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The rhizosphere
Sponsored by the DEST program:
China Higher Education Strategic Initiatives
© The University of Adelaide
Acknowledgements
This talk is based heavily on H. Marschner (1995)
Mineral Nutrition of Higher Plants; Chapter 15:
The soil-root interface (rhizosphere) in relation to
mineral nutrition
Dr Petra Marschner supplied Powerpoint slides
used as the basis of many of the following slides
Aims of this talk
Summarize properties of the rhizosphere, especially:
…that are relevant to
plant mineral nutrition
Modified from H. Marschner: Fig. 15.1
The rhizosphere
Width not to scale
• Layer of soil surrounding the
growing root that is affected by
the root
• Usually a few mm wide, up to
say 1 cm (no sharp boundary)*
• Extent depends on plant
properties; e.g.
- Root hair length & density
- Rhizodeposition (exudates etc)
- Nutrient uptake versus supply
Rovira 1960?
* ‘Mycorrhizospheres’ can extend
many cm
The rhizosphere: soil factors
Width not to scale
• Layer of soil surrounding the
growing root that is affected by
the root
• Usually a few mm, up to say 1
cm (no sharp boundary)
• Extent depends on soil
properties; e.g.
- pH & buffering
- Sorption capacity
- Nutrient supply rate
- Microbial populations
- Decomposition of exudates
Rovira 1960?
The rhizosphere: some conventions
Root
Rhizoplane
0-10 µm
Inner rhizosphere
10-500 µm
Outer rhizosphere
500-5000 µm
The rhizosphere: some conventions
Root
Rhizoplane
0-10 µm
Inner rhizosphere
10-500 µm
Outer rhizosphere
500-5000 µm
But defined ‘phases’ may not be helpful because of
gradients (no sharp boundaries)
Gradients in the rhizosphere
Longitudinal & lateral gradients
important for plant nutrition:
- especially in waterlogged soils
- concentrations & composition
- population density & composition
- especially soil bacteria
- especially mycorrhizal fungi
Gradients in the rhizosphere
Longitudinal & lateral
gradients:
Nutrient concentration
Depends on balance between soil supply
and plant uptake;
depending
in turn on:
- concentrations
& composition
- population
• Concentrations in
bulk soildensity & composition
• mobility in soil solution
• mass flow rate
• water
content &of soil
Mycorrhizal
fungi
• rateother
of uptake
into roots
• interactions with microorganisms
Ion mobilities and rhizosphere depletion
Distance moved by diffusion (mm in 6 days)
Soil volumetric water content
0.3
0.1
NO3-
30
3
K+
3
0.3
0.3
0.03
H2PO4-
Diffusion rate depends on
• Ion:
NO3->K+>H2PO4• Water content:
High > low
Rhizosphere depletion of P
is common in many soils
P depletion zones in the rhizosphere of maize
and rape: influence of root hairs
Bulk soil
150
Isotopically
exchangeable P
100
(µg ml -1)
Canola
50
Maize
1
Mean root
hair length
2
3
Distance from root surface (mm)
Hendriks et al. 1981
Accumulation of calcium & magnesium in
rhizosphere of barley
75
15
Available
Ca (mM)
Available
Mg (mM)
50
10
Mg
Ca
5
25
0
5
10
15
Distance from root surface (mm)
Roussef & Chino 1987
Gradients in the rhizosphere
Longitudinal & lateral
gradients:
pH changes
Depend on -many
factors& e.g.
