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FACTORS AFFECTING ROOT GROWTH AND DISTRIBUTION
Lv cm cm-3
b mm
25
1.1
Wheat
10-5
1.8-2.5
Corn (maize)
6-4
2.3-2.8
Soybean
2-1
3.9-5.6
Crop
Perennial ryegrass
Typical values of topsoil root-length density for some major crops, and the associated
half distance between roots assuming a uniform distribution.
Soil Chemical Properties
Nutrients (Marschner Sec 14.1, 14.2, 14.3, 14.4.1, pp 508-518)
Localized concentrations of nutrients may alter the form of a root system - nitrogen
and phosphorus have a marked effect, but not potassium.
Excessive concentration of fertilizer salts will restrict root growth due to osmotic
effects or specific toxicities such as with ammonia (NH3) or nitrite (NO2).
Safe rates have been established for fertilizers banded with or close to the seed
(OMAF Publ. 296)
Q What changes occur to roots as a result of variation in nutrient conditions in the
rooting zone ?
Morphological changes
root extension changed
localized supply altering root distribution, (but cf root cluster formation eg
proteoid roots of lupin).
Age of
plants
days
Nutrient
supply
Length of 1
laterals
Length of 2
laterals
Phosphate
uptake
µM d-1
Phosphate
uptake
rate
µM g-1 d-1
14
HHH
-
-
0.17
220
LHL
-
-
3.76
857
HHH
20.9
6.3
0.44
101
LHL
83
16.4
316
21
490
Effect of a localized supply of phosphorus on seminal root growth and uptake of
phosphate from that local zone by barley plants (after Drew and Saker, 1978). 50
µM-phosphate was supplied to three compartments (HHH) or to the central
compartment only (LHL) of divided plate. No phosphate was added to the solution
supply for the L compartments in the LHL treatment.
Nutrient effects on the relative growth of roots and shoots
Fertilizer
applied
Root weight
g m-2
Root length
km m-2
root/ shoot ratio
Site 1
mean
se
mean
se
none
79.3
7.6
9.0
1.1
1.08
P
107.3
9.7
10.4
1.9
0.69
N+P
122.9
7.0
11.3
1.3
0.41
61.2
7.6
5.4
0.9
0.96
site 2
none
P
95.0
15.3
7.7
1.2
0.75
N+P
115.6
9.5
8.7
1.7
0.72
Root growth and root to shoot ratio for barley grown at Jindiress and Breda in Syria
under dryland farming (after Gregory et al.,1984). Crops received no fertilizer or 60
kg P2O5 ha-1, or 60 kg N ha-1 + 60 kg P2O5 ha-1.
Effects on root hair development - production and elongation
Plant growth controlling substances - especially cytokinin production (N deficiency
results in decreased cytokinin production or at least decreased transport from the
roots). Evidence of aerenchyma formation in maize roots growing in limited N
supply.
Changes to membrane transport - uptake characteristics modified in zone with
localized enhanced branching.
Q What in evolutionary terms are the impacts of the changes to root growth ?
Benefits plant to grow more roots if nutrient supply is limited because it will explore a
larger volume of soil.
Compensatory growth allows efficient use of resource allocation.
Q What agronomic practices exploit the phenomenon of compensatory growth ?
Fertilizer banding
Deep placement in drought regions
Soil Acidity. (Marschner Sec. 16.3, pp 605-612 and 16.3.5, 16.3.6, pp 615-626)
The major causes of reduced root and shoot growth in acid soils are aluminum and
manganese toxicity. Direct effects of hydrogen ion concentration are of lesser
importance.
Aluminum toxicity affects primarily root growth whereas manganese toxicity affects
primarily shoot growth. Deficiencies of calcium, magnesium and phosphorus may also
be factors causing reduced growth on acid soils.
Solubility of aluminum in soil increases rapidly as soil pH decreases from 5.5 to 4.0.
The species of aluminum (A13+, A10H2+) also change. Solubility of manganese
increases as pH decreases but is also highly dependent on oxidation-reduction potential
in the soil.
Excessive aluminum inhibits root growth primarily by affecting meristematic activity.
