6.9). Among the two micro-irrigation methods (Experiment 4), sprinkler
irrigation was more efficient in lowering leaf temperature (Figure 6.8). Irrigation was helpful
preventing plant reaching dangerously high temperature levels in the short hot humid period
without rain. This is very interesting when compared with tea production in the cool dry
Tanzanian Southern Highlands where irrigation is used to raise the leaf temperature (Smith,
Burgess et al. 1994).
During periods of water stress photosynthesis is reduced either through direct influence on the
metabolic and photochemical processes in the leaf, or indirectly via stomatal closure and
cessation of leaf growth which results in decreased leaf surface area (Chen, Zhuang et al. 2010).
Among the above two mechanisms, reduction in the photosynthesis in low elevation tea seems to
be mainly caused by the influence of metabolic and photochemical process in the leaf rather than
stomata closure. This was evident in Experiment 1 results. There it was found that P n rate of
even drip-irrigated plants dropped during water-stress period. The decline of the Pn continued
during the drought period. At week 5, though there was an increase in gs, photosynthesis did not
increase with a proportionate level in both cultivars. If stomatal closure had had a significant role
in reducing Pn during the dry period, Pn should have increased with the increase of gs in week 5.
Between the two cultivars, TRI 2023 responded more to the increase in gs.
Further evidence is in the photosynthetic light response of tea cultivars after experiencing water
stress for several weeks (Figure 5.9). Among the irrigated plants the light saturated maximum
photosynthesis rate (Pmax) of TRI 2023 cultivar was 62% higher than TRI 3025. For both
cultivars, Pmax was more than 100% higher in irrigated plants than the rain-fed plants. Damage to
photosystem II is the probable cause for the less efficiency in the utilising the captured energy
(Melis 1999). As water stress reduces the capacity of leaves due to water stress, reactive oxygen
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species are produced at the cellular level (Smirnoff 1998). These reactive oxygen species cause
oxidative stress as well as intra-cellular signalling (Finkel 1998) further lowering photosynthesis.
Other evidence of lowering Pn, by processes other than stomatal control was reported in
grapevine. Maroco et al (2002) found a reduction in the activity of various enzymes of the
Calvin cycle proportional with the intensity of water stress. The above cited evidence and
relatively lower changes in gs in tea leaves (Figure 5.5) suggest that the water-stress induced
reduction of Pn in Experiment 1 was more due to direct influence of metabolic and photochemical
process, than stomatal control.
Instantaneous leaf transpiration (El) changes over the water stress period in Experiment 1 showed
significant cultivar difference. Drought resistant TRI 3025 showed 40% reduction in the El under
irrigation and a 70% reduction when rain-fed during the 10 week period.
In contrast the
reduction in the El in TRI 2023 was stronger in rain-fed plants (20% reduction under irrigated cf
48% reduction under rain-fed).
However, such variation was not observed in stomatal
conductance (gs) in either cultivar. Strong stomatal control of El was not evident during the water
stress period. Tea is a high density plant and the low elevation growing areas generally have low
wind speeds. As a result, the plant canopy is poorly coupled to the atmosphere (DaMatta and
Ramalho 2006). El hence becomes more dependent on solar radiation than the vapour pressure
deficit between stomata and ambient air (DaMatta and Ramalho 2006). This predominance of
solar radiation was also illustrated in Experiment 4 where the gs steadily declined over the diurnal
period while El showed an increase up to 1200 hours in irrigated and rain-fed plants (Figure 6.7).
9.2.2
Yield response
Yield depression is the most visible, and of course commercially significant, effect on
productivity caused by dry spells. In Experiment 1 the average weekly yield fell below more
than 50% during the January – April period compared with the rest of the year. Drip irrigation
was able to increase the yield by 92 to 118% respectively for TRI 2023 and TRI 3025 during the
driest months of February and March 2007. However these yields were still 62 to 70% lower
than the monthly average yield over the rest of the year.
A similar pattern emerged in
Experiment 4 in the comparison of drip vs sprinkler technology (Figure 6.11). The reasons for the
yield improvements during dry months in irrigated plants can be attributed to:
1.
relatively increased Pn (Figure 5.4)
2.
higher shoot weight (Figure 6.9)
3.
increased shoot extension rate (Figure 6.10).
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However, irrigation was not effective in increasing the harvestable shoot count in the plant
(Figure 6.10). As mentioned earlier, high ambient temperature, which negatively affects Pn of
even the irrigated plants (Figure 5.10) explains yield decline of irrigated plants in dry months.
One of the most notable features in these irrigation experiments was the relatively higher yield of
irrigated plants even during the wet season when the irrigation was not being applied. This was
evident in both Experiments 1 and 4. This wet season yield increase of the irrigated plants is an
additional advantage of irrigation during dry months. The carry-over effect of irrigation can be
attributed to the following reasons:
1.
Plants grown under irrigation from the field planting grow larger in terms of number of
shoot-bearing branches than rain-fed plants resulting in higher yields at maturity. Similar
results were observed from other tree crops like Japanese Plum, grown under irrigation for
4 years (Intrigliolo and Castel 2005).
2.
Irrigated plants had a higher leaf area index and non-irrigated plants higher leaf senescence
during the dry season in mature plants (Figure 4.4) as well as with young plants (Figure
7.5). So the net effect of more photosynthetic area in irrigated plants translated to higher
yields even during wet months. High LAI translates as a more favourable source to sink
ration for dry matter partitioning (Li, Yang et al.)
3.
Rain-fed plants were infested with a higher percentage of stem canker disease as observed
in Figure 5.12. This disease, which is pre-disposed by water stress, is a common cause of
die-back and death of tea (Carr 1974; Bannerjee 1993).
Further to this list is a more developed explanation in Section 9.7 which discusses the higher soil
moisture content in the irrigated plots throughout the year.
When analysing the effect of environmental factors that influenced the weekly yields during 2007
(Table 5.5), it was found that maximum temperature showed the most significant negative
relationship with weekly yield. In particular it was the maximum weekly temperatures (>35°C),
the first three months of the year that have the greatest impact, even surpassing that of rain (Table
5.3). This maximum temperature effect was visible in both cultivars in rain-fed as well as
irrigated conditions; although drip irrigation did soften the effect to some extent.
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In the dry season the differences between maximum and minimum weekly temperatures are
greater than in the wet season. Lower minimum temperatures at night (~21°C) are usually
observed in the months of December to February, than in other times in the year. These lower
temperatures will affect shoot initiation and extension. In Experiment 1 increases in minimum
weekly temperature showed a positive relationship only with irrigated plants of both TRI 2023
and TRI 3025 (P=0.02 and 0.06 respectively). Rain-fed plants did not respond to changes in
minimum temperature over the whole year. So in summary, irrigated plants were protected from
extreme maximum weekly temperatures and could take advantage of increases in minimum
weekly temperatures.
9.2.3
Tea Cultivars for irrigation and drought mitigation
Large differences in physiology, growth and yield among tea cultivars (Othieno 1978) make it
difficult to recommend tea irrigation as a blanket application. It was found in the Experiment 1
that benchmark cultivars with contrasting drought characteristics, responded differently to water
stress and irrigation. These differences between the benchmark cultivars pave the way for
selecting cultivars to tolerate water stress under rain-fed conditions or cultivars better suited for
irrigation. Such cultivar selection can be based on the similarity of physiological response to
environmental parameters.
Transpiration control is one mechanism of tree crops to mitigate water stress periods (NguyenQueyrens and Bouchet-Lannat 2003; DaMatta and Ramalho 2006). Drought tolerant cultivars
show low transpiration during water stress periods.
As a result, most genotypes, showing
drought tolerant capabilities tend to be less productive (Monclus, Dreyer et al. 2006). These
physiological traits were visible in the benchmark drought tolerant cultivar, TRI 3025 in
Experiment 1. The effort in Experiment 3 was to evaluate the drought resistant and productive
traits of some selected new tea cultivars.
Among the tested cultivars, TRI 4049 showed higher productive characters (17% higher Pn than
TRI 3025) with marginal increase of transpiration (4% compared to TRI 3025). Increasing Pn
under water deficit conditions is one of the most sought after physiological character for
improving plant productivity under drought conditions (Blum 2009).
Increasing the
instantaneous water use efficiency (Wi) is one way of achieving higher productivity in dry land
agriculture (Condon, Rebetzke et al. 2002). TRI 4049 showed 27% increase in Wi (Figure 5.22).
TRI 3014 also showed similar increase in Wi as compared to TRI 3025. However, Pn of TRI
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3014 was significantly affected by increasing ambient temperature (Table 5.10) during short
water stress period. This factor makes TRI 3014 more vulnerable to high temperature, water
stress periods in low elevation areas than TRI 4049.
High leaf temperatures (43-460C) were observed in all tested cultivars including TRI 4049 (13
February in Table 5.9). There was no significant difference among the cultivars with respect to
high leaf temperature.
