Chapter 3
Nutrient absorbing structures,
Roots
Scope of this lecture
? Root functions,for plant,soil and ecosystem
? Root structure and pathways for uptake of nutrients,
the symplast and apoplast
? Root architecture(结构 )and morphology(形态）
? Responses of roots and root systems to environmental
stress
? Root hairs and their functions
? Introduction to combined strategies for efficient
nutrient uptake
? Carbon costs of root processes [if there is time]
,Roots are natural selection’s design solution to the problems
of obtaining resources from a heterogeneous（异质，多相）,
porous（多孔的）,semi-compressible（半刚性的） medium
containing solid,liquid and gaseous phases.”
(Robinson 1991)
Fibrous roottaproot
Roots and root systems
Functions (for plants):
1.Uptake of soil nutrients
2.Uptake of water
3.Storage (e.g,carrots,radish,sugar beet)
4.Phytohormone（植物激素） production
5.Habitat for micro-organisms:
- symbiotic N2-fixing bacteria固氮细菌 (internal)
- mycorrhizal fungi菌根真菌 (internal & external)
- rhizosphere根际 bacteria & fungi (external)
6 days 10 days 17 days
Courtesy Mac Kirby CSIRO Land and Water
Roots,a dynamic system
Roots,a dynamic system
Cells dividing
& expanding
Width not to scale
Roots,a dynamic system
Main sites of uptake of
nutrients & water（根
毛区）
Cells dividing
or meristem
& expanding
or elongation
zone分生区
与伸长区
Roots and root systems
Functions (for plants):
? Uptake of soil nutrients
? Uptake of water
? Storage (e.g,carrots,radish,sugar beet)
? Phytohormone production
? Habitat for micro-organisms
? Anchorage & support
Many specialised root-types,e.g,
- aerial roots (from stems or branches)
- contractile roots (anchor seedlings)
- cluster roots
Roots and root systems
Functions (for soil & ecosystem):
? Structural stability
Lupinus albus
Hakea哈克木属 undulata
Proteaceae
Casuarina 木麻黄
属 equisetifolia
Casuarinaceae
Banksia拔克西木属 burdetti
Proteaceae
Cluster roots簇状根系
Roots and root systems
Functions (for soil & ecosystem):
? Structural stability
? Production of pores (when dead)
Courtesy Mac Kirby CSIRO Land and Water
Roots and root systems
Functions (for soil & ecosystem):
? Structural stability
? Production of pores (when dead)
? Source of C and other nutrients for soil organisms
? Source of organic matter
Distribution of organic material
Stevenson and Cole 1999
Aboveground(地上部 )
Belowground(地下部 )
Roots
Soil organisms
Litter layer(枯枝落叶层 )
Soil organic matter
Belowground Total
Hardwood forest
136 000
57 000
48 000
173 000
278 000
Tallgrass prairie
8000
136 000
6300
400 000
542 300
kg/ha
Global annual root turnover in different
ecosystems
Gill and Jackson 2003
Annual root turnover
(% of maximum root mass)
10
56
53
34
55
Trees,total root system
Trees,fine roots
Grasslands
Shrublands
Wetlands
Roots and root systems
Functions (for soil & ecosystem):
? Structural stability
? Production of pores (when dead)
? Source of C and other nutrients for soil organisms
? Source of organic matter
? Influence depth of water-table
? Potential for removal of toxic materials
(bioremediation)
Rooting depth of different plant groups
Root ?architecture?,strategies of different plant
species
Root distribution in two crop plants
Lucerne(紫花苜蓿 )
10 cm
Wheat
Fibrous root
tap root
Root distribution in deep free-draining entisols(新
成土 ) in Western Australia
Hamblin and Hamblin (1985)
Medics
苜蓿属
CloversPeasWheatLupins
200
Depth
(cm)
100
% of total root length per 10 cm
三叶草
? Root systems differ in total length,depth of
penetration,lateral spread,and root
density at each depth through the pedon,
Roots also differ in the form,thickness,
degree of branching,and development
root hairs and mycorrhizae,
? All these characteristics influence root
function,
Factors that affect nutrient uptake
Factors that affect nutrient uptake
It is important to have a large root surface area,
It is better to have many small roots and root hairs
then to have a few big roots,
For the same mass (or volume),halving the
radius doubles the area and quadruples(四倍 ) the
length,
? The volume of soil exploited can be increased by the
elaboration of root hairs (left),and by symbiotic
associations with ectomycorrhizal fungi (centre) or
vesicular-arbuscular mycorrhizae (right).