Aluminum toxicity results in short stubby roots.
There are, at least in some species, close relations between aluminum toxicity and
calcium deficiency.
Excessive manganese affects shoot growth directly rather than root growth causing
chlorotic or necrotic spots.
Plant species differ markedly in degree of adaptation to acid soils through either
tolerance or avoidance mechanisms.
Q What do we mean by soil acidity ?
Exchange capacity is important for the rate of acidification
Q How do we measure soil acidity ?
Q What is the cause of soil acidification (ie why do soils become more acid) ?
Acid rain (sulphate, chloride, ammonium ions). This has had a big impact on
forests in the north, and throughout northern Europe.
Q Why has there been such an impact in the north of Ontario ?
acid parent material, soils are coarse-textured, and have low buffering capacity
NH4+ fertilizer application
Organic acids are released from plant roots, and by the breakdown of plant residues.
This is important in soil forming processes, and is an important factor in the history
of soils.
Tropical soils may have a pH as low as 3.5 to 4.0, and be very close to an equilibrium
value
Q How do we attempt to correct acidification ?
Application of lime(stone)
Calcitic - CaCO3
Dolomitic - MgCO3
The reaction of lime:
CaCO3 + 2H+  H2CO3  H2O +CO2 
Q How is plant growth affected in acid soils ?
The key elements are Al and Mn. Al will replace Ca and Mg on clays. Al oxides
and hydroxides bind phosphate, which is then removed from the soil solution.
Their solubility increases as pH drops.
(Mn solubility is also very sensitive to soil aeration)
Q Why does Al affect plant growth ?
Al is very toxic to the root meristem. There may be a link through Ca and Mg
uptake. Al blocks Ca channels in the plasma membrane, and membrane transport
proteins in the plasma membrane. and deficiency of Ca may be particularly
important for meristematic activity. Al may also affect root cap cells, which are also
a source of growth regulators. This may explain the impact on cell extension, as
well as on cell division.
Q How does Mn affect plants ?
Shoot growth is generally affected more than root growth. The classical symptom
of Mn toxicity is the development of necrotic lesions. These spots form because the
Mn is unevenly distributed within the leaf.
Strategies for plant adaptation to acid mineral soils.
eg Some plants such as tea accumulate Al, and growth is stimulated by it.
Detrimental aspects of fertilizer application (Miller & Ohlrogge)
Addition of NO3 or Cl- to soil changes the solution concentration, but other than plant
uptake there is no process immobilizing the ions. The effect of these ions on the
osmotic potential of the soil solution is greater than for NH4+ or PO42Decrease in osmotic potential in the soil solution  water moving from the root into
the soil  decrease in root turgor.
Q If root tip is killed by elevated salt concentration, what else can happen ?
Compensatory growth of new laterals .
Ammonia volatilization
NH4+  NH3  +H+
Occurs at pH > 7, but acidification results from volatilization
When urea fertilizer is added to soil pH can increase to about 10
CO(NH2)2 + 2H2O 2NH4 CO3  NH4+ + HCO3- +OHAnhydrous ammonia and DAP can similarly increase soil pH
Safe application of fertilizers
Q Why are recommendations different for sands compared with loams?
Soil water content differences
Q Why are recommendations different for barley and corn ?
Row width - greater for corn
NB values in Publ. 296 are not exact - they have not been fully researched!
PHYSICAL FACTORS AFFECTING ROOT GROWTH
Soil Physical Properties - Structure
The structure of soil has a major effect on root development. Roots may grow well
in soils with many large pores, or if the soil can be deformed easily. Roots may also
grow well if there are sufficient structural cracks or biopores, even when soil bulk
density is high.
Mechanical Impedance (Marschner pp 528-532)
Roots will not grow into rigid pores which are smaller in diameter than the apical
meristem of the root. They can however, exert considerable pressure to enlarge or
create pores where the rooting medium is weak enough to allow this to occur.
The ability of roots to develop in soil is determined by the size and rigidity of soil
pores.
Mechanical resistance to root penetration - soil strength - is determined by the
number, diameter and continuity of soil pores, inter-particle bonding and moisture
content.