Leaf temperature build up was nearly 60C more than maximum
temperature of that particular day (~360C). In two other observation days, leaf temperature of the
cultivars ranged from 34-390C, with significant differences among cultivars. The reason for low
leaf temperature level could be low air temperature (20 February) and soil moisture
improvements after rain (06 March). Usually for the C-3 plants, including tea, increasing leaf
temperature above 300C, inhibits photosynthesis (Schrader, Wise et al. 2004). In addition to
photosynthetic inhibition, increase in leaf temperature causes large water vapour pressure deficits
on the leaf surface (Maherali, DeLucia et al. 1997) and leaves have to transpire more and may
consequentially lose water (Mitamura, Yamamura et al. 2009). As the ambient temperature
increase is an associated factor with low elevation water stress period in January – March, high
leaf temperature increases during hot dry days make all tested cultivar vulnerable for photo
inhibition and increased water stress.
Accordingly cultivar selection is an inadequate measure for mitigation of hot humid water stress
periods in low elevation areas. But an additional measure of establishment of high shade trees
(Falcataria mollucana) would control high leaf temperature build up. So, high shade trees and
drought susceptible cultivar combination would produce a formidable drought mitigation strategy
in low elevation areas, in addition to irrigation.
9.3
Water use of tea in low elevation area
Whole plant water transpiration (E) of the benchmark highly productive TRI 2023 was measured
in a wet season and dry season in Experiment 1. Transpiration is very important as it plays a key
role in the hydraulic cycle of the plant and only 1% of the water taken by plants is involved in
metabolic processes (Rosenberg, Blad et al. 1983).
Some very significant seasonal differences in plant water use were observed. Plants showed
higher E during wet season than dry season (Figure 5.14 & 5.17). Transpiration reduction in
rain-fed plants was 45% in dry season as compared to wet season. (As a parallel observation
yield reduction in dry season with compared to wet season is closer to 50%). There was an
increase in potential evapotranspiration (ET0) during the dry season. ET0 was 24% higher in dry
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season than wet season as calculated by daily values. This is similar for long term climate
average calculations as well. ET0 in months of February and March is considerably higher than
rest of the year (Figure 4.5). Increase in ET0 was mainly driven by increased solar radiation,
ambient temperature and vapor pressure deficit. The average E of irrigated plants in the dry
period was 2.7mm/day, which is 0.75 of ET0 and it was 109% higher than rain-fed plants. The
increase in drip irrigated plant was largely due to night time transpiration of the plants as shown
in Figure 5.18.
During the wet season of the year, the dominant driving factor for E is maximum temperature
(Figure 5.16). So the favorable temperature during this season can be considered as the main
reason for the increased productivity of tea in this region. Transpiration processes are closely
related to productivity as shown in our study (Figure 5.15) and in many previous studies (Yang,
Short et al. 1990; Ananthacumaraswamy, De Costa et al. 2000). The wet season of the year lasts
for nearly 9 months of the year (Figure 4.8). Maximum temperature also does not reach more
than 340C during this period. As a result, there is no threat for the photosynthetic mechanism of
the leaves or growth of the plants including shoot extension with high temperature (>35 0C)
(DaMatta and Ramalho 2006) as experienced in dry months of January to March. During the wet
months the maximum temperature is a stimulant for higher productivity without restriction of soil
moisture due to adequate rain. On the other hand, for the rain-fed plants, there is a restriction of
soil moisture to drive transpiration and to moderate the leaf to air temperature (Yang, Short et al.
1990).
It is clear, based on the results of Experiment 2 that transpiration and plant productivity are
closely correlated during wet season of the year. Wet season lasts for nine months of the year
from April to December.
The main environmental factor that determines transpiration is
maximum temperature, which does not exceed 340C on average.
9.4
Irrigation System Selection
Irrigation can be applied as either drip or sprinkler. Drip is the preferred method for hilly and
undulated tracts for a crop like tea (Sivanappan 1994). However, as the sprinkler is also popular
among farmers, due to ease of operation and low cost, performance of tea physiology and
productivity was evaluated under drip and sprinkler irrigation in Experiment 4. It was found that
increase of leaf temperature in dry days was reduced by 2-40C under sprinkler irrigation less than
drip irrigation. Sprinkler irrigation increased tea photosynthesis by 15% during the dry spell and
the total annual yield by 6% compared with drip irrigation.
These results confirmed the
193
hypothesis that different irrigation techniques have different effect on tea physiology and
productivity.
Sprinkler irrigation resulted in lowering the leaf temperature (Figure 6.8) with increased
photosynthetic rate (Figure 6.4). The mean for lowering leaf temperature in sprinkler irrigated
plants is the artificial cooling to tea leaves. Lowest transpiration was reported in sprinkler
irrigated plants among three treatments (Figure 6.6 and 6.8). In contrast with Experiment 1, there
was little difference between leaf temperatures in drip and rain-fed plants from 0600hr to 1800hr
(Figure 6.8). However leaf temperatures of both drip and sprinkler irrigated plants exceeded the
air temperature (which did not increase exceed 330C) during that period. Another reason could
be the geographical location of the Experiment 4, as compared to Experiment 1. The site for
Experiment 4 had more open space, being exposed to an adjacent public road (Figure 4.2). As
such the site probably received more scattered solar radiation which increased ambient
temperature.
Among irrigation treatments, sprinkler gave a 6% higher yield increase than drip. Reasons for
this could be: higher photosynthetic rate (Figure 6.4), higher shoot extension rate and increased
harvestable shoot number (Figure 6.10). These increased yields in the sprinkler irrigated plants
remained throughout the experiment period (Figure 6.11), unlike drip. This better growth under
sprinkler irrigation (Figure 6.12) translated into a sustained yield advantage across the dry and
wet seasons.
Experiment 4 showed that while sprinkler irrigation has the ability to maintain better leaf
temperature during dry days, high shoot density and high shoot extension rate, it could not
establish a yield advantage in a very wet year. However, it is possible that significant differences
between the treatments may occur in a dry year. Experiment 4 comparing different microirrigation techniques, was a parallel small experiment, established along main drip irrigation trial
(Experiment 1). Clearly, this is an experiment that needs data from a few more years. Sprinkler
irrigation is also a cheaper option to install than drip irrigation, so growers will naturally prefer
the most cost-effective option.
The selection of the proper irrigation systems is a focal point for the direction of the Sri Lankan
tea industry. Application efficiency of irrigation water is a critical factor with diminishing water
resources in the global irrigation industry. Based on long term research in India, it was found
that on-farm irrigation efficiency of properly designed and managed drip irrigation system was
>90% as compared to 65-70% of sprinkler irrigation (Sivanappan 1994). This factor had a huge
194
impact on preserving depletion of ground water resources and for optimal utilization of surface
water sources in India. This factor is also very important for the low elevation tea growing areas
of Sri Lanka (Eriyagama, Smakhtin et al. 2010). While the dry zone in the north of Sri Lanka is
developed for water storage in ancient and modern tanks (large earthen dams), and also supplied
from channels from the Mahaweli River, the wet zone area of Sri Lanka has no infrastructure to
harvest rain water (Punyawardena 2004).
Given the extent of tea cultivation in the low elevation area, the long term viability of sprinkler
technology is questionable. Already some sprinkler-irrigated tea cultivations in East and Central
Africa are facing increased pressure on water resources and looking for drip irrigation as the
alternative method (Möller and Weatherhead 2006).
Unlike in Experiment 4, Möller and
Weatherhead (2006) cited significant yield increase in drip irrigation as compared to over head
sprinkler
irrigation,
accounting
from
number
of
unpublished
papers
and
industry
communications.
Drip irrigation was introduced in late 1990’s as the most suitable micro-irrigation system for tea.
In addition to the advantage of water saving, there are some other advantages as well. Ability to
apply fertilizer to root zone is a much awaited practice specially for some larger tea fields. I
addition to more beneficial applications of daily or more split applications, it would save huge
cost on manual labour for fertilizer application. Fertigation is a something that cannot be
practised with sprinkler irrigation for tea. Drip irrigation allows, undertaking field operations,
like mechanical or manual harvesting, tipping infilling and weeding (Möller and Weatherhead
2006).
Hence in the longer run it is important to select drip irrigation as the suitable micro-irrigation
system for tea in Sri Lanka.
9.5
Effect on Young Tea Growth
Development during the post-nursery stage is very important for the long term productivity of tea
plants. Chapter 4 showed how short, acute water stress periods impact on the young plant
establishment in the field. There are several water stress periods within a year. For example, the
average number of rainless periods of > 5 days per year, was more than 10 during 1986-2010. In
Chapter 7 the adverse effect of exposing post-nursery plants for both short term drought and
partial supplement of crop water requirement were studied in glass house and in field
(Experiments 5-7).
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9.5.1
Effect of water stress interval on young tea
The effect of different water application intervals (daily to 14 day interval) was tested in
Experiment 5. The effect of daily irrigation on stem and root growth was significantly different
from other treatments (4, 7 and 14 day intervals). Water application at 4 day intervals reduced
stem growth by 25% and root growth by 87% as compared to daily irrigation (Table 7.5) 2. This
differential in the suppression of shoot and root growth can be expressed as the root:shoot ratio
which increased with increasing length of water stress periods.