From Marschner,Mineral Nutrition of Higher Plants” 2nd Edition,Academic Press,1995
? To get immobile nutrients efficiently,roots must
rapidly extend the rhizosphere depletion zone with
minimal cost of energy and material,Thus length and
slenderness are desirable,but if the roots are too thin
they can not penetrates the soil.
? To save the vascular tissue,multiple branching,the
formation of laterals,enable new roots in fresh soil to
join the existing conductive tissue.
? Root hairs widen the depletion zone,and mycorrhizae
efficiently extend it.
Factors that affect nutrient uptake
? Most immobile nutrients are more available in surface
horizons because surface horizons are exploited most
effectively by densely spaced,fine,active growing
roots near the soil surface,with ample hairs or
mycorrhizae,
? But shallow roots are useless during drought,
? Shoots should penetrate deep to rescue mobile
nutrients and tap subsoil water,
Factors that affect nutrient uptake
Architectural & morphological adaptations to the environment
? Drought,extensive root systems
? Poor soil nutrient status,extensive root systems; long
root-hairs
? Soil compaction (pressure)
? Water-logging & poor aeration,aerenchyma (air channels)
Some plants are ‘plastic’,show short-term responses to
environmental stress
Growth of maize under drought stress
Stasovski and Peterson 1991
Day
21
34
Watered
Drought stress
Watered
Drought stress
Shoot
114
44
230
48
Root
40
28
106
45
Root/shoot
0.4
0.6
0.5
0.9
Dry weight (g)
Drew and Saker 1975
Nitrate supply and root growth of barley
1 mM
0.01 mM
0.01 mM
Depth of placement of N fertilizer and barley
root distribution in a sandy soil
Gliemeroth 1975
From Drew and Saker (1978) Journal of Experimental Botany 29,435-451
L a te ra l ro o t len g th (cm )R o o t
zo n e
U n ifo rm
s u p p l y
L o c a li s e d
s u p p l y
B a s a l 40 14
M id d le 27 332
A p ic a l 18 11
Localized P supply and root growth of barley
P deficiency and cluster root formation in white lupin
Rengel 2003
From Watt and Evans (1999) Plant Physiology 121,317-323
P deficiency and cluster root formation in white lupin
N
treatment
(mM)
Dry weight Root:shoot
ratioShoot Root
0.05 0.8 0.45 0.56
0.5 3.5 1.39 0.40
5.0 9.2 1.82 0.20
Investment of assimilates in roots is increased in
response to nutrient deficiencies
Soil compaction influences root growth
Simeon Materetchera
Increasing pressure
wheat
Solution culture Soil SoilGrowth medium:
The effect of soil mechanical impedance on
the seminal roots and laterals growth of barely
压力（ kPa）
0 50
Seminal root
length（ cm） 8.6 2.0
Distance of youngest lateral to the to
tap root tip（ mm） 30 4
laterals
Number of laterals on the one tap root 19 10
The number of laterals per cm tap root 3.5 6.7
Soil compaction influences root growth
Root length
Habib Nadian
Trifolium subterraneum
Root diameter
Mycorrhizal plants
Non-mycorrhizal plants
The plant on the
left was not
infected by
mycorrhizae,
The plant on the
right is infected by
mycorrhizae
? Soil penetrability depends on water
content and bulk density.
? Increasing the water content softens the
soil and helps penetration,except near
saturation,when poor aeration begins to
inhibit root growth.