When root growth is impeded there is an increase in the osmotic potential within the
cells. The increase probably occurs because of the reduced growth rather than a
physiological response to the impedance. Turgor pressure in the zone of cell
expansion may also increase (Clark et al., 1996).
Physical factors alone cannot account for the marked reduction in root elongation
produced by a relatively small resistance. There is good evidence of physiological
response mechanisms.
Impedance affects apical cells and their subsequent elongation. Elongation will not
return to the unimpeded rate until cells formed after the impedance is removed reach
the elongation stage.
Roots sense physical contact and react to it very quickly. A temporary reduction is
barley root elongation rate was observed for about 10 minutes after a root tip made
contact with a physical object. If the object offered little resistance, root elongation
increased to the original rate after about 20 min. If the root cap was removed, roots
were not sensitive to contact, suggesting an important role for the root cap in the
response to mechanical impedance.
Results from a number of studies suggest that changes in cell wall properties are
important in the response of roots.
Mechanical Resistance to Penetration
Q What are the origins of the mechanical resistance ?
The soil reacts to the compressive and shearing forces induced by penetration of the
root.
The reaction depends on the water content of the soil, how soil particles are arranged,
and how resistant is the bonding between particles
eg Impact of water content on the strength of a silty soil compacted to two densities in
the field.
Bulk density
(g cm3 )
1.46
Volumetric water content
1.55
Cone resistance (MPa)
0.24
1.5
0.30
0.6
0.28
3.0
0.30
1.9
Q How do roots respond to mechanical impedance ?
Increased cell turgor in expanding cells - but not in fully expanded cells
reduced extension, shorter cells
increased radial expansion of cells
eg Effect of mechanical impedance on cell expansion in barley plants 7 days old.
Measure
Applied pressure (kPa)
0
27
Epidermal cells
: length (µm)
: diameter (µm)
31
: length (µm)
: diameter (µm)
14
164
78
56
127
67
31
Cortical cells
changes in orientation of cell division
root hair development
modification of root branching patterns
eg Effect of mechanical impedance on lateral root development in barley plants 7 days
old.
Measure
Applied pressure (kPa)
0
50
Number per root
19.2
10.5
Number per cm branched root
3.5
6.7
length (mm)
5.0
9.0
modification of normal development in laterals - eg sugar beet
Q What information is there about the mechanisms underlying these responses ?
recovery of roots after a period of impedance delayed
changes in the root cap cells
contact of root cap with a barrier causes rapid response with reduced elongation:
Modelling the Response to Mechanical Impedance
Hettiaratchi and O'Callaghan (J. Theor. Biol. 1974, 1978) proposed a model to
describe root extension under mechanical constraint. It is essentially an engineering
approach, and tends to reflect the changes rather than predict behaviour.
The first model was proposed by Greacen and Oh (Nature 1972). It assumed
that roots grown under mechanical constraint were not able to adjust their cell water
relations as efficiently as under water deficits. This model was not consistent with all
the data they published with the model.
No models have been developed that deal with the changes in the branching of
roots as well as the changes in cell expansion.
Soil Temperature (Marschner pp 532-535)
Root growth can be adversely affected by both sub- and supra-optimal soil
temperatures. Work with monocots has often been confused because the shoot
meristem remains below ground for a considerable time. Hence the effects on roots
may also include indirect effects due to differences in shoot growth between treatments.
At both high and low temperature the rate of cell extension is slowed. Changes in
anatomical features result from low temperatures eg lignification of late metaxylem
vessels. These observations suggest changes in enzymatic activity, possibly
influenced by changes in the formation of plant hormones such as ABA and cytokinins.
Root growth depends on the supply of carbohydrate from the shoot. In monocot
species the soil temperature governs shoot growth for a longer period than for dicots
because the shoot apex stays below the ground surface for the early stages of
vegetative growth rather than being lifted above the surface. In cool soils root growth
may be more constrained in monocots than dicots because the expansion of the shoot
is limited by soil temperature, whereas shoot growth in dicots will depend on air
temperature.
