Plant Physiology - Musiyenko, M. M. 2001

Plant Root Nutrition
The Root System as an Organ of Absorption and Metabolism

In Plant ROOT Nutrition, just as in Photosynthesis, one of the most striking Properties of the plant Organism is manifested—its autotrophy, that is, The ability to build its body from inorganic substances. Moreover, plant nutrition itself ensures a constant cycle of matter and energy transfer, closely linking the mineral and living worlds.

The root system, serving as a specialized organ for Water absorption, also ensures the uptake of Mineral Substances. The function of the root is not limited solely to absorbing and transporting substances to the above-ground Organs, as the root system is also an organ of active and specialized METABOLISM. Finally, the root ensures the release of substances of various nature and biological significance into the environment. The physiological Functions of the root are closely related to its Anatomical Structure. A formed root system is a rather complex organ with a well-differentiated internal structure (Fig. 114).

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Fig. 114. Root structure diagram: a — root cap, b — meristem zone, c — elongation zone, d — root Hair zone, e — central cylinder; 1 — pericycle, 2 — immature phloem elements, 3 — mature phloem elements, 4 — endodermis without Casparian strips, 5 — immature xylem elements, 6 — rhizodermis, 7 — primary cortex, 8 — endodermis with Casparian strip, 9 — mature xylem elements, 10 — root hairs

Differentiation of root Cells begins in the meristematic division zone. In the elongation zone, these processes accelerate—the rhizodermis (epiblem), the first conducting elements of the proto- and metaphloem, pericycle, etc., appear. The division and elongation Zones of the growing root are the most active zones for the absorption of water and mineral substances. The rhizodermis forms root hairs on the outer surface of its cells. A root hair grows by its tip, where metabolic processes are most active and where nutrients are constantly supplied. The root hair zone is the most active absorbing part of the root.

As the rhizodermis with root hairs dies off, a new protective tissue—the exodermis—arises On the surface of the root from the primary cortex. The Cells of the primary cortex carry out active and passive Transport of substances to the central cylinder of the root, and also perform the function of synthesizing and accumulating various reserve substances. Along the path of substances coming from the soil solution, There is a series of barriers (Fig. 115).

Fig. 115. Radial Symmetry of the root: 1 — rhizodermis, 2 — root hair, 3 — exo-, mesodermis, 4 — endodermis, 5 — Casparian strip, 6 — pericycle, 7 — phloem, 8 — xylem

The first of these is the rhizodermis, and the second is the inner layer of primary cortex cells—the endodermis, the lateral walls of whose membranes have special thickenings—Casparian strips, impregnated with Lignin and suberin, which block the movement of the solution. However, they are not present in all endodermal cells; among them, passage cells are encountered that are unlignified and located in the zone of lateral roots; through them, ion transport occurs without hindrance. The endodermal zone is a physiological barrier where the protoplast "controls" the flow of ions.

The function of the endodermis is not exhausted by this, as it also delays and regulates the influx of substances from the central cylinder into the peripheral Tissues. Another barrier on the path to the central cylinder of the root is the pericycle, consisting of active meristematic cells. The pericycle is the outer layer of the axial cylinder. Various substances accumulate in it, including phytohormones that stimulate The formation of lateral roots.

Thanks to such physiological barriers, the solution entering the root stele is significantly transformed and therefore differs from the initial soil solution. "Semi-finished products" are synthesized in the root, which are later easily incorporated into BIOSYNTHETIC PROCESSES IN the stem or leaves.

The central cylinder of the root is characterized by a radial arrangement of the primary conducting tissues—phloem and xylem (Fig. 116). Thus, the Structural Features of the root largely determine

Fig. 116. Main zones of the root: A — root zones, B — localization of primary root Meristems

its functions. According to K.M. Sytnyk, plant leaves and roots differ polarly in oxidation-reduction potential tension. Leaves have low tension, while roots have the opposite. Maintaining this potential at a level corresponding to the normal vital activity of plants is ensured by the appropriate exchange of products from their own synthetic function.

Back in 1949, D.A. Sabinin substantiated THE CONCEPT OF the synthetic function of the root: the root not only absorbs mineral elements but also transforms them during transport into the above-ground organs. The synthesizing activity of the root is based on assimilates coming from photosynthetic organs. His proposition regarding the synthesis of physiologically active substances (Cytokinins) in roots was confirmed.