Soil water influences root growth
Root hairs
? Can be up to 13 mm long (e.g,
barley & wheat)
? Grow to full size within a few
hours
? Short-lived
? Growth responds to external
factors
Bentgrass (Agrostis
tenuis) seedling
Raven et al,(1998)
Genotypic differences in root hair response to
varying phosphate level
No P
Low P
High P
DOR 364 G 19833
XL Yan et al 1997
Phaseolus (bean)
Interactions,water,P supply & root hair density in
maize
Mackay and Barber 1985
0
Ro
ot
ha
ir
de
ns
ity
(n
um
be
r/
cm
ro
ot
)
500
1000
1500
P supply (mg/kg soil)
0 50 100 150 200
Water content
(%)
22
27
32
Strategies of P uptake in cowpea genotypes in a soil
with low P availability
Krasilnikoff et al,2003
Shoot biomass (g/pot)
Root length (m/pot)
Root hair length (mm)
Rhizosphere soil
(% of total soil volume)
Total P uptake (mg/pot)
Rate of non-Olsen P depletion
(pg P/mm3 d)
IT90K-59
19
984
0.23
3.0
56
13
Danila
7
81
0.38
4.5
24
0.2
IT82K-716
21
101
0.30
4.8
53
7
High P uptake
Long roots
Short root hairs
P mobilisation
High P uptake
Short roots
Long root hairs
P mobilisation
Low P uptake
Short roots
Long root hairs
No P mobilisation
- ignoring the mycorrhizal strategy!
Lambers 1987
Some costs,fate of photosynthates
translocated to roots (crop plants)
Exudation
Growth
Respiration
N2 fixation
Mycorrhizas
% of total C assimilated
5
11-35
12-29
5-23
7-10
Respiration:
1-4% for biosynthesis
10% for maintenance
10% for ion uptake
Costs relating to nutrient uptake
Conclusions
? Roots are the major site for nutrient uptake
? ?Efficiency? of uptake of nutrients relate to many aspects of
root structure & architecture
? Some plants respond to changes in soil nutrient levels by
changes in root growth & architecture
? This is one of a range of strategies for maximizing efficiency
of nutrient uptake,they all have ?costs?
? In general,efficiency of uptake depends on processes at
the root surface and in the rhizosphere (including the
?mycorrhizosphere?)
Nutrient absorbing structures,
Roots
Scope of this lecture
? Root functions,for plant,soil and ecosystem
? Root structure and pathways for uptake of nutrients,
the symplast and apoplast
? Root architecture(结构 )and morphology(形态）
? Responses of roots and root systems to environmental
stress
? Root hairs and their functions
? Introduction to combined strategies for efficient
nutrient uptake
? Carbon costs of root processes [if there is time]
,Roots are natural selection’s design solution to the problems
of obtaining resources from a heterogeneous（异质，多相）,
porous（多孔的）,semi-compressible（半刚性的） medium
containing solid,liquid and gaseous phases.”
(Robinson 1991)
Fibrous roottaproot
Roots and root systems
Functions (for plants):
1.Uptake of soil nutrients
2.Uptake of water
3.Storage (e.g,carrots,radish,sugar beet)
4.Phytohormone（植物激素） production
5.Habitat for micro-organisms:
- symbiotic N2-fixing bacteria固氮细菌 (internal)
- mycorrhizal fungi菌根真菌 (internal & external)
- rhizosphere根际 bacteria & fungi (external)
6 days 10 days 17 days
Courtesy Mac Kirby CSIRO Land and Water
Roots,a dynamic system
Roots,a dynamic system
Cells dividing
& expanding
Width not to scale
Roots,a dynamic system
Main sites of uptake of
nutrients & water（根
毛区）
Cells dividing
or meristem
& expanding
or elongation
zone分生区
与伸长区
Roots and root systems
Functions (for plants):
? Uptake of soil nutrients
