Here is an elaborated discussion on:- 1. Introduction to Roots 2. Functions of Root 3. Initiation and Growth 4. Systems 5. Efficiency.
Introduction to Roots:
Roots as major vegetative organs supply water, minerals and substances essential for plant growth and development. Despite these vital contributions they are often, probably too often, taken for granted because they are not visible —unfortunately, “out of sight, out of mind.”
Research on roots is relatively limited, compared with that of other plant organs, due to a great extent to the difficulties involved in their study. However, there is more opportunity to promote plant growth by changing the root environment than by changing the shoot environment.
Air, water, and mineral phases of the rhizosphere (root environment) are relatively easy to alter by agronomic practices; soil temperature can be influenced by tillage and mulching, moisture by irrigation and nutrient status by fertilization. The shoot atmosphere of crop plants, on the other hand, is difficult, or practically impossible, to change. Root studies probably should be given more emphasis than is done at present.
Functions of Root:
Vigorous root growth is normally required for general vigor and growth of tops. If roots are damaged by biological, physical, or mechanical disturbances and become less functional, top growth will likely be less functional.
Roots serve the plant in the following important ways:
(1) Absorption
(2) Anchorage
(3) Storage
(4) Transport
(5) Propagation
They are also a primary source of certain plant growth regulators.
The absorption of water and minerals occurs primarily through the root tips and hairs, although older and heavier parts of a root absorb some. Older roots perform the necessary functions of substance transport and storage, analogous to substance transport to and from leaves by stems and branches.
Anchorage is more than just holding the plant in place. Roots themselves need to be anchored against the force exerted by terminal parts that penetrate dense soil zones.
Roots often serve as the primary organ for storage of food reserves, especially in dicots. Dicot roots are well endowed with cortex, pith, or similar parenchymous tissue (e.g., sugar beet, alfalfa, and other plants with fleshy roots). Grass roots are fine and, by comparison, have little storage capacity.
Roots of numerous species can be used for propagation because of their capacity to develop adventitious shoots and to store food reserves that support new growth. Many obnoxious weeds, notorious for this type of propagation, can resist eradication by tillage.
Roots are believed to be the primary source of the growth regulators gibberellins and cytokinins, which influence overall plant growth and development.
Root Initiation and Growth:
Root length results from elongation of cells behind the apical meristem; breadth greater than that of enlargement of apical cells results from lateral meristem or cambium formation, which initiates secondary growth from the cambium meristem. Length and girth growth is generally analogous to that of shoots.
However, lateral branching is not analogous, since it arises from the pericycle deep within older or differentiated tissues, a morphogenesis markedly different from the surface origin of branches from the apex in the shoot.
Based on ATPase enzyme activity indicative of a high metabolic rate characteristic of meristems, a sub apical meristem has been located a few millimeters from the root tip. ATPase activity in soybean roots was observed to begin near the tip and continue for 27.5 mm, with a maximum at 3.5 mm. Root elongation was greatest in a zone between 5.0 and 15.0 mm. The zone of differentiation, including root hairs, xylem, phloem, pericycle, and other specialized cells, began at about 15 to 25 mm. The faster a root grows, the greater the length of the zone of differentiation.
New cells from the root apical meristem may be partitioned to root extension or to renewal of the root cap. The root cap plays an important role in protecting the root meristem from physical damage during soil penetration and possibly in guiding its direction. The sloughed root cap cells also provide lubrication for the growing tip, substrate for microbes, and additional soil organic matter. The root cap also produces abscisic acid, a plant growth substance.
The root apical meristem differs from the shoot apical meristem in its relatively low DNA, RNA, and mitotic activity. In case of damage or decapitation, the quiescent center regenerates a new meristem and restores the geotropic character within 36 hr at a favorable temperature. Root extension and cap renewal can then continue as before.
Extension:
Root meristems are capable of continuous, indeterminate growth that results in root extension for potentially indefinite periods. Growth may occur for the entire growing season or longer, amounting to penetration of up to 2 m per season. Excised roots were found to grow for 40 to 50 wk, but only if the sucrose content of the medium was relatively low and the culture solutions changed frequently.