All trees have an optimal
root:shoot ratio to match their physiological ability to access water with the availability in the
environment (Steinberg, Miller(Jr) et al. 1990). For tea, the optimal root:shoot ratio is identified
as 1.5 (Bannerjee 1993). Under daily irrigation this ratio was maintained at 1.5 but with water
stress it increased to about 2.
This glasshouse experiment used the same soil as in the field experiments and this soil is
particularly low in organic carbon (Table 5.1). Soils with low carbon content are prone to high
soil compaction (Gomez, Singer et al. 2002). The presence of gravel content can also be seen in
this soil. These two factors can cause the soil temperature to increase in this hot humid climate,
which slows the root growth (Lopushinsky and Max 1990).
After a 14 day watering interval only 55% of leaves remained on the plants (Table 7.4). Even
under a 4 day watering regime plants only 73% of leaves remained on the plants. Similarly leaf
loss after short watering periods has been observed in mature commercial fields under sprinkler
irrigation near Ratnapura. The physiological reason for this response could be as follows.
Ethylene is the main hormonal factor controlling the physiological process promoting leaf
abscission after a period of water stress (Tudela and Primo-Millo 1992).
The ethylene is
generated by the oxidation of 1-aminocyclopropane-1-carboxylic acid (ACC) which originates in
the roots. However, in addition to ethylene, abscisic acid (ABA) is also implicated in the process
of leaf abscission (Goren 1993). Water stress induces both ACC and ABA accumulation in roots
and arrested xylem flow (Gómez-Cadenas, Tadeo et al. 1996). Shortly after rehydration root
ABA and ACC returns to pre-stress levels and restores normal xylem flow promoted ABA and
ACC transport to leaves, triggering leaf abscission (Tudela and Primo-Millo 1992; GómezCadenas, Tadeo et al. 1996).
2
This sensitivity of young tea to short periods of water stress was evident in another experiment, not reported within the body of this
thesis (Appendix 2), where water application rates were studied in potted tea plants (C.sinensis var sinensis) in a growth room.
Transpiration was reduced with increasing water stress particularly under higher ambient temperature (35°C).
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9.5.2
Effect of partial irrigation on young tea
As described earlier, the wet zone of Sri Lanka does not have rain water harvesting structures like
irrigation tanks. As a result, severe water shortages even for domestic purposes are common
during prolonged rainless periods. One option for larger estates is to irrigate the young tea fields
by transporting water by tankers attached to field tractors (Figure 9.1).
In such scenario,
controlled application of irrigation water is essential to reduce irrigation cost.
Figure 9.1 Water tanker used for sprinkler irrigation commercial tea field in Ratnapura, Sri Lanka January, 2010
(Note: Shade plants were not grown up to provide satisfactory shade in the drought)
Partial or regulated deficit drip irrigation is an irrigation strategy based on limiting wastage from
soil evaporation and drainage and applying water so that plant water deficits occur when adverse
effects on productivity are minimized (Goldhamer and Viveros 2000). It was widely used in fruit
crops like peach and pear (Domingo, Ruiz-Sánchez et al. 1996). Partial irrigation has also been
used for cost-effective tree establishment on degraded lands (Khamzina, Lamers et al. 2008).
Experiment 6 simulated partial irrigation regimes on young tea in a glasshouse. In growth terms,
both plant stem girth and leaf number were significantly affected when the daily irrigation
application was reduced from 100% to 25% of plant water requirement (Table 7.6). Stem girth
was reduced by 33% and leaf number reduced by 45%. However, there was no significant
difference in branch development, perhaps due to short period of study.
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In the total plant development also, both stem growth and root growth were significantly affected
under partial irrigation. However, significant reduction occurred only when the irrigation amount
was reduced to 50% from 100% (Table 7.7). Stem growth reduction was 36% and root growth
reduction was 46% for the reduction of irrigation level from 100 to 50%.
This glass house experiment suggests that 75% partial irrigation can be applied without
significant negative effects on the growth of young tea. Translating this finding into field
plantings requires some caution especially in the sensitive first year. The glasshouse is a much
gentler environment with partial shade, protection from wind and relatively stable humidity. The
practice of partial irrigation when establishing tea needs to be further developed in the field.
9.5.3
Raised beds to enhance irrigation in young tea
Conventional practice is to establish tea on flat ground. Soil compaction is observed in tea fields
due to frequent human traffic to the field, especially for harvesting. Soil compaction decreases
total porosity and increases volumetric water content and soil strength (Greacen and Sands 1980).
Experiment 7 was conducted to evaluate the performance of post-nursery plants in the first year
of field planting under raised beds and irrigation. The raised beds constructed in the experiment
reduced soil compaction (as measured by dry bulk density) by 12% under rain-fed treatment
compared with normal flat ground. After only one month of irrigation the soil settled so that the
bulk density was only 6% lower than flat ground.
Raised-bed planting even improved the root growth significantly under rain-fed conditions. Fine
root content was increased by 120%, while coarse leaf content increased by 96% over rain-fed
flat ground. Increased porosity may be the reason for better root growth under raised beds. On
the rain-fed raised-beds a high leaf loss and dead twigs was observed in many plants. As a result
net leaf area gain is low (Table 7.9). This is likely due to the enhanced exposure to radiation and
advection coupled with low soil moisture content.
In this simple experiment, it showed that there are some advantages in planting young plants in
raised-beds, to enhance the irrigation effect. Raised beds were able to lower the soil compaction,
and it was reflected in improved root growth in raised beds. So lowering of soil compaction is a
matter for concern to achieve optimum soil bed irrigation. However, this is a practice that should
be adapted with cautious. Essentially, this type of practice is more suitable for flat lands, as with
possible soil erosion in sloppy lands.
There are some issues associated with system, like
additional cost for establishment, potential more weed emergence, interaction with tea pluckers
and potential adverse effect in case of if irrigation could not be applied in dry season.
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9.5.4
Final comment on irrigation in young tea plants
The importance of mitigating water stress during the post nursery period was shown through
Experiments 5-7. Better root development and prevention of water stress induced leaf abscission
are the two major gains of irrigation. Growers in Sri Lanka are concerned that irrigating tea from
field establishment will encourage surface rooting, making them ‘softer’ and more vulnerable.
However, we now know this concern is unfounded. The root growth of 9-year old tea in
Experiment 4 which had been irrigated since establishment was 18-34% better than the rain-fed
plants. This makes sense as root growth is favoured by high nutrient and moisture availability
(Persson 1978). It is possible that plants irrigated from establishment may become less tolerant
to water stress if irrigation is discontinued when the plants mature. This has also been shown to
be false, at least for oak afforestation programs in the Mediterranean (Siles, Rey et al. 2010). In
Sri Lanka, which struggles with maintaining an optimal replanting rate (Anon. 2011), irrigation
in the first one or two years of establishment should become a standard recommendation. This
may be done with portable sprinkler systems that can be moved onto another re-planting field.
However, this research program has shown that there is great merit in establishing permanent drip
irrigation systems.
9.6
Further agronomic considerations
The sections in this Chapter hitherto discussed the results of field and glass house experiments
conducted at Ratnapura, Sri Lanka. The Section 9.6 is dedicated for discussing some of the
significant observations, apart from formal results and irrigation planning.
9.6.1
Carry-over effects into the wet season
Maintenance of optimum soil moisture in the plant root zone is very important for the tea crop, as
it produces new shoots for harvesting throughout the year. Also exposure of plants to dry spells
can induce a reproductive phase (Mueller, Scudder et al. 2005). The irrigation systems proposed
for the low elevation growing areas are to only apply water over the January to March period
when these intense dry spells occur. An important observation in this research was that yields of
irrigated plots were elevated even in the wet season when the pump was turned off (Experiment
1, Figure 5.11). Similarly both yield and elevated soil moisture was observed in irrigated plots
over the wet season in Experiment 4 when the pump was off (Figure 6.3). How can this be
explained?
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In Experiment 4 monthly soil moisture of rain-fed and irrigated plots was measured up to 60cm
depth in 2008 January - 2009 March period (Figure 6.3). In this soil type, 50% moisture
depletion of available water content(AWC) limits the root water intake (Anandacumaraswamy
2008). Some however suggest that for tea crop, this limit is 40% (Allen, Pereira et al. 1998).
Reduction in the AWC to 50% was observed in some rainy months like April, July and
December (Figure 6.3). Even without irrigation in such months, it is important to cite possible
reasons for the moisture build up in irrigated plots;
1.
Irrigated plants showed a higher canopy growth (Figure 6.13) than rain-fed plants. Since
tea is cultivated as a bush, high canopy means, more stem water harvesting in rain events
(Llorens and Domingo 2007).
2.
Irrigated plants showed a higher structural root density, particularly in lower soil depths of
40-60 cm (Figure 6.14). Higher root density ensure more rain water infiltration to the root
zone (Martinez-Meza and Whitford 1996).
3.
Irrigated plants have a higher leaf area growth, which prevents soil evaporation, preserving
soil moisture during non rainy days.
