Although they are often ignored because of their “out-of-sight” nature, roots are an integral part of plants, playing important functions in regulating whole-plant growth. Studies demonstrate that maintaining a healthy root system is essential for managing turfgrass, especially under stressful conditions. Understanding the basics of root growth and function is helpful for developing effective management programs for maintaining high-performance turfgrasses.
Turfgrass roots have essentially three major functions:
Absorption: The primary function of roots is absorption of water and nutrients from the soil. Roots provide shoots with water and nutrients.
Anchorage: Roots anchor the plant in place to keep it from being washed away or blown away, or from being toppled.
Hormone synthesis: Roots are believed to be the primary source for cytokinins and gibberellins, which regulate shoot growth and development. The different root architectures or structures provide roots with efficient mechanisms for performing the various functions and controlling whole-plant growth.
Monocotyledonous plants (monocots), such as turfgrass, tend to have fibrous root systems. The growth and development of these root systems involve production of new roots, root elongation and branching. A typical fibrous system can be found in the grass family. A fibrous root system consists of one or several primary roots steming directly from the seed (seminal roots), as well as roots developed adventitiously from the lower stem nodes (adventitious roots, or crown roots). Both seminal and adventitious roots then produce lateral roots. Subsequently, any of these initial laterals may produce further laterals, and so on, for few or many further orders of branching (see “Turfgrass Roots,” page G6). These roots remain active for long periods, and some of them support the plant during the entire course of its life. Generally for the grass family, crown roots (adventitious roots) begin development by the three-leaf stage, and all root support to the plant prior to this stage depends on the seminal root system. Also, during different stages of plant development, each of the two groups of roots supports different allotments of the shoots. Seminal roots support mainly the primary shoot, although some support is also given to the tillers. Adventitious roots, however, are connected only with one or with very few of their mother tillers. Seminal roots are more important for the survival of whole plants than are adventitious roots.
Soil environments can alter root distribution. Soil resources (water, fertilizer) are often unevenly distributed. Many plant species respond to these unpredictable conditions with morphological and physiological plasticity of roots, which is their ability to exploit available resources by increasing root growth. It is now widely accepted that plants can alter root distribution patterns and rates of nutrient uptake when a localized supply of nutrients is elevated. Root plasticity plays an important role in plant adaptation to heterogeneous environments. Plants exhibiting rapid and highly plastic responses in root growth and development may, under certain circumstances, be at a selective advantage because they can rapidly utilize the available resources. Although the relative importance of altered root morphology (root density, length, root hairs, etc.) vs. uptake kinetics is still debatable, it seems clear that many plant species are capable of rapidly adjusting both their morphology and physiology in the acquisition of limiting essential resources that become available in a localized patch of soil.
Root distribution, which is influenced by root birth, growth and death, strongly responds to spatial variations in water availability. Phenotypically, roots tend to proliferate or extend in localized wet zones in a soil profile. Species also exhibit distinct rooting patterns, which can have evolved to a particular climatic region. For example, in hot deserts and other arid regions where soil surface is wet periodically due to sporadic, light rainfall during the growing season, it is common to find that many plants have extensive, shallow root systems that appear to be appropriate for the adsorption of water following light rain. Prairie plants and cold desert plants growing in environments where there is often relatively abundant water at depth may have very deep root systems. When soil is dry in the surface, production of roots increase considerably in the lower layer where water is available. The ability of roots to follow moisture into deeper layers of the soil profile conditions the ability of a plant to tolerate or avoid short and long periods of drought.
Sporadic light rainfall often occurs in many semi-arid and arid regions following a prolonged period of drought, which can lead to brief periods of high water availability, resulting in increased plant physiological capacity. Improved shoot growth after soil rewetting may be largely determined by the ability of the root to resume water and nutrient uptake. Therefore, how fast root growth and water uptake respond to re-supply of water following a period of drought stress is also an important aspect of growth plasticity of roots. Rapid regrowth of existing roots and production of new roots are important for rapid exploitation of water and nutrients following rainfall or irrigation events. This ability may be expected to confer superior productivity under transient drought conditions typical in semi-arid regions. The continued response to increased water availability is made possible by the appearance of new roots. In some species, new roots are produced within hours after rewetting. The new root growth increases the absorption rate and expands the root system, which increases contact with wet soil.