? Uptake of water
? Storage (e.g,carrots,radish,sugar beet)
? Phytohormone production
? Habitat for micro-organisms
? Anchorage & support
Many specialised root-types,e.g,
- aerial roots (from stems or branches)
- contractile roots (anchor seedlings)
- cluster roots
Roots and root systems
Functions (for soil & ecosystem):
? Structural stability
Lupinus albus
Hakea哈克木属 undulata
Proteaceae
Casuarina 木麻黄
属 equisetifolia
Casuarinaceae
Banksia拔克西木属 burdetti
Proteaceae
Cluster roots簇状根系
Roots and root systems
Functions (for soil & ecosystem):
? Structural stability
? Production of pores (when dead)
Courtesy Mac Kirby CSIRO Land and Water
Roots and root systems
Functions (for soil & ecosystem):
? Structural stability
? Production of pores (when dead)
? Source of C and other nutrients for soil organisms
? Source of organic matter
Distribution of organic material
Stevenson and Cole 1999
Aboveground(地上部 )
Belowground(地下部 )
Roots
Soil organisms
Litter layer(枯枝落叶层 )
Soil organic matter
Belowground Total
Hardwood forest
136 000
57 000
48 000
173 000
278 000
Tallgrass prairie
8000
136 000
6300
400 000
542 300
kg/ha
Global annual root turnover in different
ecosystems
Gill and Jackson 2003
Annual root turnover
(% of maximum root mass)
10
56
53
34
55
Trees,total root system
Trees,fine roots
Grasslands
Shrublands
Wetlands
Roots and root systems
Functions (for soil & ecosystem):
? Structural stability
? Production of pores (when dead)
? Source of C and other nutrients for soil organisms
? Source of organic matter
? Influence depth of water-table
? Potential for removal of toxic materials
(bioremediation)
Rooting depth of different plant groups
Root ?architecture?,strategies of different plant
species
Root distribution in two crop plants
Lucerne(紫花苜蓿 )
10 cm
Wheat
Fibrous root
tap root
Root distribution in deep free-draining entisols(新
成土 ) in Western Australia
Hamblin and Hamblin (1985)
Medics
苜蓿属
CloversPeasWheatLupins
200
Depth
(cm)
100
% of total root length per 10 cm
三叶草
? Root systems differ in total length,depth of
penetration,lateral spread,and root
density at each depth through the pedon,
Roots also differ in the form,thickness,
degree of branching,and development
root hairs and mycorrhizae,
? All these characteristics influence root
function,
Factors that affect nutrient uptake
Factors that affect nutrient uptake
It is important to have a large root surface area,
It is better to have many small roots and root hairs
then to have a few big roots,
For the same mass (or volume),halving the
radius doubles the area and quadruples(四倍 ) the
length,
? The volume of soil exploited can be increased by the
elaboration of root hairs (left),and by symbiotic
associations with ectomycorrhizal fungi (centre) or
vesicular-arbuscular mycorrhizae (right).
From Marschner,Mineral Nutrition of Higher Plants” 2nd Edition,Academic Press,1995
? To get immobile nutrients efficiently,roots must
rapidly extend the rhizosphere depletion zone with
minimal cost of energy and material,Thus length and
slenderness are desirable,but if the roots are too thin
they can not penetrates the soil.
? To save the vascular tissue,multiple branching,the
formation of laterals,enable new roots in fresh soil to
join the existing conductive tissue.
? Root hairs widen the depletion zone,and mycorrhizae
efficiently extend it.
