FACTORS AFFECTING ROOT GROWTH AND DISTRIBUTION 

 
 
 
Crop Lv cm cm-3 b mm
Perennial ryegrass 25  1.1 
Wheat 10-5 1.8-2.5
Corn 6-4 2.3-2.8
Soybean 2-1 3.9-5.6
 
    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 (OMAFRA 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 primary laterals 
    cm
Length of secondary laterals 
   cm
Phosphate uptake 

     µM d-1

Phosphate uptake rate 
    µM g-1 d-1
14 HHH - - 0.17 220
LHL - - 3.76 857
21 HHH 20.9 6.3 0.44 101
LHL 83  490 16.4 316
 
    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
site 2
none 61.2              7.6 5.4               0.9 0.96
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 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 ? 
 

  • Compensatory growth of new laterals . 
 

Ammonia volatilization 

          NH4+ -> NH3 (Vol) + 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- +OH- 
    Anhydrous 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 cm3 )

Volumetric water content Cone resistance (MPa)
1.46 0.24 1.5
0.30 0.6
1.55 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)
27
Epidermal cells 
     : length (µm) 
     : diameter (µm)
164 
  31 
78 
56
Cortical cells 
     : length (µm) 
     : diameter (µm)
127 
  14 
67 
31 
 
 
  • 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: removal of root cap nullifies the effect. 
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 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 elongation reduced reduced
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 elongation less less
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
 
  This 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: 

    1. Gravity perception which is thought to be a physical process involving the falling of a statolith (amyloplast or starch grain). 
    2. A translation of the physical signal into a chemical substance within the apex. 
    3. Translocation of the chemical substances in the apex in an asymmetric fashion 
    4. 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. (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). 

 
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