Plants in regions where rainfall events are short and sporadic may favor roots that can readily proliferate near the soil surface so that water can be captured before it is evaporated (e.g., hot deserts). Species adapted to relatively wet sites where mineral nutrients are strongly limiting and patchily distributed may also tend to build fine root systems. On the other hand, areas where there are more distinct wet and dry seasons may favor root systems which promote development of a deep root architecture, which should favor substantial investment in large-diameter framework roots that can sustain growth through dry surface layers.
Evidence that predictable patterns to the vertical distribution of nutrients (and non-limiting water conditions) can influence the evolution of root architecture is illustrated by genotypic variation among turfgrass root systems. Turfgrasses that have been selected for high performance under drought stress tend to grow a higher proportion of their total root system near the soil surface than less nutrient-efficient cultivars or wild species, even under uniform nutrient distribution. The more shallow growth can be caused by many factors, including more lateral root initiation near the soil surface.
ROOT FUNCTION IN WATER UPTAKE
Water uptake from the soil is a crucial function of a root system and largely determines the water status of the whole plant. Water uptake capacity depends on root morphological characteristics (e.g., root length density and root distribution) and physiological properties (e.g., viability, osmotic adjustment and hydraulic conductivity).
Water uptake rate of root systems generally is considered to be proportional to root length density (RLD). However, this relationship of total root length to water uptake may not hold true in some cases, depending largely on plant species, soil water availability and soil depths. Water uptake is positively correlated with RLD when soil is moist, but not well-correlated when soil is dry, especially when water is available only deeper in the soil profile. Under soil drying conditions, water uptake correlates better with rooting depth than with RLD. Deep rooting has been considered an important trait of drought resistance in various species. Development of a deep root system could be related to a faster elongation rate of roots under drying conditions. However, when limited soil water is stored in deeper soil profiles, faster root extension into deeper soil profiles may be detrimental for plants because of rapid depletion of water. In contrast, water will be conserved if the plant has a sparse and poorly permeable root system with a slow rate of extension. Deep roots not only enhance water utilization in deeper soil profiles but also appear to act as a water transport system and can deliver water absorbed from deep in the soil profile to the surface dry soil at night (hydraulic lift).
As soil dries, root hairs increase in length and number. Increases in root hairs in dry soil have a pronounced effect on total root surface area. This response may be an adaptive mechanism to maintain liquid continuity around the growing roots and to provide greater root surface for nutrient absorption, because the rate of nutrient diffusion to the root decreases in drier soil. Root hairs can be sites for extensive mucilage production. Mucilage can enhance the ability of the hair to attach to soil particles, and thereby prevent air gaps from developing between the soil and root surface when the soil dries; reduce water efflux from plants into drying soils; and ultimately delay root desiccation. Extensive development of root hairs enhances water uptake and facilitates water retention under soil drying conditions.
ROOT FUNCTION IN NUTRIENT UPTAKE
Nutrient uptake is also an essential function of the root system. Mineral nutrients required by plants enter entirely or predominantly via roots. Each essential nutrient element must be taken up in an amount equal to or above the minimum needed to produce the amount of biomass that the conditions of growth allow. Being able to acquire these nutrients in soil depends upon plant and soil factors. Plant root uptake mechanisms restrict and select ions taken up to some degree, but this regulation is not absolute. Shoot physiological processes and reactions in which mineral nutrient are involved, such as in energy metabolism, protein biosynthesis, and internal transport, may affect plant growth rates and, thus, nutrient acquisition. The rate of growth and activity regulates nutrient uptake by controlling nutrient demands. Various reactions and processes in soils determine the accessibility or the supply of nutrients. Nutrient acquisition also involves rates of solubilization relative to root growth and mycorrhizal development, interactions between various ion species, pathways and rates of movement through soil pore species.