concentrations
composition
- population density & composition
• Form of N nutrition
• Soil pH buffering capacity
• Production of organic & amino acids
Mycorrhizal &
• Microbial
activity
other fungi
Effect of N form on the rhizosphere pH of barley
200 kg N/ha
H+ uptake (or
OH- release)
during NO3assimilation
H+ release
during NH4+
assimilation
NO3-
NH4+
Römheld 1986
Soil nitrate concentration & rhizosphere pH of maize
75
125
200
NO3-N (kg/ha)
400
Römheld 1986
Rhizosphere pH of chickpea with NH4+ supply in
soil and different CaCO3 addition
% CaCO3
1.5
3.0
6.0
Römheld 1986
Increasing soil pH &
buffering
Rhizosphere pH of different plant species supplied
with 200 kg nitrate/ha
Sorghum and
chickpea
Barley
Lentils
Not all species increase pH
Cowpea
Römheld 1986
Rhizosphere pH and P depletion in soil
7
pH
100% NO3
80% NO3
20% NH4
6
5
1
2
P concentration (g P/g soil)
canola
300
280
260
3
4
1
2
mm distance from the root surface
Lower rhizosphere pH
improves P availability
3
4
Gahoonia & Nielsen 1992
P supply and cluster root formation and
rhizosphere pH of white lupin
N supplied as NO3-
Cluster roots
formed at low P;
pH decrease is
due to organic
acid extrusion to
mobilise P; - not
associated with
NO3- assimilation
No P
Foliar P
Römheld 1986
Gradients in the rhizosphere
Longitudinal & lateral
gradients:
Redox potential:- especially in waterlogged soils
•
- concentrations & composition
decreases in waterlogged
soil (low
O2)
- population density
& composition
• increases solubility of Mn & Fe
• can lead to production of phytotoxic
organic products.
Mycorrhizal &
• Plants
adapted
other
fungi to waterlogging (e.g rice)
have ‘oxidation zone’ (to 5 mm) due to O2
transport from shoot
Gradients in the rhizosphere
Longitudinal & lateral
gradients:
- especially in waterlogged soils
- concentrations & composition
Root products:
composition and
- population density & composition
concentrations
Many functions:
•Nutrient mobilisation
•Soil detoxification (e.g. Al)
Mycorrhizal &
•Substrates
for microorganisms
other fungi
•Stimulation or repellence of microorganisms
•etc
Release of organic material (rhizodeposition)
Sloughed off (removed by friction):
• Cells and cell debris
Organic material exuded (from living cells):
• High molecular weight:
- mucilage (polysaccharide & polyuronic acids)
- enzymes
• Low molecular weight:
- sugars
- organic acids
- amino acids
- phenolics
- others
CO2 (weak acid) - of organic origin
H+ - of organic origin (in exchange for mineral cations: C+)
Main sites of root exudation
Organic & amino acids
Low molecular weight are important in
mobilizing mineral
nutrients
mucilage
Not to scale
Release of organic material (rhizodeposition)
Amounts and composition are affected by
• Plant species & age
• Soil type & properties
• Nutritional status of the plant
• Temperature
• Light intensity and duration
• Presence of microorganisms
Major components of plant root exudates
Sugars
Amino acids
Organic
others
Enzymes
acids
Glucose
Fructose
Maltose
Galactose
Ribose
Xylose
Rhamnose
Arabinose
Raffinose
Oligosaccharides
Leucine
Isoleucine
Valine
Aminobutyrate
Glutamine
Alanine
Asparagine
Serine
Glutamate
Aspartate
Glycine
Phenylalanine
Threonine
Tyrosine
Lysine
Proline
Methionine
Cystathione
Oxalate
Malate
Acetate
Propionate
Butyrate
Valerinate
Citrate
Succinate
Fumarate
Glycolate
Proteins/
Flavones
Adenine
Guanine
Scopoletine
Cyanogenes
Flavonglycosides
Cinnamic acid
Chlorogenic acid
Invertase
Amylase
Protease
Peroxidase
Differences between species: organic acid
exudation of legumes under P deficiency
nmol/g root fresh wt/ 12h
Total
Fumaric
Citric
Malic
Malonic
Soybean
3