9.6.2
Irrigation scheduling
Irrigation scheduling in this trial was based on the Class A Pan evaporation data and 5 rainless
was the trigger to start the irrigation. This method was followed in the absence of any previous
research on irrigation scheduling in tea in this hot humid environment. The gap of 5 consecutive
non-rainy days is just a locally assumed indication of ‘a short dry spell’ for other perennial crops,
such as cocoa, coffee, pepper and nutmeg in the wet zone of Sri Lanka (Sumanasena 2008). It
has been selected on the basis of letting the soil dry for around 25-30% of reduction in the
available soil moisture in the root zone. However, the current research program found that, in
addition to the soil moisture replenishment, irrigation plays an important role in maintaining
favourable leaf temperatures. The effect of even 4 day water stress period cause a significant leaf
abscission in Experiment 5. For this reason it may be advantageous to commence the irrigation
within two or three days of rainfall ceasing to get the best advantage of irrigation in tea.
Instead of scheduling irrigation based on plant water use and rainfall, measurement of leaf
temperature can also be used successfully to identify crop water stress (Howell, Hatfield et al.
1984). When considering the difficulties of operating equipment (e.g. soil moisture meters ) in
this compacted soil, leaf temperature measurements can be developed as a quick method to
identify the plant water stress at a rapid time.
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The nominal water application rate of Netafim RAM 17D drip dripper is 2.2 mm/hr. The wetting
pattern of the root zone was not tested under this water application rate in the study. As there is a
chance under drip irrigation for wet and dry regions to develop in the root zone (Bielorai 1982),
plants may still have a chance to experience water stress (Romero, Pablo et al. 2004). So it is
necessary to measure several water application rates to identify the correct water application rate
according to the soil type.
9.6.3
Shade trees
The sites for both Experiment 1 and 4 were without shade trees. Planting of shade trees is
recommended practice under rain-fed tea cultivation in Sri Lanka. In Latin America, similar
plantation crops such as coffee and cocoa used to all have shade trees in the early 20th century,
but in some sites shade-free intensively managed monocultures have been found to work (Alvim
1977). In 1969 shade trees were removed from a significant area of highland tea estates in Sri
Lanka. This led to an immediate yield increase, but then a gradual decline resulting from
sunburn and dieback (Fuchs 1989). In low elevation growing areas, the relative merit of shade
trees has not been tested. This region has a higher cloud cover than the highlands and it is
possible that tea could still be grown without shade trees. Some small holders would prefer not to
have shade trees because of problems such as wasp attack and falling limbs. Nevertheless, shade
trees are still the standard practice. This practice is typically using medium sized trees of
Gliricidia spp which are pruned to height of 3-4m during rainy months.
The experiments in this research program were designed intentionally without shade to exclude
the competition to light, water and nutrients (Beer and Catie 1987), and also to minimize the
disturbance to water application in the sprinkler irrigation trial. Shade trees would particularly
confound results in the wet season of the year, when it is already quite cloudy. However, as this
research program has revealed the significance of high temperatures on low elevation tea, it
would be appropriate follow-on research to study the interaction of shade trees, which will
moderate air temperature, with drip irrigation.
Importance of shade tea presence to harvest rainwater as stemflow and throughfall in tea field is
discussed in Appendix 1. Breaking precipitation as stemflow is important for minimizing the soil
erosion (Morgan, Quinton et al. 1998). But the larger water drips, dropping as throughfall disturb
soil particles (Hidalgo, Raventos et al. 1997) and cause erosion (Figure 9.2).
Accordingly,
albizia plant plays a major role in harvesting more rainwater as stem water, due to its higher
relative dominance (with high basal area). This is particularly important in collecting rain water
201
to soil moisture storage during isolated rains in dry periods. However, there are no reports to
analyze the competition of albizia roots on tea plant for soil moisture during water limited dry
seasons.
Figure 9.2 Presence of soil particles can be seen at tea stems after heavy rain events
9.7
Financial Evaluation
Financial evaluation was conducted to answer the question under what financial conditions is tea
irrigation feasible in Sri Lanka’s low elevation tea growing areas. The evaluation was based on
drip irrigation with two different cultivars with the result that under present economic conditions,
only TRI 2023 (the benchmark high yielding but drought susceptible cultivar) is profitable for a
high investment like drip irrigation.
The Internal rate of return for TRI 3025 is 7% as against 23% of the TRI 2023. IRR analysis
gives a quick comparison for the farmer to decide on investment by comparing it with available
lending rates. The maximum available rate of return is 10% for commercial agriculture ventures
in Sri Lanka. However, sometimes under special agriculture project finance schemes, there is a
chance for a grower to obtain an agriculture loan at a subsidised rate of 8%. In tea such loans are
available for new planting fields but not for irrigation. The main reason for lack of credit facility
for irrigation is the lack of awareness about the positive results of irrigation, among growers as
well as among lending agencies.
The interest rate of 8% usually comprise of 2-3%
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implementation charge by the local banks. Donor agencies like Asian Development Bank,
Central Bank of Sri Lank or World Bank dispatch agriculture loans usually at interest rate of 56%. Still there is a chance to claim financial viability for drip irrigation, if there is a mechanism
to finance the investment by donor agencies to farmers directly through a mechanism like farmer
organizations at a rate of 5-6%. This would be very beneficial for the small farmers who own
less than one or two hectares.
Net present value of drip investment for TRI 2023 is Rs 391779 and for TRI 3025 it is Rs -57898.
The index of profitability of the NPV technique is to accept all projects with positive NPV
(Cuykendall, White et al. 1999). Based on current climatic conditions, only investment for TRI
2023 is financially viable.
In the present analysis, NPV was calculated based of the drip
irrigation yields from 2001-2009. There were no major droughts in this period like there were in
1983 and 1992. However, based on the evidences of climate change, there is a high vulnerability
for rain-fed cultivations to experience more frequent severe water stress periods, with increasing
ambient temperature (Mongi, Majule et al. 2010).
So it is worthwhile to calculate future
irrigation investment in tea, considering the additional drought stress possible.
Cultivar difference in response to irrigation is a known factor in tea (Salardini 1978; Stephens
and Carr 1991; Kigalu, Kimamboa et al. 2008). In Experiment 1, yield increase under irrigation
is 26% for TRI 2023 as compared to 16% of TRI 3025. Nevertheless, 16% yield increase under
irrigation is significant achievement for higher productivity. Currently tea growers are not using
TRI 2023 because it is no longer recommended for rain-fed cultivation. They are using less
productive tea clones which have been bred for drought tolerance. This research program has
shown that growers considering drip irrigation would be better advised to either: bring back
TRI2023; or introduce TRI 4049 which was shown to have relatively higher P n and Wi in
Experiment 3. In any case to continue to irrigate with currently used drought-tolerant varieties is
financially sub-optimal.
The main limitation for financially feasibility for drip irrigation is the very high investment cost.
The investment cost for the drip irrigation system alone (without pump) is Rs 450,000 per
hectare. This is approximately 30% of the average land price of one hectare of mature tea in the
area. There is a chance to lower the high investment cost of drip irrigation for farmers with large
extent of land and plantation companies who owns regional large tea estates, by investing in large
areas. Irrigation cost will be lower for larger extent since certain component cost (e.g. pump and
filters) remain same irrespective of area covered (Sivanappan 1994).
203
Increase in labour wage rate is a threat for future investment. During the period from 2000 to
2009, average agriculture wage rate increased by 213%. For the workers attached to plantation
companies, there is a fixed wage rate which usually determined by every two year. But for the
medium scale tea farmers, who has the most potential for irrigation, pays the wage rate usually
based on the market wage rate. Such farmers sometimes tend to pay little higher wage rate than
average to attract workers for the work and to ensure long term service. So it is important to
determine the financial feasibility of such growers, based on their actual wage rate.
In summary it can be concluded that for TRI 3025, IRR is lower and positive Net Present Value
was obtained only from TRI 2023. Among the variable factors associated with financial analysis,
wage increase is the most possible scenario. In this case, if the wage rate is increased to Rs
700/day, as requested by some labour unions, an irrigated tea cultivation would not be financially
feasible unless the farm gate green leaf price also increases to Rs 54/kg for TRI 2023 and Rs
70/kg for TRI 3025.
9.8
Summary
This chapter has discussed the tea plant response to irrigation in low elevation tea growing areas,
in physiological, yield and financial terms. It has focussed on explaining the direction and
variations in which the experimental data took, and possibilities for future research.
The
following and final chapter will conclude the thesis with an explication of how these 8
experiments satisfy the aim and objectives of this program of research.
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Chapter 10
Conclusion
This research program has evaluated the effect of short-term water stress on the agronomic and
physiological characteristics of low-grown tea, the responses to irrigation, and the financial
practicality of introducing irrigation in the low elevation tea growing areas of Sri Lanka. The
study revealed: the particular significance of high air temperatures in low-elevation tea; the
importance of cultivar selection for irrigation; and the importance of reducing water stress of
young establishing tea plants. In summary it has shown that, within well-defined practical and
financial parameters, there are strong grounds for promoting drip-irrigation of tea in this region.
