Factors Affecting Root Growth and Distribution | Botany

Differences in rooting habit, although inherent, are also highly influenced by soil environment, both directly and indirectly. Above-ground factors affect­ing shoot growth, especially the transport of carbohydrates to the roots, can have a major effect on root growth, as can rhizosphere factors (e.g., moisture, temperature, nutrient levels, toxic substances, soil strength, and biological agents).

Factor # 1. Genotype:

Large differences in rooting exist among genotypes, as does the opportu­nity for breeding and selection. Apparently most root charac­teristics are quantitatively inherited, that is, controlled by a number of genes. These inherent differences then interact with the soil environment. 

Large and highly heritable differences were observed in the ratio of branch roots to main roots of maize inbred lines, Sorghum roots were more geotropic, with more secondary branching. ‘Harosoy 63’ cultivar of soybean had a more extensive root system and twice the root surface of ‘Aoda’.

Depth distribution of roots varies widely among forage species. Compared with bluegrass (Poa travialis) and perennial ryegrass, orchardgrass had 20 to 30% less root mass in the top 10 cm and a greater percentage of roots in deeper strata.

The mechanisms of genetic control of rooting are complex, but as in the shoot, the action of growth hormones is indicated. Auxins (IAA) promote root growth but only in low concentrations. The requirement for auxin is evidenced by the leaf factor that is necessary for the rooting of cuttings. Many species require some leaf tissue or an active bud, apparently to produce the diffusable growth-promoting subtance(s).

Girdling the stem piece nullified the leaf factor effect. A rooting cofactor, isolated and identified as catechol and pyrogallol, acted synergistically with IAA to pro­mote rooting. Ethylene, produced during germination of certain species, re­tarded root growth; cytokinins tended to inhibit rooting.

Roots of stressed plants were lower in cytokinins, which suggests that the reduction in cytokinins and re­duced supply to leaves may contribute to drought senescence of stressed leaves. Auxin, gibberellin, and cytokinin acted independently or in combina­tion to control root growth in radish. It seems evident that growth hormones constitute the “chemical messenger” in the ex­pression of genotype.

Factor # 2. Plant Competition:

The competitive advantage of barley over wild oats is due, at least in part, to the greater number or density of barley seminal roots. In drilled rows of ‘Marquis’ wheat, for example, the number of crown roots was greatly reduced, from 73 to 12, compared with spaced plants; the total root length was reduced from 70,000 to 900 m per plant. Competition from close spacing had only a minor effect on the seminal roots.

Increasing the plant population of maize from about 12,000 to 62,000 plants per ha resulted in a 72% decrease in root dry weight per plant. However, the total root weight per ha increased up to a population of about 50,000.

The competitive ability of many species appears to be due to the secretion of toxic or inhibitory substances by roots, a phenomenon known as allelo­pathy. A thick, vigorous tall fescue sod is practically immune to weed encroachment, which has been attributed to root density and resulting root competition.

Root competition probably is an operative factor, but it has recently been demonstrated that tall fescue roots excrete allelopathic chemicals. Quite likely tall fescue is allelopathic to competing species but not to itself, allowing dense sod formation from its own roots while suppressing growth of competitors.

Factor # 3. Defoliation:

The old saying “To prune the shoots is to prune the roots” is a valid assessment, since roots are dependent on shoots for assimilate. Pruning the top of sudangrass periodically at 10 cm reduced root weight by 85%. Pruning the roots also reduced root and top growth. Clipping at frequent intervals significantly reduced root weight of blue panicgrass.

The effect of clipping varies with species and is related to the amount of photosynthetic area remaining after defoliation, which may still maintain a critical leaf area index (95% of light absorption). For example, close and continuous clipping can be practiced on a low-growing species such as creeping bentgrass.

Most perennial species exhibit cyclic rooting, that is, annual dieback and regeneration of a portion of the root system. As defoliation occurs in autumn from frost the root zone is still warm and supportive of respiration, which exhausts food reserves and results in root dieback.

The annual cyclic dieback is a plausible explanation for the high humus content under tall grasses of prairie sod, since the top growth of species in this ecosystem is killed by frost and the fine roots have little food reserve storage. A cool-season species like orchard- grass exhibits less dieback and regeneration and appears not to build humus content so rapidly.