Factors that affect nutrient uptake
? Most immobile nutrients are more available in surface
horizons because surface horizons are exploited most
effectively by densely spaced,fine,active growing
roots near the soil surface,with ample hairs or
mycorrhizae,
? But shallow roots are useless during drought,
? Shoots should penetrate deep to rescue mobile
nutrients and tap subsoil water,
Factors that affect nutrient uptake
Architectural & morphological adaptations to the environment
? Drought,extensive root systems
? Poor soil nutrient status,extensive root systems; long
root-hairs
? Soil compaction (pressure)
? Water-logging & poor aeration,aerenchyma (air channels)
Some plants are ‘plastic’,show short-term responses to
environmental stress
Growth of maize under drought stress
Stasovski and Peterson 1991
Day
21
34
Watered
Drought stress
Watered
Drought stress
Shoot
114
44
230
48
Root
40
28
106
45
Root/shoot
0.4
0.6
0.5
0.9
Dry weight (g)
Drew and Saker 1975
Nitrate supply and root growth of barley
1 mM
0.01 mM
0.01 mM
Depth of placement of N fertilizer and barley
root distribution in a sandy soil
Gliemeroth 1975
From Drew and Saker (1978) Journal of Experimental Botany 29,435-451
L a te ra l ro o t len g th (cm )R o o t
zo n e
U n ifo rm
s u p p l y
L o c a li s e d
s u p p l y
B a s a l 40 14
M id d le 27 332
A p ic a l 18 11
Localized P supply and root growth of barley
P deficiency and cluster root formation in white lupin
Rengel 2003
From Watt and Evans (1999) Plant Physiology 121,317-323
P deficiency and cluster root formation in white lupin
N
treatment
(mM)
Dry weight Root:shoot
ratioShoot Root
0.05 0.8 0.45 0.56
0.5 3.5 1.39 0.40
5.0 9.2 1.82 0.20
Investment of assimilates in roots is increased in
response to nutrient deficiencies
Soil compaction influences root growth
Simeon Materetchera
Increasing pressure
wheat
Solution culture Soil SoilGrowth medium:
The effect of soil mechanical impedance on
the seminal roots and laterals growth of barely
压力（ kPa）
0 50
Seminal root
length（ cm） 8.6 2.0
Distance of youngest lateral to the to
tap root tip（ mm） 30 4
laterals
Number of laterals on the one tap root 19 10
The number of laterals per cm tap root 3.5 6.7
Soil compaction influences root growth
Root length
Habib Nadian
Trifolium subterraneum
Root diameter
Mycorrhizal plants
Non-mycorrhizal plants
The plant on the
left was not
infected by
mycorrhizae,
The plant on the
right is infected by
mycorrhizae
? Soil penetrability depends on water
content and bulk density.
? Increasing the water content softens the
soil and helps penetration,except near
saturation,when poor aeration begins to
inhibit root growth.
Soil water influences root growth
Root hairs
? Can be up to 13 mm long (e.g,
barley & wheat)
? Grow to full size within a few
hours
? Short-lived
? Growth responds to external
factors
Bentgrass (Agrostis
tenuis) seedling
Raven et al,(1998)
Genotypic differences in root hair response to
varying phosphate level
No P
Low P
High P
DOR 364 G 19833
XL Yan et al 1997
Phaseolus (bean)
Interactions,water,P supply & root hair density in
maize
Mackay and Barber 1985
0
Ro
ot
ha
ir
de
ns
ity
(n
um
be
r/
cm
ro
ot
)
500
1000
1500
P supply (mg/kg soil)
0 50 100 150 200
Water content
(%)
22
27
32
Strategies of P uptake in cowpea genotypes in a soil
with low P availability
Krasilnikoff et al,2003
Shoot biomass (g/pot)
Root length (m/pot)
Root hair length (mm)
Rhizosphere soil
(% of total soil volume)
Total P uptake (mg/pot)
Rate of non-Olsen P depletion
(pg P/mm3 d)
IT90K-59
19
984
0.23
3.0
56
13
Danila
7
81
0.38
4.5
24
0.2
IT82K-716
21
101
0.30
4.8
53
7
High P uptake
Long roots
Short root hairs
P mobilisation
High P uptake
Short roots
Long root hairs
P mobilisation
Low P uptake
Short roots
Long root hairs
No P mobilisation
- ignoring the mycorrhizal strategy!
Lambers 1987
Some costs,fate of photosynthates
translocated to roots (crop plants)
Exudation
Growth
Respiration
N2 fixation
Mycorrhizas
% of total C assimilated
5
11-35
12-29
5-23
7-10
Respiration:
1-4% for biosynthesis
10% for maintenance
10% for ion uptake
Costs relating to nutrient uptake
Conclusions
? Roots are the major site for nutrient uptake
? ?Efficiency? of uptake of nutrients relate to many aspects of
root structure & architecture
? Some plants respond to changes in soil nutrient levels by
changes in root growth & architecture
? This is one of a range of strategies for maximizing efficiency
of nutrient uptake,they all have ?costs?
? In general,efficiency of uptake depends on processes at
the root surface and in the rhizosphere (including the
?mycorrhizosphere?)