A summary of temperature effects on root growth
Root activity
Soil
Temperature
Below optimum
Above optimum
Comment
remova
Cell division
? reduced
reduced
Cell elongation
reduced
reduced
Cell radial
expansion
Cell maturation
increased
?
Closer to apex for
some cells,
suberized closer
to apex.
Slower for late
metaxylem in
wheat
Root elongation less
Closer to apex
Root branching
depressed
depressed
Carbohydrates
carbohydrates
may accumulate
Nutrients
uptake may be
slower
limitations may
contribute to
reduced growth
large NO3
supply may
further
decrease
growth
Growth control
substances
cytokinin
production
depressed
geotropism
affected
The length of the meristem
and zone of expansion will be
shorter. Changes in cell wall
extensibility may reflect as
much as be the cause of
these effects.
These may largely reflect the
change in cell elongation
Temperature effects can be
expected because of effects
on enzymes and enzyme
systems.
less
unclear whether this is the
result of the difference in
length
At lower soil temperatures
and fast rate of evaporation,
can slow shoot growth
Almost certainly affected,
especially if meristem activity
changes
geotropism
affected
Aeration is dealt with as a separate topic.
Tropic Responses of Roots
The direction in which roots grow is clearly important to the plant. It determines the
extent and distribution of the root system and hence the efficiency with which water and
nutrient content of the soil is exploited. Hence it is not surprising that the direction of
root growth is closely regulated.
The main root axes of a plant generally grow in a downward direction - positive
gravitropism - although examples of upward growth - negative gravitropism - exist
(eg."breathing roots", of swamp plants). Lateral roots, however, grow in a more
horizontal direction.
The change in direction of root growth occurs because of differential elongation of
cells in the zone of root elongation. Curvature occurs because of one or a combination
of decreased rate of elongation of cells on the lower side of the root or increased rate of
elongation of cells on the upper side.
Gravitropic response occurs as a chain of four processes:
gravity perception which is thought to be a physical process involving the falling of a
statolith (amyloplast or starch grain). a translation of the physical signal into a
chemical substance within the apex. translocation of the chemical substances in the
apex in an asymmetric fashion a response of the cell growth to the chemical
substance.
The first two steps appear to occur in the root cap in which amyloplasts are
prevalent. The cap also appears to be the site of production of the growth substances.
IAA, ABA and ethylene have all been suggested as the growth substance involved. All
have been shown to be distributed non-uniformly across the root under gravity
stimulations.
Calcium appears to be involved. Under a gravitational stimulus, Ca moves to the
lower side of the root cap. This concentration of Ca may alter the action or
concentration of the growth inhibitor. The presence of mucilage appears to be
essential to the movement of these substances.
Temperature appears to influence the geotropic response of roots, at least of corn.
Corn roots grow in a more vertical direction when exposed to a high temp. (33C) for a
short time period (see Sheppard and Miller). These effects may explain several
observations on root distribution in the field. For example research in France has
concluded that soil temperature at the time of emergence (or shortly after) of nodal roots
of maize (corn) accounted for difference in root trajectory between location, year,
sowing date and presence or absence of mulches. Roots that emerged in cool soil
grew in a more horizontal direction than roots that emerged in a warmer soil. (Tardieu
and Pellerin 1991. Plant and Soil 131:207-214).
Hydrotropism, ie curvature toward a zone of greater water content, has been
suggested. However, much greater vapor pressure gradients than occur in soil are
required to cause curvature. The observed curvature toward higher soil moisture may
be explained, at least in part, by the effect of temperature on gravitropism.
There also appears to be a mechanism for control of the direction of horizontal
growth. Horizontally growing roots of maple (Wilson 1967, Botanical Gazette
128:79-82), and corn seedlings (Bandara and Fritton 1986, Plant and Soil 96:359-368)
resume the original direction of growth after being deflected by a barrier. This
response has been called "exotropy". Little is known about the mechanism for this
response. Konings has reviewed research on geotropism since the 1950's (Konings, H.
1995. Gravitropism of roots. An evaluation of progress during the last three decades.
Acta Botanica Neerlandica 44:195-223).