Factor # 4. Soil Atmosphere:

The atmosphere of roots is usually quite unlike the atmosphere of shoots. Both oxygen (O₂) and carbon dioxide (CO₂) levels in the rhizosphere may differ greatly from ambient atmosphere and both can have a direct effect on root growth. Generally, the effect of either one is modified by the presence of the other. Nitrogen is an inert gas and has no negative effect.

Oxygen is essential for metabolic processes, including active absorption and transport. In soybean, the O₂ requirement and nutrient uptake were greater during vegetative stages than during reproductive stages. Soil water removal by barley roots was increased by increasing O₂, which suggests that water uptake is active or perhaps additional roots are stimulated.

Some species, such as rice, can absorb ade­quate O₂ through the leaves and transport it to the roots via the aerenchyma (air cells); therefore O₂ is not always required in the rhizosphere. Maize was observed to have this capability but evidently not suffi­ciently for normal growth under prolonged flooding. Oxygen in the rhizo­sphere has indirect effects, such as stimulation of microorganism activity, which in turn influences nutrient availability to roots. Some CO₂ is evidently beneficial for optimum root growth.

Concentrations of CO₂ up to 2%, or nearly 10-fold greater than ambient atmosphere, stimulated root growth of barley and peas, but 8% retarded root length. The effect of CO₂ depended on the partial pressure of O₂ in the root atmosphere. In general an O₂ level one-third that of normal air (21%) was adequate for growth, unless the CO₂ concentration was too high.

Factor # 5. Soil pH:

Soil pH outside the range of 5.0-8.0 has a potentially direct effect in limiting root growth; within this range, as occurs under most field conditions, the effect is usually indirect. Soil pH of less than 6.0 increases the solubility of aluminum, manganese, and iron, which can be toxic and limit root growth.

Plant breeders have been successful in selecting aluminum-tolerant lines in a number of crops. Tolerant lines raise the pH in the immediate vicinity of the root. Species and cultivars vary in capacity to alter the pH of the immediate root environment.

Factor # 6. Soil Temperature:

Optimum temperatures were generally lower for roots than for shoots, which is consistent with natural growth; during spring root temperatures under a sod or vegetation are lower than the above-ground tem­peratures. Temperatures optimal for species growth vary widely, of course.

Increasing the root temperature by hot water pipes favored warm-season grasses, such as sudangrass, more than cool-season grasses, such as tall fescue. Temperature affected the growth of roots more than the growth of shoots. The direction of growth is highly related to temperature, as indicated previously.

Factor # 7. Soil Fertility:

Roots require adequate mineral nutrients for growth and development, as do other plant parts. Because they are closer to the source than shoots, roots have the first opportunity for minerals and water, although they have the last opportunity for assimilate formed in the shoots. For this reason, a water or mineral deficiency generally affects roots less than tops (decreasing the S-R ratio) unless it interferes directly with photosynthesis (e.g., an iron deficiency, which reduces chlorophyll). A light deficiency also interferes directly with photosynthesis, resulting in shoot priority (increasing the S-R ratio).

Generally fertilization favors expression of the inherent tendencies of roots. Maize roots tend to proliferate in zones containing organic matter and fertilizer (e.g., a fertilizer band), particularly if the band contains N and P. Brouwer (1966) suggested that the determining factor is not the presence of fertilizer elements in the root environ­ment per se but rather the overall nutrient status of the plant. Fertilized maize in a Muscatine soil rooted to a depth of 1.7 m, compared with 1.4 m for unfertilized maize.

Roots that contact a fertilizer band may become injured, exhibit deformi­ties, and be shorter than untreated roots. Apparently, seminal roots and first order branches are deformed or killed in a fertilizer band, or by other chemicals in sufficient concentration to be toxic, but higher orders of roots may proliferate more as the fertilizer concentration in the band declines with time.

Increasing the N level favors top growth in relation to root growth, that is, increases the S-R ratio. Thus high N may allow top growth to usurp available carbohydrates; the increased top growth causes more shading of lower leaves, which further aggravates the situation. In addition, a greater N supply tends to increase the auxin levels, which may inhibit root growth.