High sucrose levels promoted ageing and shortened root extension. Grasses (Agropvron) adapted to dry areas elongated at a rate of as much as 15 cm per wk. At 49 days there were large variations in total root length among species as follows- A. desertorum, 73.8 cm; A. intermedium, 72.5 cm; A. cristatum, 48.9 cm; and tall fescue (Festuea arundinacea), only 12.2 cm.
These values reflect genetically controlled morphological variations that impart differences in drought tolerance. Rate of root growth is generally believed to decrease with maturity. With soybean, total root lengths per unit of leaf area were 630 m. m-2 leaf area at the V6 stage, 1190 m. m-2 at V12R2, and 345 m .m-2 at V15R5. In another study soybean root length increased for 70 to 80 days, remained constant to 100 days, and decreased thereafter. Even though rooting density may decrease with maturity, rooting depth continues to increase until stage R7 in soybeans.
The decline of root density during pod fill in these studies appears to be of particular importance physiologically. Reduced mineral uptake at the time of maximum demand is indicated Senescence of the vegetative parts and redistribution of minerals and assimilate to fruits may be a consequence or may itself be the cause of reduced root growth. Also, the loss of roots means loss of new tips and meristematic activity in the root and probably a decline in cytokinin export from roots to shoots. The cytokinin decline may be the mediating mechanism in senescence.
Lateral Roots:
Lateral roots have their origin in meristems that form in the pericycle several centimeters from the root tip. The lateral or new root breaks through the endodermis and cortex as cell division and elongation push the new root tip toward the root surface.
In dicots lateral root formation is opposite the points of the xylem star (the pattern of xylem formation in a cross section of the root). The xylem star of sugar beet root has two points and hence two rows of lateral roots. A four- point xylem star in soybean root gives rise to four rows of lateral roots. Cotton roots have four, five- or six-point xylem stars.
Formation of lateral roots is under genetic control but is also highly influenced by environment.
Genetic control can result from three factors:
(1) Production of β-inhibitor in the root tip, which is related to apical dominance.
(2) Production of growth-promoting substances in the shoot, which are transported to the roots (e.g., auxin, thiamine, nicotinic acid, and adenine)
(3) A balance or interaction between growth-promoting and growth-inhibiting substances.
Injury to or removal of the root tip removes apical dominance and promotes lateral root formation. In cultured, excised root sections, auxin promoted formation of lateral roots on root sections of field bindweed. Carbon dioxide and gibberellic acid promoted lateral root formation, as did the so-called “stoppered bottle” effect, which is believed to be due to ethylene production but could be due to carbon dioxide.
Differentiation:
Specialized cells or tissues are first evident in the undifferentiated tips in the formation of root hairs, lateral extensions of epidermal cells. Root hairs may achieve a length of several millimeters and number 200. mm-2. Their life span is about 50 hr at moderate temperatures and less at high temperatures.
A new root hair zone a few centimeters in length is formed as new growth increments are produced. Root hairs produce mucigel, which invites microbial activity. Most important, theoretically root hairs provide an extremely large surface area to interface with large volumes of soil fractions for mineral uptake.
A few millimeters from the root tip the amorphous cells begin to differ in size, shape, and structure, becoming specialized or differentiated. The central or vascular cylinder, consisting of xylem and phloem tissues, is ringed by a specialized, one-cell-thick layer of cells, termed the pericycle. The thin-walled parenchyma cells of the cortex are bounded on the inside by the endodermis and on the exterior by the epidermis.
Dicot roots usually have the capacity to grow in diameter from the vascular cambium. A balance of auxins and cytokinins admitted at the basal end of root sections of radish was found to be essential for secondary thickening from the vascular cambium.
Either natural or synthetic auxin (including 2, 4-D), combined with cytokinin, was equally effective in stimulating vascular cambium activity and secondary thickening. In addition to losing root hairs, older, differentiated root sections may lose absorptive capacity by becoming suberized (impregnated with phenolic compounds).
Root Systems:
In a homogeneous and barrier-free rooting medium, which is rare or nonexistent in nature, root growth produces geometric configurations- a hemisphere, cylinder, cone, or inverted cone, depending on the genotype. This configuration and its components at any point in the life cycle are referred to as the root system.