There were five specific objectives to this study and the conclusions to each of them are as
follows:
A. To quantify the changes in physiology and yield as affected by the water stress and
recovery by irrigation
Water stress during the main seasonal dry season, January to March, caused key
physiological processes (photosynthesis, transpiration, stomatal conductance and
leaf water potential) which are related to productivity, to be depressed even under
drip irrigation. Cultivar TRI 2023 (benchmark for high productivity, low drought
tolerance) responded better to irrigation in maintaining high physiological activity,
than TRI 3025 (benchmark for moderate productivity, high drought tolerance).
Overall, drip irrigation resulted in 21% annual yield increase (P<0.001) over rainfed cultivation.
Higher physiological response to irrigation of TRI 2023 was
materialized as higher yield increase, resulting in 16% higher yield than TRI 3025
under drip irrigation. Cultivar difference in response to irrigation is a decisive
factor for irrigated tea cultivation. New cultivars for irrigation can be screened on
the basis of their physiological responses under high ambient temperatures in
humid, but water-stressed, conditions. For example in this study, TRI 4049 is a
strong contender as a more productive clone in hot humid drought periods with 27%
higher water use efficiency than TRI 3025.
B. To evaluate the water use of tea and environmental parameters that govern the water use
in low elevation tea growing areas
Air temperature is the dominant parameter governing transpiration in the wet season
(r2=0.62, P<0.0001), which is the key for higher productivity in the region.
205
However, high air temperature suppresses physiological activities in the dry season.
There was a strong difference in tea plant transpiration in dry and wet seasons.
Average daily transpiration of rain-fed plants was 2.3(±0.3) and 1.3(±0.2) mm/plant
in the wet and dry seasons respectively. Irrigated plants showed 109% increase in
transpiration than rain-fed plants in the dry season.
C. To evaluate plant performance in response to different micro-irrigation methods
Sprinkler irrigation produced 6% higher yield than drip irrigation, based on the very
wet year of 2008. Higher assimilation rate (15%), shoot extension rate (31%) and
higher shoot count (51%) were observed than drip, translating into higher yield in
sprinkler-irrigated plants in dry month.
Maintenance of 2-4% lower leaf
temperature in midday hours under sprinkler irrigation, compared with drip
irrigation, was the key for a higher assimilation rate. Irrigation treatments showed to
counter the negative effects of hostile climate parameters (temperature, irradiance,
vapor pressure deficit and potential evapotranspiration), which increase during dry
season. However there are practical problems with sprinkler irrigation in the low
elevation tea growing areas; undulating topography and presence of shade trees
affect the uniformity of application.
D. To quantify the effect of soil moisture limitation on young tea plant growth.
Irrigation proved effective in establishment of the young tea plant establishment
through glasshouse and field experiments.
Ideally, young plants require daily
irrigation during the dry season. Decreasing irrigation frequency from daily
irrigation to 4 day intervals reduced the stem growth by 20% and root growth by
46%. In the glasshouse, it is possible irrigate young plants to 75% of plant water
requirement with no significant effect on growth. However, deficit irrigation of 50%
reduced stem growth by 34% and root growth by 45%. Caution should be applied
when applying deficit irrigation in the field. Establishing young plants on raised
beds in the field enhances growth through increased soil porosity. Irrigated plants on
raised beds grew more roots (12 and 8% increase in fine and coarse roots) and a
60% increase in leaf area, compared with irrigation on flat ground. Rain-fed plants
on raised beds tended to shed more leaves possibly due to greater exposure to
radiation and advection.
206
Together, these field and glasshouse experiments with young tea point to the
possibility of an ideal establishment regime to be: daily irrigation but irrigating to
75% of plant water requirement; on raised beds, or ground otherwise prepared to
increase soil porosity.
E. To undertake a simple valuation of the practical financial feasibility of irrigating tea.
Financial evaluation of drip irrigation showed that an investment in drip irrigation is
feasible under the assumed parameters of production cost and green leaf price. The
choice of the right cultivar, i.e. one that responds well to irrigation, is crucial.
Cultivars selected for drought tolerance are not advised. Under the present
economic climate (considering 2007 as base year) and field yield data from 1999 to
2009, drip-irrigated TRI 2023 cultivar returned Net Present Value of Rs 391,779
and Internal Rate of Return of 23%. Sensitivity analysis showed that there is good
flexibility in the face of fluctuations of the main cost components. The systems is
feasibly against a possible wage rate increase to Rs 700/day (2011 rate = Rs
540/day), and where TRI 2023 should fetch a farm gate price of Rs 54/kg (2011
price range = Rs50 – 65 /kg).
This research program showed that irrigation can be applied as a successful tool in the field to
increase productivity and to protect the crop during acute short water stress periods. Climate
change predictions for the region are more rain, greater variability of rainfall and higher
temperatures. As this research has shown that it is high temperature during short dry periods that
impact most on yield, irrigation will still have a future in Sri Lanka even under a predicted higher
annual rainfall regime. As the wet zone does not have water storage infrastructure, the next step
in promotion of irrigation in the low elevation growing areas will be to demarcate the fields
which already have water resources for irrigation and the areas that have potential to develop
water resources.
207
208
Appendix 1
Rain Partitioning in a Low Elevation Tea Field
A1.1 Introduction
Tea is grown almost entirely in Sri Lanka as a rain-fed crop. So the precipitation is the main
contributor, recharging soil moisture storage.
The precipitation is redistributed in tree
environment as through fall and stemflow (Martinez-Meza and Whitford 1996). Precipitation,
intercepted by leaves and branches collects in the canopy until it is evaporated, drip to soil or
routed through canopy (Johnson and Lehmann 2006).
Evaporated fraction is termed as
interception and varies by storm and tree species characters, sometimes ranging up to 30% of
precipitation (Rutter, Kershaw et al. 1972).
Stemflow is the water that collect in canopy and routed down the trunk (Johnson and Lehmann
2006). Throughfall is the water that routed towards trunk but falling to ground before reaching
trunk, due to blockages or discontinuities along flow paths (Crockford and Richardson 2000).
Understanding throughfall and stemflow is important to understand the above ground process of
partitioning rainfall in tea environment.
The processes important in relation to soil moisture storage of plant root system are deep
drainage, soil evaporation, runoff, subsurface flow and capillary rise (Allen, Pereira et al. 1998).
While the subsurface flow and capillary rise can be considered to have a minimal impact in tea
environment, it is important to understand the drainage and runoff to estimate amount of rainfall
which is termed as “effective”(Dastane 1974).
Even though there are many attempts to understand the above ground rain partition and water
balance in forests and in other annual and perennial crops, there was no attempt to understand the
rain water partition and water balance in low elevation tea growing areas, where mixed
cultivation of shade and tea cultivations are available. This is an attempt to understand the above
ground rain partitioning, runoff, soil evaporation and deep drainage in a low elevation tea field.
A1.2 Materials and Methods
A1.2.1 Study Area
The study was conducted at Field No 01, St Joachim Estate, Ratnapura Sri Lanka (6040’ N,
80025’E, 29 m amsl). Details of the location and climate were given in previous chapters. The
209
field consist of more than 15 years old mature tea, belongs to cultivar TRI 2026. This is a
cultivar with large leaves and very popular among growers in the region. The field consisted of
large area of Gliricidia maculata (Gliricidia) as medium shade tree and another separate block
consisted of Falcataria molucana (Albizia) as high shade tree. Tea plant stemflow and
throughfall were however measured at open spaces, where shade canopy was not intercepting the
rainfall. Data collection was conducted during South West monsoon period of 2007 and 2008 in
between May-June months. Soil evaporation was measured during 11 rainless days in between
January- March, 2008.
A1.2.2 Rainfall
Rainfall data was collected from an Automatic Weather station (Measurement Engineering,
Australia) installed at the field site. Weather station was placed in between Experiment 1 and 4
sites (described in Chapter 5 and 6), where shade trees were not available.
Rainfall data
measured through a tipping bucket rain gauge and recorded to Starlogger data logger. Rain
gauge was placed above the tea canopy to receive full amount of precipitation. Rainfall amount
was measured after each rain event during day time in some selected days. When two rain events
were measured in a day, a gap of at least 4 hours was observed for the second event in the day.
Four hour gap is considered to be enough for the plant to dry water of the previous rain event.
A1.2.3 Stemflow
Stemflow was measured in 24 tea plants, 10 gliricidia plants and 6 albizia plants. Tea plants
grown with gliricidia as a shade plants was selected for the data collection as it has a minimal
effect of rainfall interception than albizia. To measure stemflow plastic circular collar was
attached to the stem of trees, covering full circle. It was placed tea trunk (before branching) at
tea plants and at a 0.5-1m height at gliricidia and albizia plants. Plastic collar was stapled and
sealed to tree trunk using non leaking adhesive. Water flowing through was collected in plastic
containers. The container was to be buried into soil near the tea plants as the collar was fixed at
lower height. After each rainfall event, the amount of water collected was measured and the
container was emptied. The canopy cover of each plant was measured approximately to calculate
the unit rainfall.
A funnelling ratio (F) is used to express the plants ability in collecting stemflow. It shows the
tree canopy divergence of rain water to trunk. The funnelling ratios of tea and gliricidia were
plotted for some rain events according to the following equation (Herwitz 1986):
210
‫ ܨ‬ൌ
ܵ‫݈݋ݒܨ‬
‫ ܣܤ‬ൈ ܲ݃
Where SFvol is the stemflow volume, BA is basal area of the tree and Pg is the incident rainfall at
the top of canopy.