However, N fertilizer increases the total dry weight of roots. Cereal crops that have a high early level of N and a diminish­ing concentration as the season progresses generally produce a greater leaf area early and more photosynthate later for the roots. Nitrogen-fertilized maize plants have been observed to have a greater root development and to use considerably more water in drought conditions. Nitrogen fertilization seems to promote deeper and more profuse rooting early in the season, probably due to increased leaf area and more assimilate for root growth.

Phosphorus-fertilized plants develop more roots than do unfertilized plants, but this is probably not a direct effect; P availability first increases photosynthesis, which in turn increases root growth. Also, extracts from P- fertilized roots had less auxin activity and theoretically less inhibition than extracts from N-fertilized roots. However, P caused a direct increase in root hair proliferation. Phosphorus did not have to be at the site of growth to provide normal development, which suggests that P in the subsoil may have no advantage over P in the surface layer in promoting deep rooting.

Considerable work has been done to evaluate the best N-P ratio in fertil­izer mixtures, especially for applications made at planting. A 1:5 ratio of N to P in a fertilizer band appeared to be optimum for maize root development.

Potassium (K) seems to have no direct effect on rooting in either elonga­tion or branching. It is, however, important to certain physiological functions of the root; inadequate K may cause a weak translocation system, poor cell organization, and loss of cell permeability. In general the effects of K and other fertilizers are primarily indirect, increasing root growth only after increasing top growth.

Weaver (1926) was interested in the potential benefit of deep fertilization to stimulate root growth. Experimental work since then has produced both positive and negative results. However, deep fertilization experiments have most commonly been done in conjunction with deep tillage, and the effects of each have not been clearly separated.

Factor # 8. Water:

Water is essential for root growth, evidenced by the fact that roots do not grow through dry soil layers. However, roots have what might be regarded as a water-stress adjustment mechanism whereby solutes accumulate in the tip and elevate the turgor pressure, which can sustain growth for a limited time. Soil moisture stress significantly reduced root weight of blue panic grass, and root length of soybean was significantly reduced by water potentials less than —2 bars, or 16%.

A moisture-deficient soil also modified rooting patterns: a smaller per­centage of the total roots were found in the surface layer (0—15 cm) and a higher proportion in deeper strata. Irrigation reversed this trend.

Factor # 9. Mechanical and Physical Forces:

Roots encounter mechanical resistance to growth from a variety of causes, such as particle size, lack of aggregation, soil strength, and compac­tion. Decreasing porosity or increasing bulk density decreased root growth. Root penetration and proliferation were greater in undisturbed soil cores of less than in cores of three soils higher in clay content and bulk density. Root growth was not confined to interpedal spaces, however, suggesting that soil strength affects root entry into soil space. High soil bulk density, as in compacted sandy loam soil, greatly reduced the root growth of tobacco (Table 10.3).

Maize roots were observed to penetrate about 2 m in a coarse-textured glacial till but seldom more than 1 m in fine- textured soils. Increasing the bulk density of the soil medium from 1.65 to 1.96 g. cm^-2 not only reduced soybean root growth but altered the root anatomy by increasing the thickness of cell wails and casparian strip and by distorting the shape of the central cylinder. Such anatomical aberrations indicate impairment of the absorption function. Root density of maize and cotton was related to soil strength, and the relationship of rooting density to absorption of soil moisture was linear.

Machinery rolling between plant rows caused compaction and reduced water availability; the first pass with machinery accounted for the greatest percentage of compaction. Whether compaction reduced root growth and water uptake by mechanical impedance, by reduced root growth due to reduced O₂ supply, or by reduced active water absorption due to re­duced O₂ was not clarified.

Some researchers attributed restricted root growth in compacted soils to mechanical impedance. Others have pointed not only to mechanical impedance but to lack of O₂. While both factors have direct and indirect effects, a limiting O₂ supply appeared to be more important over a wider range of conditions.

The fact remains that low porosity or high bulk density due to soil compaction causes restricted root growth and function. Much research effort has been given to methods of shattering com­pacted or hardpan areas, but results have been variable and benefits usually short-lived.