The cycle of matter in a plant is a single link in root nutrition, and it is strictly controlled by the requirements of the plant and the nutrient sources available to it. This cycle is associated with both the absorptive and Excretory Functions of the root. It has been proven that almost all types of water-soluble compounds are excreted through the root system. The question of the ecological expediency of such secretions naturally arises. These secretions can be an adaptive response to changing environmental conditions for the rhizosphere and soil microflora, and a portion of them is reused by the same or a neighboring plant organism of the biocenosis. They lie at the root of The phenomenon of allelopathy (mutual influence) of plants that are part of the phytocenosis. Active soil allelopathic agents are triterpenes: erythrodiol and serratiol. It is worth noting that such toxic plant substances as Chalcones and dihydrochalcones (phloridzin) are rapidly destroyed in the soil, losing their activity.

Even the concept of allelochemicals has emerged, i.e., products arising from the interaction of plants and soil that can be used as a form of natural herbicides. The creation of a natural herbicide system can become one of the biotechnological models that will avoid The Use of synthetic herbicides with increased toxicity in crop production.

It has been proven that various plants exhibit Specificity in root secretions, which is often associated with the so-called soil fatigue.

Soil Fatigue

Long-term cultivation of a single plant species in the same place causes an effect manifested in a sharp decrease in plant productivity. One of the many causes of this phenomenon is the excessive development of microorganisms that produce various phytotoxins. The biological and Chemical Nature of phytotoxins has not yet been finally clarified. It is known that these substances accumulate in fertile, organic-rich soils.

Phytotoxins belong to BIOLOGICALLY ACTIVE SUBSTANCES and have an oligodynamic effect. In small doses, they can completely or partially inhibit The Development of soil organisms or plants. Phytotoxins of microbiological origin have a diverse chemical nature and varying physiological effects.

The general biological group of mycotoxins includes the so-called aflatoxins (named after the producer fungus Aspergillus flavus). Regarding animal organisms, aflatoxins have a carcinogenic effect. To a certain extent, they also affect plants, in particular acting as antihormones relative to gibberellin and auxin, or as Enzyme Inhibitors, and also causing disruptions in RNA informational activity. It has been established that under The Influence of aflatoxin, the chlorophyll content in green plant organs can decrease.

The phenomenon of soil fatigue is not observed in natural biocenoses. It is a typical sign of anthropogenic Changes in the soil ecosystem. Monoculture as an ecological factor disrupts natural processes on the scale of the soil ecosystem. The comparative COMPOSITION OF THE soil microflora of tired and natural soil indicates that many microorganisms in tired soils produce substances toxic to plants. Tired soil is characterized by the excessive development of microorganisms harmful to plants.

Research shows that 15–45% of microorganisms isolated from tired soils produce phytotoxins. Evidence of the indirect influence of factors leading to soil fatigue is the excessive development of Bacteriophages that lyse nodule Bacteria. Rhizobiophages destroy bacteria of the genus Rhizobium, reducing nitrogen-fixation productivity. Large crop losses in monocultures are caused by pathogenic organisms that multiply without significant limitations under these conditions.

Fatigued soil is unhealthy soil that exhibits signs of deep-seated pathogenic changes. Soil fatigue is one of the major challenges in global agriculture, currently monitored by the UN FAO. According to this food commission of the United Nations, approximately 1 million 250 thousand hectares of arable land consist of exhausted soil. As a result of soil fatigue, 25% of the total biological losses of the global crop yield are lost annually (P. Nadtochei, F. Volvach, V. Hermashenko, 1997).

Root exudates can alter the water regime of both the soil and the plant, Respiration intensity, Transpiration, carbohydrate assimilation, etc. The toxicity of secreted substances is most sensitively manifested in the rhizosphere zone.

Rhizosphere

A phenomenon of major importance for the Organization OF THE soil ecosystem, known as the rhizosphere effect, is closely associated with root systems. In the primary sense of the word, the rhizosphere is the root-accessible soil layer, which can be of varying depth. The Concept of the "microbial rhizosphere" emerged somewhat later and refers to the site of concentration and interaction of microorganisms with plants. This rhizosphere belongs to a narrow layer of soil surrounding root hairs, where microflora develops most intensively on the products of root exudates. Three zones are distinguished within the sphere of plant-microorganism interactions: the rhizosphere proper, the rhizoplane, and the histosphere. The rhizosphere zone is where microorganisms exert their effects on plants, whereas in the histosphere zone, conversely, the influence of root systems on microorganisms predominates.