The average stem areas of the plants were used to calculate the basal areas. The average stem
areas at breast height (~1.2m) were calculated for 6 albizia plants and 12 gliricidia plants to
calculate the basal area. For tea, 24 plants were sampled at the just above base to calculate
average stem basal area.
A1.2.4 Throughfall
Throughfall was measured 12 collectors placed beneath tea canopy, under gliricidia shade.
Throughfall collectors were made of PVC pipes with 12cm diameter and 40cm height.
Collectors were randomly placed in the field and location of each collector was changed daily
after measurements to minimize sampling error.
A1.2.5 Soil evaporation
Soil evaporation was measured using 16 micro-lysimeters, distributed in the site during 11
rainless days. The micro-lysimeters were made up of steel cylindrical tube with 10cm diameter
and 15cm height. There were openings at either side of the tube. Tubes were driven carefully
into soil in a nearby field, where measurements were not taken. They were then removed
carefully with intact soil inside the tubes. For that surrounding soil was removed and a sharp cut
was made at the bottom to detach the soil. Once removed, the bottom side was covered with a
muslin cloth. Micro-lysimeters were then buried beneath tea bushes carefully with a minimum
disturbance to soil in tube or surrounding. Water loss was calculated as the evaporation from the
soil by weighing the micro-lysimeter at 800 hours each day and reducing the previous days’
weight. Solar radiation of the day was recorded separately as the water loss through the soil was
related to the solar radiation available during the day (Hanks 1991).
A1.2.6 Drainage
A flux meter was buried at a depth of 60cm below the soil surface. It consisted of large opening
with a 25cm diameter in a conical shape. The bottom of the opening consisted of a funnel and
funnel neck was completely filled with a cotton rope. Before burying in the soil the top part of
the flux meter was filled with soil from the same site. This arrangement facilitated soil water to
211
seep down through the cotton rope and a spoon type flow gauge was fixed beneath to measure the
water flowing down. A flow gauge was fixed to a data logger and drainage was measured after
each rain event.
A1.2.7 Runoff
Surface runoff after rain events was measured using two steel runoff plots. The runoff plots were
rectangular steel structures, with 20cm walls. The lower wall of each plot had a flume leading
the runoff water out. Runoff collectors were buried 50 cm into soil and large containers were
placed in a soil pit beneath the openings. Water collected to as the runoff was measured after
each rain event.
A1.3 Results
The industry-recommended number of tea plants and shade plants and their basal area is given in
Table A1.1. Albizia showed the highest basal area in the tea field. Since gliricidia plant too had
small diameter stem (only 3 times larger than tea stem), it showed the lowest basal area.
Table A1.1 Standard plant density in a one hectare tea field and basal area of plants
Plant
Tea
Density/ha
Stem area
2
(m /plant)
Basal area
2
(m /ha)
-3
25.81
6.27 X 10
-3
1.63
4.97
298.04
12500
2.06 X 10
Gliricidia
260
Albizia
60
Table A1.2 shows the frequency of plants and relative dominance of the plants as the percentage
of the total basal area of the field, under three different shade plant combinations. Since tea is a
densely planted crop, it has the highest frequency under three different shade scenarios. Tea
shows a 94% dominance when it is grown under gliricidia. But in other two shade combinations,
albizia shade plant is dominant in basal area, though its frequency is small.
Relationship between funnelling ratio of tea and gliricidia plants and rainfall is given in Figure
A1.1. Tea clearly shows a reduction in funnelling ration with the increase of rainfall depth.
Though it is not quite significantly visible as for tea, funnelling ratio of gliricidia plant too
decreased with rainfall depth.
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Table A1.2 Frequency of plants (%) and relative dominance (%) according to basal area under different shade plant
combination in a tea field
Gliricidia only
Shade
Frequency
type
%
Relative
dominance
Albizia only
Frequency
%
%
Gliricidia + Albizia
Relative
dominance
Relative
Frequency
dominance
%
%
%
Tea
98.0
94.0
99.5
8
97.5
7.9
Gliricidia
2.0
6.0
-
-
2.0
0.6
-
-
0.5
92
0.5
91.5
Albizia
250
tea
gliricidia
Funneling ratio
200
150
100
50
0
0
5
10
15
20
25
30
35
Rain (mm)
Figure A1.1 Relationship between funnelling ratio and rainfall according to tree species
Table A1.3 shows the relationship between rainfall and different rain partitioning components in
a tea field. In a completely covered tea field, where chances are few for direct incidence of
rainfall receiving the soil, only around 40% of rainfall is collected to soil. Out of the soil that
received to soil, nearly 8% is lost as the drainage from root zone.
Though soil evaporation was measured in a dry season due to practical limitations in the wet
season, it also showed a considerably high value in wet season. The average solar radiation of
the 11 rainless days, sampled, was 207(±8.5) Wm-2. Accordingly, average soil evaporation
during dry season in >1mm even within a tea canopy.
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Table A1. 3 Relationship between rainfall and partitioning and soil moisture movement in a tea field. Throughfall and
stemfall is given only for tea plants. n= no. of rain events sampled/. (Note: Soil evaporation is expressed in
relation to solar radiation measured in Wm-2)
Slope
r
2
Probability
Throughfall (n=36)
0.106
0.70
0.0001
Stemfall (n=35)
0.322
0.82
0.0001
Drainage (n=22)
0.084
0.43
<0.0001
Runoff (n=21)
0.038
0.51
<0.0001
Soil evaporation (n=11 days)
0.017
0.74
0.0007
Parameter
A1.4 Discussion
The study evaluated the pattern of different tree species in a tea field, contributing to stemflow of
the precipitation. It gives an idea about rain partitioning and effective rainfall in the field.
Stemflow is the main rain contributor to the soil moisture storage.
Different tree species
contribute differently to stemflow, based in their canopy characters. Among them, basal area is a
most significant character (Herwitz 1986). According to basal area of shade and tea plants, when
the tea is grown with gliricidia, its contribution to stemflow is much lower than tea. But in the
presence of albizia as the shade tree, it represents the largest basal area contributing much
stemflow. Even though, the relationship between basal area and stemflow was not studied in this
study, relationship between basal area and stemflow was established for other trees like, red oak,
sugar maple and American beech (Carlyle-Moses and Price 2006).
Funnelling ratio of the tea and gliricidia shows a negative relationship with increasing rain event.
This is can be due to dripping of more water from the plant before diverting to the stem, due to
heavy water flow in high rain storms. Because in higher intensity rainfalls result in increased
flow velocities along branches that may exceed the flow capacities of the branches (Herwitz
1986). In compared to gliricidia, tea plant was more efficient in harvesting small rain events with
higher funnelling ratio. Presence of comparatively (with stem sizes) higher crown area, branch
inclination and shorter distance to travel may contribute to higher capacity of tea plant to collect
more stem flow (Steinbuck 2002).
The simple water balance model shows a picture about the effective rainfall in low-grown tea
field.
According to the findings, nearly 40% of the rain enters soil as the stemflow and
throughfall. However, in this study, the rain events measured were <40mm. Heavy rain events
were very difficult to sampling with over flowing water collectors. The contribution in such
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large rain events (>40mm) may be higher. Also the rain falling on the open spaces, like in
between rows need to be considered. Even under small rain events, >10% is lost from root zone
as drainage and runoff. Even though measures like mulching soil with organic matter can be
used to minimize the runoff, there is no mechanism to control the drainage. Soil mulching has
another advantage to reduce the high soil evaporation in dry days. This is important for irrigated
tea field as well, since there is a high chance to lose irrigation water as soil evaporation in dry
season.
This analysis is important in calculating the soil water balance especially in some inter-monsoon
dry periods, when isolated low rain events are occurred. In such cases it is worthwhile to arrange
mechanisms to harvest maximum rain amount to root zone and minimize losses.
A1.4 Summary of Conclusions
Albizia as a shade tree has higher dominance in harvesting precipitation as stemflow.
Tea plays a major role in producing more stemflow in low rain events.
Funnelling ratio reduced with expanding the rain event.
After rain event, only 40% of rain reaches soil as stemflow and throughfall.
Runoff and drainage losses accounts for nearly 10 of the soil moisture balance in tea root
zone.
During, dry days, soil evaporation accounts for >1mm moisture loss.
215
216
Appendix 2
A2 Tea Plant Behavior under Water Stress on Different Temperature Regimes
A2.1 Introduction
Tea is mainly grown in high rainfall areas of the tropics and subtropics with its preference to
warm wet climates. Other than rainfall, the next most important factor in deciding the
geographical distribution of the crop is air temperature. An ambient temperature regime of 18300C is considered to be optimal for the best growth and yield of tea (Squire and Callander 1981).
