FACTORS AFFECTING ROOT GROWTH AND 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 (OMAFRA Publ. 296)
Q What changes occur to roots as a result of variation in nutrient
conditions in the rooting zone ?
localized supply altering root distribution, (but cf root cluster formation eg proteoid roots of lupin).
Crops received no fertilizer or 60 kg P2O5 ha-1, or 60 kg N ha-1 + 60 kg P2O5 ha-1.
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 ?
Q What is the cause of soil acidification (ie why do soils become more acid) ?
Dolomitic - MgCO3
(Mn solubility is also very sensitive to soil aeration)
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 PO42-
Decrease 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 ?
Q Why are recommendations different for sands compared with loams?
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 ?
(g cm3 )
|Volumetric water content||Cone resistance (MPa)|
|Q How do roots respond to mechanical impedance ?
|Measure||Applied pressure (kPa)|
: length (µm)
: diameter (µm)
: length (µm)
: diameter (µm)
|Measure||Applied pressure (kPa)|
|Number per root||19.2 10.5|
|Number per cm branched root||3.5 6.7|
|length (mm)||5.0 9.0|
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.
|Root activity||Soil Temperature||Comment|
|Below optimum||Above optimum|
|Cell division||? reduced||reduced||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.|
|Cell radial expansion||increased||?|
|Cell maturation||Closer to apex for some cells, suberized closer to apex.
Slower for late metaxylem in wheat
|Closer to apex||These may largely reflect the change in cell elongation
Temperature effects can be expected because of effects on enzymes and enzyme systems.
|Root branching||depressed||depressed||unclear whether this is the result of the difference in length|
|Carbohydrates||carbohydrates may accumulate||limitations may contribute to reduced growth||At lower soil temperatures and fast rate of evaporation, can slow shoot growth|
|Nutrients||uptake may be slower||large NO3 supply may further decrease growth|
|Growth control substances||cytokinin production depressed||Almost certainly affected, especially if meristem activity changes|
|geotropism affected||geotropism affected|
Tropic Responses of RootsThe 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:
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. (33C) 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).
|If you would like to download this file please click|
Back to the Introduction