One of the CHARACTERISTICS OF THE rhizosphere is the density of microbial populations, as well as the content of oligodynamic compounds. The quantitative ratio among organism species in the rhizosphere is formed on a trophic basis. Root exudates serve as an ideal nutrient substrate for autotrophic microflora. They are rich in various nitrogen and carbon compounds, Trace Elements, Amino Acids, Vitamins, Enzymes, and growth regulators. These are concentrated in the root zone of the soil, where the greatest species and numerical diversity of microflora has been established.

The interaction between root systems and microflora manifests as the so-called rhizosphere effect, expressed as The ratio of the number of Microorganisms in the rhizosphere (R) to those outside it (S). This ratio is not uniform for different plant species and depends on many factors, most importantly the plant species and soil conditions (Table 14).

Table 14. R/S ratio of soil organisms in the rhizosphere of winter wheat at various Stages of Ontogeny

The most common actinomycetes and fungi in the rhizosphere of agricultural plants include aerobic and anaerobic, autotrophic and heterotrophic, proteolytic and auxotrophic forms. Rhizosphere microorganisms require various biologically active substances for their development. At the same kiel, many of their species are capable of producing virtually all known physiologically active substances.

SOIL AS THE MEDIUM FOR Root nutrition of plants

Soil acts as an active medium for plant nutrition, consisting of organic, mineral, and organomineral components that yield plant-available nutrients under the influence of abiotic and biological processes. The latter constitute the primary component characterizing soil fertility. It is determined by the soil's capacity to supply plants with water, air (oxygen), heat (for roots), and favorable physical and physicochemical conditions for Plant GROWTH AND DEVELOPMENT. Soil fertility is its fundamental qualitative trait that distinguishes soil from rock and passive substrate.

During root nutrition, plants primarily absorb chemical elements from the soil (macro-, micro-, and ultramicronutrients), the supply of which depends on the exchange capacity of the solid-phase adsorption complex and the concentration of the soil solution. The production of Mineral nutrition elements for plants within the soil occurs mainly through the Mineralization of organic components and plant residues (ammonification, nitrification), Nitrogen Fixation, and the decomposition of mineral substances by microorganisms. The reserves of plant mineral nutrients in the soil are also replenished via atmospheric precipitation, surface and ground waters, and anthropogenic activity (application of mineral fertilizers). Applied organic fertilizers are likewise mineralized to form salts.

The cycle of substances in nature—both biological (assimilation of absorbed ions by the plant and their return to the soil with biomass) and geological—has a profound impact (both positive and negative) on the supply of nutritional elements. The latter is driven by the leaching of soluble substances via surface and downward water currents. Losses of plant mineral nutrients from the soil are facilitated by their transition into gaseous form (denitrification) and release into the atmosphere, as well as by water and wind erosion. A significant portion of nutrients removed with the harvest drops out of the minor biological cycle. For instance, the nutrient removal from the soil by field crops per 1 ton of production (kg per 1 ha) is as follows:

Therefore, maintaining a non-deficit nutrient balance in the soil is a crucial task for regulating the nutritional regime.

Soil fertility, and consequently the content of mineral and organic nutrients, varies zonally. From the northwest to the southeast of Ukraine, the following zones succeed one another: sod-podzolic soils, gray forest podzolized soils, leached typical soils, ordinary and southern chernozem, and chestnut soils combined with solonetzic and solonchak soils of the dry steppe. Among cultivated lands in Ukraine, chernozems and closely related meadow-chernozemic soils occupy the largest area at 72.7%, followed by podzolized soils at 12.1%, sod-podzolic soils at 7%, and fertile chestnut soils at 3.9%.

Polissya soils include sod-weakly podzolic sandy, clayey-sandy soils; sod-medium podzolic sandy loam and gleyed sandy loam soils. These soils contain low levels of nutrients, unlike meadow, meadow-bog, and bog soils, which have satisfactory to good nutrient supplies and represent valuable arable land.