Tea is consumed mainly as either black (fermented), green (non-fermented) or oolong (semifermented) beverage (Hampton 1992; Kamunya, Wachira et al. 2009). The production type of
the tea is mainly spread around different tea producing countries. Sri Lanka is mainly producing
orthodox black tea. Among total Sri Lankan tea production, 94% belong to orthodox black tea
category and 98% of total tea production of low elevation tea growing areas of Sri Lanka is
orthodox black tea, in 2008 (SLTB 2009).
The price of the tea is determined by the quality that is determined through the chemical
composition of the black tea (Wright, Mphangwe et al. 2000). To determine the quality of the
made tea, chemical composition of the fresh green leaf can be used satisfactorily (Robertson
1992; Obanda and Owuor 1995; Wright 2005). The quality of the black tea is determined by (a)
environmental influences on the quality, (b) manufacturing practices and (c) genetic make-up of
the cultivar (Ramaswamy 1964; Astill, Birch et al. 2001; Wright 2005).
An experiment was conducted examining the hypothetical interaction of soil moisture and air
temperature, under the controlled conditions. It studies the relationship in Chinese variety tea
plants, (Camellia sinensis var. sinensis) rather Assam origin tea (Camellia sinensis var. assamica)
as appropriate stock was not available in Australia at the time. The experiment measures the plant
growth, photosynthesis, transpiration and changes in quality components water use efficiency of
green tea grown over a range of four water stress regimes by two temperature regimes.
A2.2 Method
A2.2.1 Plant material
For the experiment, 15 month old Camellia sinensis var. sinensis plants were used. Plants were
provided by a shade house nursery in NSW, Australia (Paradise Plant Nursery of Kulnura, New
217
South Wales). After the delivery to experiment site, plants were transplanted into 12 litre
containers filled with University of California mixture (pH 5.5) and grown for two months in a
glass house prior to imposing the temperature treatments. Two weeks before commencement of
the treatments, plants were uniformly pruned to 60cm height.
A2.2.2 Experimental design
The experimental design was a randomized complete block design where four water stress
treatments were assigned for four blocks in two contrasting temperatures.
Each treatment
consisted of 3 plants in a plot and 5 blocks were prepared inside the growth room. This would
ensure equal light availability for all treatments and plots. As the growth chamber (manufactured
by Phoenix Biosystems, Australia), could only be set at one temperature regime at a time, the two
separate sets of plants were used for the two temperature regimes, each for 21 days.
A2.2.3 Temperature regimes
A low and a high temperature regime were applied which simulate the minimum and maximum
temperature levels experienced in low-grown tea areas of Sri Lanka. For the low temperature
regime, the growth chamber temperature was kept at 250C during day time (0900 -1900 hrs) and
at 180C during night period (1900 – 0900 hrs). For the high temperature treatment, the growth
chamber temperature was kept at 330C during daytime (0900 -1900 hrs) and at 260C during night
time (1900 – 0900 hrs).
A2.2.4 Water stress regimes
Water stress was applied using different irrigation frequencies, from daily irrigation to zero
irrigation for the entire trial period of 20 days. The objective of this water frequency is to
simulate the different level of short term drought spells available in low elevation tea growing
areas of Sri Lanka, where rain normally falls as few isolated rain events in dry season.
Accordingly the four water stress regimes were:
1. Watering the plants daily 100ml (W1)
2. Watering plants at 5 day interval 500ml (W5)
3. Watering plants at 10 day interval 1000ml (W10)
4. Plants were not watered for 20 days (W20).
218
A2.2.5 Light regime
Six light bulbs of 400 watt lamps (Phillips) suspended 0.4m above the tea plants provided the
light to the growth chamber. Day length was set as 10hrs and 30 minutes and the light intensity
level was varied across the day to approximately simulate diurnal fluctuations in the field (Table
A2.1). Maximum Photosynthetically Active Radiation(PAR) of 600µmolm-2s-1 was received by
the plants for 7 hours during day time.
Table A2.1 Available Photosynthetically Active Radiation from 0900 to 1930 hours during the treatment period
Time
(hours)
Light Intensity
PAR
-2 -1
(µmolm s )
0900 – 1030
60%
360
1030 – 1730
100%
600
1730 – 1930
60%
360
A2.2.6 Measurements
Leaf physiological activities of photosynthesis (Pn), leaf transpiration rate (El), stomatal
conductance (gs) and leaf temperature (Tl) were measured at two day intervals using ADC
LCApro gas exchange system.
(Results are given as average value for the entire period).
Topmost mature leaf of the plant was used for the measurements, recorded between 1300-1400
hours. Three samples were taken from each replicate, making 15 records for each treatment.
Plant water use of the water stressed plants (W20) was measured by weighing the pots each day.
To measure the shoot growth, shoot extension of selected active shoots were measured at the start
and end of trial period. At the end of 20 days, harvestable young shoots were plucked and
analysed for the quality components after drying in a micro wave oven, using HPLC. The
samples were freeze-dried and packed in dry ice before sending to the Gosford Horticultural
Research Centre, New South Wales Department of Primary Industries for analysis. Samples
were analysed for theanine, caffeine and catechins.
SAS (Version 9.s) statistical software (SAS Institute, Inc,) was used for statistical analysis.
219
A2.3 Results
A2.3.1 Leaf photosynthesis (Pn)
Both increasing irrigation interval and air temperature negatively affected the leaf photosynthesis
(Figure A2.1). There was a significant difference among W1 and W5 treatments under both
temperature regimes, showing the importance of daily rainfall or irrigation.
When the air
0
temperature level was kept at 25 C during day time, plant Pn was higher for all irrigation
intervals. However, the negative effect of increasing irrigation interval acted on a similar pattern
in both air temperature levels.
Photosynthesis rate (Pmol CO2 m-2s-1)
4.5
low temperature (r 2 = 0.85, P = 0.07)
high temperature (r 2 = 0.85, P = 0.07)
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0
5
10
15
20
25
Irrigation interval (day)
Figure A2.1 Average photosynthetic rate of mature tea leaves at mid-day during treatment period, according to
watering frequency
A2.3.2 Stomatal conductance (gs)
Higher stomatal condition was observed under high air temperature level in all 4 irrigation
treatments (Figure A2.2). Under both temperature regimes, well watered plants showed the
highest gs. Other irrigation intervals, did not show significance difference among each other in
both high and low air temperature levels.
Even the 5 day water stress caused significant
reduction in gs than daily watering plants.
220
Stomatal conductance (mol H2O m-2s-1)
0.045
low temperature (r 2 = 0.63, P = 0.2)
high temperature (r 2 = 0.75, P = 0.1)
0.040
0.035
0.030
0.025
0.020
0.015
0
5
10
15
20
25
Irrigation interval (day)
Figure A2.2 Average stomatal conductance of mature tea leaves during midday, according to frequency of watering
A2.3.3 Leaf transpiration (El)
Transpiration rate (mmol H2O m-2s-1)
1.2
low temperature (r 2 = 0.6, P = 0.2)
high temperature (r 2 = 0.9, P = 0.05)
1.0
0.8
0.6
0.4
0.2
0
5
10
15
20
25
Irrigation interval (day)
Figure A2.3 Average transpiration rate of mature tea leaves during the study period at mid-day, according to different
watering frequency
Similar to gs, high air temperature has caused more transpiration of water from leaves (Figure
A2.3). Water consumption increased more than 100% when increasing the ambient temperature
level. In both temperature regimes, well watered plants showed highest average El. However
effect of irrigation interval was not significantly different among each other.
221
A2.3.4 Leaf temperature (Tl)
Leaf temperature (Tl) response to irrigation interval was similar in both air temperature levels
(Figure A2.4). In both occasions, lowest leaf temperature was recorded from well watered plant.
However, when the air temperature level was kept at 250C, Tl was kept above 2-30C, above
ambient temperature level. But at 330C air temperature level, daily irrigated plants showed lower
Tl than air temperature. All other irrigation treatments kept Tl above 330C. Deviation of Tl from
air temperature level (330C) was however confined to <10C.
36
34
330C
Leaf temperature (0C)
32
30
28
26
250C
24
low temperature
high temperature
22
20
W1
W5
W10
W20
Treatment
Figure A2.4 Leaf temperature of mature tea leaves during mid-day during treatment period according to various water
frequency. Reference line shows the high (330C) and low (250C) air temperature
A2.3.5 Water use efficiency (Wi)
Instantaneous leaf water use efficiency (ratio of photosynthesis to respiration) is shown in Figure
A2.5. Highest Wi was shown in low air temperature regime for all irrigation intervals. Except
for W20, for all other irrigation intervals, Wi was >100% higher at 250C. Difference in Wi
among irrigation intervals were lesser at high ambient temperature levels.
222
0.8
low temperature (r2 = 0.9, P = 0.03)
high temperature (r2 = 0.8, P = 0.1)
0.7
Water use efficiency
0.6
0.5
0.4
0.3
0.2
0.1
0
5
10
15
20
25
Irrigation interval (day)
Figure A2.5 Average instantaneous water use of tea plants according to water application frequency
A2.3.6 Shoot extension rate
2.0
1.8
low temperature
high temperature
Shoot extension (mm/day)
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
W1
W5
W10
W20
Treatment
Figure A2.6 Shoot extension rate during the experiment period for two air temperature levels
223
At high ambient temperature level of 330C, shoot extension rate was >100% higher for all
irrigation intervals. There was no significant difference among irrigation intervals at either air
temperature levels. On average, well watered plants showed a higher shoot extension rate.