1
1
1
-
Chickpea
66
7
36
13
7
Peanut
47
24
9
13
-
Pigeon pea
6
1
1
4
-
Species
Ohwaki and Hirata 1992
Ethanol soluble sugars
µg g fw -1
Effect of plant age on sugar exudation from
maize
30
20
10
20
40
Plant age (days)
60
Matsumoto et al. 1979
Organic 14C exudation along wheat roots
750
Radioactivity (cpm)
Lateral root emergence
500
250
0
0
5
10
15
20
Distance from the root tip (cm)
based on Rovira and Davey 1974
Soil types: exudation of organic acids by chickpea
100
Organic acid composition in
rhizosphere (% of total)
Organic acid concentration in
rhizosphere (mol/g root)
150
75
0
A
B
C
D
E
F
50
0
A
B
C
Soil
Soil
Succinate
Malonate
Veneklaas et al. 2003
D
Citrate
E
F
Root exudates improve solubility of lowsolubility mineral compounds
H. Marschner (1995); Fig. 15.10
Soil mechanical impedance increases root
exudation in barley
Nutrient solution Nutrient solution
alone
+ glass beads
Plant dw (mg/plant)
Shoot
Root
57
32
52
36
Exudation (mg/plant)
Amino acids
Carbohydrates
Total
0.1
1.5
1.6
0.2
3.0
3.2
5.0
1.8
9.0
3.7
% of root dw
% of total plant dw
Barber and Gunn 1974
Release of enzymes
Bürkert 2003
Acid phosphatase activity and organic P depletion in
the rhizosphere of wheat and clover
Relative units
Acid phosphatase activity
Organic P concentration
Wheat
Clover
1
2
3
4
1
2
3
4
mm distance from the root surface
Tarafdar and Jungk 1987
Gradients in the rhizosphere
Longitudinal & lateral
gradients:
- especially in waterlogged soils
- concentrations & composition
- population density & composition
- especially soil bacteria
-especially mycorrhizal & other fungi
Distribution of microorganisms along roots
Many soil microorganisms
utilise root exudates.
Microorganisms can be
beneficial (e.g. improving
nutrient availability) or
harmful (e.g. competition for
soil nutrients, or root
disease)
mucilage
20
5.0
15
4.5
10
% coverage
log cells mm-2
Bacterial colonisation of maize root surface
5
4.0
Root tip
Root hair Lateral root
zone
zone
Schönwitz and Ziegler 1989
Density (log 7 g-1)
Bacterial population in the rhizosphere of
different plant species
300
Rhizosphere
Bulk soil
200
100
0
Clover
Oats
Linum Wheat Maize Barley
Rovira and Davey 1974
Effects of P-solubilizing bacteria
Dry weight (mg) of lavender in alkaline soil
Treatment
0 rock phosphate
+0.5% rock phosphate
Sterile soil
97
99
+ bacteria*
133
227
[* Pseudomonas & Agrobacterium]
Azcon et al. (1976)
Effects of P-solubilizing bacteria and
mycorrhizal fungus
Dry weight (mg) of lavender in alkaline soil
Treatment
0 rock phosphate
+0.5% rock phosphate
Sterile soil
97
99
+ bacteria
133
227
+ Glomus
148
233
+ Glomus & bact.
293
403
Azcon et al. (1976)
Interacting rhizospheres: wheat and lupin with
separated or intertwining roots
Dry weight
g/pot
P uptake
mg/pot
Wheat
Lupin
Wheat
Lupin
Separated
20
33
19
42
Intertwining
38
28
42
41
Root systems
Horst and Waschkies 1987
Conclusions
• The rhizosphere is the interface between
soil and roots
• Its properties depend on many processes
in plants and soil
• A ‘healthy’ rhizosphere – in physical,
chemical and biological terms – is
fundamentally important in influencing
mineral nutrition of plants
Buckwheat:
Römheld
Nitrate uptake and pH increase
Apoplast
Cytoplasm
Plasma membrane
H2 O
H+
pH
H+
] OH- + R-NH2
[Or:]
[Nitrate reduction]
[OHNO3-
NO3-
+ R [Organic C]
pH ‘Balance-sheet’ only
Ammonium uptake and pH decrease
Apoplast
Plasma membrane
NH4+
Cytoplasm
NH4+
+ R [Organic C]
[assimilation]
pH
H+ + R-NH2
H+
H+
pH ‘Balance-sheet’ only
Rhizosphere pH of buckwheat with NO3 supply
200 kg N/ha
pH decrease due
to extrusion of
organic acids not associated
with NO3assimilation
Römheld 1986