Gray forest soils contain 2-4% humus, relatively high amounts of mobile phosphoric acid, moderate supplies of available potassium, and are deficient in mobile nitrogen.

The soils of the Forest-Steppe—podzolized, leached, and typical chernozems—vary in their nutrient availability. Chernozems of the forest-steppe and steppe zones generally contain abundant nutrient elements: N — 0.2-0.5%; P₂O₅ — 0.15-0.3%; K₂O — 2-2.5%. Soil reaction is neutral to slightly alkaline. Despite their high potential fertility, chernozems contain few nutrients that are readily accessible to plants. Consequently, fertilizers should be applied to such soils, and The activity of the soil microflora within them should be stimulated.

Among mineral fertilizers, phosphatic ones are of primary importance. To improve the agrophysical properties of chernozems, their lost structure must be restored and their natural structure preserved.

Chestnut and meadow-chestnut soils of the dry steppe zones contain 3-4% humus, with total reserves of phosphoric acid at 0.1-0.2% and potassium at 1-2%. The soil reaction is slightly alkaline.

The foundation of soil fertility is humus. Alongside humic acids (humin, humic acids and their salts, and fulvic acids and their salts), it comprises CARBOHYDRATES (5-14%), proteinaceous substances (6-8%), and others. The percentage of actual humic acids and their salts relative to organic matter ranges from 46-49 in peat-bog soils, 46-52 in podzolic soils, 50-70 in brown forest soils, 57-65 in humus-carbonate soils, 81-82 in meadow-bog soils, and 71-91 in chernozems. Humus content is lowest in podzolic, brown, and sandy soils (1-2% of total soil mass) and highest in typical chernozems (8-12%) and peat-bog soils (15-20%).

Humus contains virtually all soil nitrogen, about half of the phosphorus, 60-90% of the sulfur, and a significant portion of other nutrients. It has been established that humus contains calcium, potassium, magnesium, sulfur, and iron. Humus retains phosphorus in a plant-available state and enhances the root system's uptake of nutrients from the soil solution. During the slow oxidation of humus, carbon is released as carbon dioxide, which facilitates the dissolution of certain soil minerals, rendering them more easily assimilable by plants.

Nearly 98% of soil bioelements are bound in organic residues, humus, and sparingly soluble Inorganic Compounds. This serves as a nutrient reserve that is mobilized very slowly through humus mineralization and weathering processes.

Soil structure, weathering, humification, and, above all, the mobilization of mineral nutrition elements and Ion Exchange are influenced by the acid-base buffering capacity of the soil. Acid-base conditions determine the form of an element's compounds and the sign and magnitude of the charge of its particles in solution. Soil buffering is a multifunctional property of soil resistance against changes not only in the concentration of the soil solution and its redox state, but also its capacity to resist shifts in the activity of various components driven by changes in moisture, Temperature, and other external factors (P.P. Nadtochei et al., 1997). The acid-base buffering capacity of soil serves as an integrative indicator of the agro-ecological status of soils.

Highly acidic soils release large amounts of H⁺ and Al³⁺ ions, while the content of available Ca²⁺, Mg²⁺, K⁺, and PO₄^3- ions is reduced. In more alkaline soils, by contrast, iron, manganese, and PO₄^3- ions are bound in poorly soluble compounds and are therefore barely accessible to plants (Fig. 117).

Fig. 117. The availability of a particular nutrient to a plant is directly related to its solubility, which in turn depends on soil pH. The width of the horizontal band on the graph illustrates the solubility of each element at various pH levels.

Thus, when humus is present in the soil, plants can absorb more nutrients from the solution than without it. Furthermore, on humus-rich soils, plants show a stronger response to increased Applications of mineral fertilizers. Apparently, humus acts as an ion carrier, accelerating their movement from the soil solution into the root system. Humus improves the physical and Chemical properties of the soil, enhances microbial activity, increases fertilizer efficiency, and stimulates mineral nutrition. All these factors contribute to soil fertility, which promotes the growth and development of crops. Preserving and enhancing natural soil fertility is a vital objective of agriculture. To make the most efficient use of soil nutrients, it is necessary to create optimal conditions for the soil's water and air regime. When moisture is insufficient, plants cannot assimilate even sparse nutrient reserves, whereas excessive moisture leads to significant and unproductive nutrient losses.