Plants may access nutrients by root proliferation through the soil, but generally, for most nutrient uptake, mechanisms of transport to the roots are involved in meeting with growth needs. Nutrient acquisition is assumed to be governed primarily by nutrient availability in the soil. Similar to soil moisture, nutrient distribution is also uneven in the soil in natural environments. Roots must respond to the heterogenous distribution of nutrients in order to supply sufficient nutrients to the shoot. Nutrient uptake rate per unit root mass or length is often higher for roots in nutrient-rich patches than those in uniformly rich or uniformly poor soil or solution. Higher specific rates of uptake may be a function of both higher nutrient concentrations in the patch and higher uptake capacity of the roots in the patch. Roots growing in nutrient-enriched patches exhibit elevated nutrient uptake capacity than roots in un-enriched soil.
ROOT FUNCTION IN GROWTH REGULATION
As discussed earlier, root morphological and physiological characteristics play important roles in water uptake and supply to shoots. The plant is able to sense the water status in the soil and respond accordingly. Essentially, as soil moisture status declines, water uptake by the plant is reduced, leading to a reduction in the water content, water potential and turgor of leaves. Stomata and leaf growth react to these changes in leaf water status and turgor. Undoubtedly, this is the major mechanism by which plants sense large changes in soil moisture.
In recent years an increasing number of reports from several independent sources indicate that roots communicate to shoots information on soil moisture status by non-hydraulic means, which are mostly hormonal. You can see evidence of a root signal when one half of the root system is grown in a pot containing soil that is fully hydrated, while the other half is grown in a separate pot containing soil that has been depleted of moisture. In spite of the high leaf water status, reduction in stomatal conductance or leaf expansion is observed when a root system is partially exposed to drying soil, which illustrates that a root signal is evidence. Therefore, roots influence shoot water relations and growth not only by supplying water, but also by providing a feed-forward signal to the shoots. The interpretation of this accumulating evidence is that soil drying generates some kind of a chemical signal (“root signal”) that moves up the plant, probably through the transpiration stream, to regulate growth and physiology of the shoot.
Abscisic acid (ABA) in roots has been found to be the chemical messenger that mediates plant responses to drought. Stomatal closure can be induced by increases in leaf epidermal ABA content. ABA is synthesized in roots in drying soil, which move through the transpiration stream to the leaves within in a very short time, where it induces stomatal closure. It appears that as greater parts of the root system are exposed to a drying soil, more ABA is being produced by the whole root. Water-stressed roots accumulate ABA more quickly and with greater sensitivity than leaves. Roots seem to be able to “measure” the degree of soil drying and send a corresponding chemical message to the leaves where stomatal conductance and transpiration are reduced.
While ABA produced in the root may affect leaf growth via the effect on stomatal closure and gas exchange, it may have also a direct effect on growth processes independently of stomatal closure. ABA is well known for its growth inhibition properties. Growth inhibition of plants as a result of water-deficit stress may be a result of ABA accumulation, although turgor loss or reduced assimilation of carbon as a result of decrease stomatal conductance, or direct effects on photosynthesis could contribute. Nevertheless, the root signal constitutes an early alert system, before the shoot experiences any hydraulic effect of the drying soil. Such an early alert system as conditioned by root exposure to a drying soil may be important for the survival of plants and their species in natural habitats.
While ABA signals in controlling stomatal closure is the most widely reported response of plants to ABA, many other processes of growth and development can also be affected by ABA in the absence of stress, including leaf area, plant height and root development. For a plant growing in a water-limiting environment, which results in raised endogenous ABA levels, some of these responses to ABA would help to maintain plant water status by reducing the demand for water, such as reduced plant size.
Root systems are essential parts of a plant. They have diverse morphology and architecture, and serve a multitude of functions. Root morphological characteristics are genetically controlled, but can be modified by environmental factors. The environment surrounding root systems, such as water and nutrient status in the soil, is highly heterogeneous both in time and space. Root systems have the ability to react to the heterogeneity. They possess phenotypic plasticity, which allow efficient water and nutrient uptake. The degree of plasticity varies largely with plant species and root morphological characteristics. Roots react to soil moisture status not only by morphological plasticity through water uptake, but also can send chemical signals to shoots. Soil drying generates some kind of a chemical signal, commonly believed as abscisic acid, that moves up from roots to shoots, probably through the transpiration stream, to regulate growth and physiology of the shoot.
Bingru Huang is an associate professor in the Department of Plant Biology and Pathology at Rutgers University (New Brunswick, N.J.).
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