Several factors contribute to characteristic differences in the architecture of root systems, such as fineness, branching habit, and geotropism. Soil factors also strongly influence root growth and the architecture of the system.
I. Dicots:
The root system of dicot species generally consists of a large, positively geotropic, primary root with fine branching laterals. Fineness increases with branching order, that is, tertiary branches are finer than secondary branches. Often the primary root (taproot) has so much secondary thickening that it obscures the fine secondary or lateral branches (e.g., carrot). Between the extremes of the typical taproot and the typical fibrous system (e.g., grass plant) are a number of intermediate types.
Species such as radish and turnip have an unusually large swelling or secondary thickening in the hypocotyl area of the primary root. The swelling of the taproot of sugar beet and carrot is more generally distributed along the length of the root. The inverted cone of these roots has a thick cortex adapted to carbohydrate storage.
The perennial forage legume alfalfa typifies a taproot system, whereas birds foot trefoil has a more or less branching taproot. Branching taproot systems are common in all legumes since they can be induced by soil barriers or injury to the primary root apex.
Early in the season lateral roots of soybean were found to be much less positively geotropic than the primary root. The angle formed by laterals with the subtending root was generally obtuse. Later in the growth cycle these and any newly formed roots became strongly positively geotropic, growing vertically from an acute angle with the mother root.
Mitchell and Russell (1971) described the ontogeny and rooting pattern of soybean in Iowa (group II) as having three definable phases:
1. Vegetative Growth:
From 0 to 31 days; a positively geotropic primary root to a depth of 5 to 60 cm and horizontal laterals, primarily in the top 10 cm of soil.
2. Pod Filling:
From 67 to 80 days; new and positively geotropic laterals, secondary and tertiary branching; nodules present on the primary root and coarser laterals; about 85% of the total root weight in the top 15 cm of soil.
3. Rapid Pod Fill:
From 80 to 100 days; primary root growth rate decline, lateral root growth on a strong, positively geotropic course to a depth of 120 to 180 cm; root weight increase in top 8 cm and below 120 cm.
II. Monocots:
Roots of monocots (grasses) are fine and lacks cambium for secondary thickening. Collectively these are referred to as a fibrous root system.
The monocot root system has two phases:
1. Seminal roots (also referred to as seed roots) emerge, along with the radicle, from the scutellar (first) node of the seed embryo axis. The scutellum (cotyledon) and subtending node remain embedded in the seed, hence the term seed roots. In wheat the coleoptilar, or second, node and the scutellar node remain in the seed, since emergence is from elongation of the second internode.
Therefore this node also contributes to the seminal root number. Seminal roots of wheat comprise the radicle and one to seven roots from these two seed nodes. Number varies widely with genotype. Differences in seminal root number appear to contribute to adaptation and competitive advantage, particularly in certain environments.
2. Adventitious roots, also referred to as nodal, coronal, or crown roots, emerge from the basal nodes of the grass shoot just below the soil surface. In grasses three to six nodes without internode growth form the crown, which gives rise to successive whorls of adventitious roots referred to as the crown roots. Since grass emergence is by elongation of the first internode or mesocotyl (in the case of wheat, the second internode), the crown is positioned near the soil surface regardless of seeding depth.
Mesocotyl elongation stops just below the soil surface due to the phytochrome-red light control mechanism in the emerging coleoptile. In maize aerial adventitious roots also emerge from four or more nodes well above the soil surface. These are usually referred to as the prop, or brace, roots.
The first nodal roots to appear on grasses were found to be fine and only slightly positively geotropic, growing downward at an angle of about 25° from horizontal and densely branched. Nodal roots that arise later are thicker and more positively geotropic, generally growing at an angle of about 45° from horizontal. The last-formed nodal roots are coarse and grow vertically.
These coarse, brace roots may produce fine branches upon entering the soil and become functional in absorption as well as anchorage. Other observations revealed that in maize the direction of growth was highly related to soil temperature surrounding the seed. The angle of radicle growth was 30° (from horizontal) at 18°C and 61° at 36°C.
The fineness of grass roots depends on the order of the internode of origin and, in the case of lateral roots, on the branching order. Very fine seminal roots arise from the scutellar node.