A2.3.7 Quality parameters
Composition of the some of the chemical constituents that are important in determining made tea
quality is shown in (Table A2.2). They are caffeine, total catechin level and catechin gallate to
catechin
ratio
[epicatechin
gallate(ECG)
+
epigallocatechin
gallate(EGCG)
to
epigallocatechin(EGC) ratio]. In general the under the high air temperature, concentration of the
caffeine was higher than under low air temperature. Though statistically not significant at P=0.05
level, watering interval showed some effect increasing leaf caffeine. More caffeine content was
detected in plant leaves when grown at high air temperature levels. However, amount of total
catechin was lower at high air temperature levels.
Total catechin concentration at low
temperature was 28.9(±0.7)mg/g while with high temperature it was 22.1(±1.0)mg/g.
The ratio between ECG+EGCG/EGC varied differently with air temperature. There was no
significant difference among watering treatments and the catechin gallate to catechin ratio under
low temperature conditions. But with high temperature conditions, watering frequency had a
significant relationship with watering frequency. Catechin gallate to catechin ratio increased
>300%, when the irrigation interval was increased from daily to 5 day interval at high
temperature. At the same high temperature level, when the plants were kept without watering for
20 days, the catechin gallate to catechin ratio increased to nearly fivefold level.
Table A2.2 Main components of made tea quality according to different watering treatment in mg/g
Caffeine
Total Catechin
(ECG+EGCG)/EGC
Treatment
low
high
low
high
low
high
W1
11.9(±0.9)
15.4(±0.9)
30.0(±3.5)
24.9(±2.2)
5.0(±0.5)
8.4(±1.2)
W5
12.5(±0.7)
16.5(±0.9)
29.9(±4.1)
21.9(±1.3)
6.1(±1.1)
27.0(±4.7)
W10
14.0(±1.0)
16.7(±0.8)
28.7(±2.1)
21.7(±1.7)
4.8(±0.4)
25.9(±5.1)
W20
13.3((±0.8)
18.0(±0.6)
27.1(±2.7)
20.1(±1.1)
5.6(±0.7)
37.0(±3.1)
ns
ns
ns
ns
ns
11.8
LSD (0.05)
224
A2.4 Discussion
In past, experiments were conducted to evaluate the suitable temperature for the tea plants
photosynthesis using different clones (Barbora 1994; Joshi and Palni 1998). Also the effect of
water stress and seasonal changes in photosynthesis has been studied (Squire 1977; Smith,
Burgess et al. 1993; Lin 1998; Hajra and Kumar 1999). However in this experiment, the effect of
higher temperature and water stress were studied together.
A2.4.1 Effect on physiology and growth
Photosynthesis is an indicator of plant stress (Nilsen and Orcutt 1996). Both increasing irrigation
interval and air temperature had a negative effect on Pn. Since there was similar PAR on both
temperature regimes, air temperature can be considered as the environmental parameter that
negatively affected Pn. The results support the argument that at high air temperature levels, Pn is
controlled due to changes to leaf photosynthetic capacity (Lin 1998). The results contradicts the
argument that Pn is controlled by stomatal activities (Barbora 1994). However, plants are very
sensitive to irrigation frequency under both air temperature levels. Best irrigation management to
keep higher Pn is the daily irrigation or rain.
But in overall, Pn was lower than in field conditions, as reported in field experiments and
experiments in elsewhere (Hajra and Kumar 1999). This could be due to the low availability of
PAR within the chamber. The available average light intensity inside the chamber leaf surface is
370(± 7.2) µmolm-2s-1, which is much lower than the saturation light level of 735 µmolm-2s-1
(Sakai 1987).
High ambient temperature caused the plants to lose more water, as shown by high gs and El. So
the correct moisture maintenance is more crucial in hot environments like, low elevation tea
growing area. Also higher transpiration is related to higher dry matter production in the plant,
which ultimately resulted in high yield. This was evident in this experiment through increased
shoot extension rate and in previous experiments. But even 5 day irrigation interval caused
transpiration process to reduce significantly than daily watering.
Leaf temperature, which is an important parameter controlling photosynthesis and the yield,
when reached to excessive levels (Hadfield 1968), have behaved differently under two
temperature levels. In both temperature levels, well watered plants have been able to maintain a
lower leaf temperature value. However when the air temperature was kept at 330C, leaf
temperature has been higher than air for water stressed plants. But when the air temperature is
225
kept at 250C, all plants leaf temperature values have been kept above the air temperature. Daily
plant watering is important to maintain a favourable leaf temperature level, in areas with high
ambient temperature like low elevation tea growing areas of Sri Lanka. Otherwise leaf
temperature could reach to critical values of 350C or above as reported by (Bannerjee 1993).
Under the 5 day watering treatment (W5), the leaf temperature seems to be little higher than other
treatments, under both temperature levels. The plants of W10 and W20 treatments, may have
adapted other techniques like, folding the leaves to minimize the temperature damage. With
relation to irrigation under high temperature levels, it can be hence assumed that regular watering
is the best way to minimize the leaf temperature increase.
According to experiment, it showed that Wi was lower and less sensitive to irrigation interval at
high ambient temperature.
But, in contrast, daily watering has leads to higher water use
efficiency under low temperature. So in low temperature tea growing locations, like mid and
high elevations of Sri Lanka, daily irrigation during dry season can be used to increase low
productivity.
Shoot extension rate was not affected by the irrigation interval at either air
temperature levels. This may be due to short duration of the experiment period. But, the high
temperature increased shoot extension rate.
A2.4.2 Effect on quality
The shoot chemical composition of the young tea shoot grown under two different temperature
levels were studied as a response to different irrigation intervals.
The results indicate a
significantly different behaviour of main important chemical constituents, with relation to their
growing environment and moisture stress as imposed by different watering regimes. In overall
the results indicate the quality of the black tea produced can be controlled by controlling soil
moisture and air temperature. One popular method of quality improvement in made tea was
accomplished by different cultivar selection within a given region (Wright 2005; Owuor, Obanda
et al. 2008). But this experiment, quality aspect of made tea can be altered with the maintaining
the water stress level in the field.
The caffeine content is the chemical compound that involves in tea cream and it may influence
the briskness of the tea (Robertson 1992). Though there is a trend of increasing caffeine content
with increasing duration of water stress, difference was not significant among watering interval.
However, as there is a consumer concern about the amount of caffeine content in tea, irrigation
can be used to control the increasing caffeine content under hot growing conditions.
226
The quality index of ECG+EGCG/EGC was found to be directly related to sensory properties of
green tea and is used to assess the quality of tea in China (Liang, Liu et al. 1990). It is also used
for assessing the seasoning variation of fresh shoots (Yao, Caffin et al. 2005). Under high
temperature conditions, watering frequency, the ratio increased significantly. The high ratio was
quite extra ordinary except for daily watering treatment, as such high variation were not found
among seasonal variation in field level (Yao, Caffin et al. 2005). However, this ratio is not so
significant in black tea production as it is mainly used to detect the quality of green tea
(Harbowy, Balentine et al. 1997). Even though, low elevation tea growing areas of Sri Lanka
produce almost entirely black tea, understanding the quality changes are important in future
product diversification.
A2.4.3 Effect of physiological process on quality
Caffeine content changes according to Pn and El is given in Figure A2.7. Both increasing, Pn and
El, reduced the caffeine content. As the concentration of caffeine is higher under high air
temperature, irrigation interval is more important in hot growing areas. As the transpiration
process had little impact from the irrigation interval, maximizing Pn by daily irrigation is the best
possible way to reduce the caffeine content. Also in field conditions, as the measurement of P n
and El is easy and not damaging the plant, detection of Pn and El can be used to predict the made
tea quality. Even large area can be sampled with minimal damage in a short time.
19
(a)
(b)
low temperature (r2 = 0.69, P = 0.1)
high temperature (r2 = 0.92, P = 0.03)
18
Caffeine (mg/g)
17
16
15
14
low temperature (r2 = 0.93, P = 0.02)
high temperature (r2 = 0.98, P = 0.005)
13
12
11
1
2
3
4
Photosynthesis rate (Pmol CO2 m-2s-1)
0.4
0.6
0.8
1.0
1.2
Transpiration rate (mmol H2O m-2s-1)
Figure A2.7 Relationship between average photosynthesis during the experiment and caffeine content (a) and
transpiration and caffeine content (b)
227
A2.5 Summary of Conclusions
Plant photosynthesis rate had significant negative relationship with increasing water stress
days under high temperature conditions
When the water stress days were increased, stomatal conductance too fell, but did not
show a strong relationship.
However transpiration had a close linear relationship with watering frequency under high
temperature condition.
Plants showed a high water use efficiency under low temperature conditions, and there
was a strong negative relationship too with watering frequency under low temperature
levels.
The shoot extension was significantly higher under high ambient temperature levels.
Made tea quality has strong relationship with drought stress in high temperature levels.
228
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