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Possible effects of climate
change on crop-weed
Professor Andreas J. Karamanos
Faculty of Crop Science
Agricultural University of Athens
Definition of climate change
 the
long-term change in the average
weather conditions prevailing in a region
 they are expressed as changes:
- in the average air temperature
- in precipitation
- in winds etc.
 External
( changes in solar or cosmic
radiation, earth’s orbit etc.)
 Human activities (greenhouse gases)
Greenhouse effect
Global warming:
- increase in average air temperature from 1.5 –
5.5οC between the years1990-2010
 Parallel impacts:
-distortion in the movement of barometric
- distortion in the hydrologic cycle
- ice melting
- increase in the frequency of extreme incidents
Climate change aspects affecting
 Increase
in air temperature
 Increase in CO2-concentration
 Change in the amount and distribution of
 Change in evapotranspiration
 Increase in the frequency of extreme
weather incidents (droughts, floods etc.)
forecasts in the Mediterranean
Time-series of rainfall intensity in
Increase in [CO2] and plant physiology
Positive effects on photosynthesis
Increase in [CO2] and plant physiology
Decrease in photorespiration (only in C3-plants)
 Increase in net photosynthesis
Namely, increase in overall carbon balance
Increase in [CO2] and plant physiology
Decrease in transpiration by 25-50% (partial
stomatal closure)
 Increase in water use efficiency (WUE, g of dry
matter / g of transpired water)
Elevated [CO2] and WUE
Increase in temperature and plant
Acceleration of most physiological processes
Increase in temperature and plant
Increase in respiratory dry matter consumption
Increase in temperature and plant traits
Decrease in average grain weight of temperate
Increase in temperature and plant traits
Increase in growth rate
Effects of temperature on phenology
Increase in temperature and plant traits
Vernalization in temperate crops
Combined temperature and [CO2]-effects on
Combined temperature and [CO2] - effects
Water shortage effects
 Less
intense under higher [CO2]
 Evident on all physiological processes
 Different degrees of sensitivity among
physiological processes
 Plant differentiation according to their
adaptability to drought
Reasons for change in the weed / crop
competition patterns
 Differential
effects of [CO2] on C3 and C4
more beneficial to C3 plants
- 14 of the 18 “worlds worst weeds” are C4 plants
- 12 of the 15 major crops are C3 plants
perennial weeds more difficult to
control due to the stimulation of growth of
rhizomes and storage organs
Ranges of increase in biomass of C3 and C4
weeds and crops grown under
doubled [CO2] (Patterson, 1995)
95 – 272 %
107 – 494 %
56 – 161 %
98 – 182 %
Reasons for change in the weed / crop
competition patterns
 Differential
effects of temperature increase
(2 to 5oC) on weeds and crops:
- expansion of warm season weeds to higher
latitudes and cooler areas (e.g. Cassia,
Amaranthus, Sesbania, Crotalaria, Rottboellia,
Imperata, Panicum, Striga, etc.)
- competitive advantage (higher growth rates)
of warm season weeds against temperate crops
Reasons for change in the weed / crop
competition patterns
 Differential
effects of water availability to
crops and weeds:
- less severe water stress-effects in raised
[CO2] levels (reduction in transpiration,
development of osmotic adjustment, higher root /
shoot ratio)
- they depend on the drought resistance
mechanisms to be developed by crops and
Interventions on weed control
Choice of the appropriate crop species/cultivar
 Adjustment of planting time/season
 Possible effects of the elevated [CO2],
temperature, and water availability on chemical
 Adjustment of the irrigation/fertilization schemes
The increase in [CO2] and temperature will affect the
weed-crop competitive interactions. Water availability
might be a less decisive factor.
The effects on crop production will depend on:
- the local weed flora
- the C-assimilation pathway and the temperature
requirements of the crop and weed species
The interacting effects of many factors influence
weed/crop competition in a complex way and make an
overall prediction on the consequences on crop
production risky.
Thank you for your attention