Whorls arising from successive nodes are progressively coarser and brace roots are probably 10 times the circumference of seminal roots. On the other hand, successive orders of lateral branches from these primary roots reverse the order of size and become progressively finer. Laterals will be long if the primary root is short and vice versa.
Contribution of Seminal and Crown Root Systems:
The question of the longevity and contribution of seminal roots to the total system seems unresolved. Although it is generally believed that seminal roots are short-lived and make a minor contribution, some studies have shown that they are long-lived and make a major contribution. Both views are correct, depending on the species and environment.
Pavlychenko (1937) observed that in a number of cool-season cereals under western Canadian conditions the seminal roots not only were important but were the sole root system because crown roots did not develop in drought years. Competitive ability early in the season was related to the development of seminal roots. For example, ‘Han- nachen’ barley at 80 days had 6.6 seminal roots per plant, compared with 3.0 for wild oat and 4.6 for wheat, and was the most competitive of the three.
Even in heavily wild oat-infested soils, a reasonably good crop could be expected from barley, which Pavlychenko attributed to an early competitive advantage due to more seminal roots. Later in the season, at 80 days, barley and wheat had 11 to 13 crown roots, respectively, compared with 17 for wild oat, a shift in number and probably in competitive ability.
Spacing of plants did not appear to affect the number of seminal roots, but close spacing drastically reduced the number of primary and secondary branches on crown roots. Total root length of plants in drilled rows was greatly reduced, compared with that of individually spaced plants.
Boatwright and Ferguson (1967) observed that the early tillering, phosphorus uptake, and grain yields of wheat were all significantly greater if plants had both seminal and crown root phases, since removal of either phase decreased their values. However, grain yields were greater from plants with crown roots alone than from plants with seminal roots alone.
Seminal roots of the perennial grass timothy were of little functional significance, since plants with adventitious roots alone performed as well as plants with both root phases. The absorptive capacity of seminal roots of timothy were estimated to be 50-fold greater than that of adventitious roots. Therefore the seminal roots might be expected to be important in the early growth phases.
The importance of seminal roots to wheat and certain other cool-season cereals is more apparent, since more seminals are produced because of the emergence from elongation of the second internode. However, the potential for crown root numbers is correspondingly decreased by one whorl as a result of one less node in the crown. The early advantage from more seminal roots seems to outweigh this loss.
There is general agreement that in maize under field conditions the seminal roots is short-lived and makes a relatively small contribution to the total because:
(1) The mesocotyl disintegrates after a few weeks, separating the seminals from the plant
(2) The magnitude of the weight, volume, and length of adventitious roots is enormous, compared with seminal roots.
Nevertheless seminal roots are important to maize, especially for early support. And the fineness and frequency of branching of the seminal results in high uptake efficiency, important in the early stages.
Root Efficiency:
While older roots are vital to the plant, absorption is severely reduced because:
(1) Root hairs are no longer present.
(2) Old roots often have deposits of phenolic substances.
(3) Older roots occupy exploited soil spaces.
The last is true for minerals but may not be true for water, which is periodically recharged.
New roots, primary or lateral, push into unexploited soil spaces and develop root hairs in large numbers, resulting in a large surface area. Successive whorls of crown roots showed optimal activity on successive dates, which was believed to be due to root impermeability with age and/or environment exhaustion.
In view of the large number, length, density (cm root length. cm-3 soil), and surface area of root hairs, they would seem to be the most effective component of the system for mineral uptake. However, root hairs are thought to be scarce under natural conditions in which roots are infested by mycorrhiza. The mycelia of mycorrhiza greatly increase active root surface and mineral uptake so the loss of root hairs to mycorrhiza may not be important.
Deep penetrating roots may grow into unexploited moist soil layers, which generally have a low content of certain minerals. On the other hand, new roots and branches near the surface find-a higher mineral content but in areas frequently low in moisture.
Since mineral nutrients, especially N and phosphorus (P), are usually concentrated in the plow layer, a plant that is frequently watered should not need deep rooting. In fact it might be better to invest the assimilate in fruits or harvest products, and under irrigation this is usually the natural course of development.
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