Plant Physiology - Musienko M.M. 2001

Physiology of Excretion in Plants
Excretory Function of the Root System. Allelopathy

ROOT systems release almost all types of Water-soluble Organic compounds. This excretion process occurs continuously under normal conditions and is clearly a normal function of the plant Organism. Sugars and Amino Acids are mandatory components of root exudates, whereas Vitamins, Enzymes, and volatile organic compounds are detected less consistently. The quantity and composition of these exudates are determined by the species and variety CHARACTERISTICS OF THE plants. For instance, the root exudates of legumes are significantly richer in amino acids than those of cereals.

Using isotope tracing, fluorescence, and chromatographic analysis, researchers have demonstrated the ability of roots to release species-specific substances. For example, apple trees secrete Phenolic Compounds through their roots, notably phloridzin. Its accumulation in the soil inhibits the growth of young saplings in areas where trees of this species have previously grown. The phytotoxic effect of phloridzin and its degradation products has been confirmed in laboratory conditions at a concentration of 100 ppm. This amount of phloridzin can be extracted from 1 kg of apple tree roots. The products of phloridzin decomposition by soil microorganisms include phloretin, phloretinic acid, phloroglucinol, p-hydroxycinnamic acid, and p-hydroxybenzoic acid. The Root System of the widespread weed Agropyron repens secretes the phytotoxic compound agropyrene, while oat exudates contain lactone-type substances.

The total amount of root exudates is difficult to quantify precisely. For example, corn roots release 0.1–0.3% of the carbon from leaf-derived assimilates as organic compounds, while legume roots release 0.5–0.7%. Recently, studies using radioisotope tracer Methods have fostered the understanding that the release of organic carbon and numerous ions by plant roots is remarkably intensive. C¹⁴ supplied to the leaves as CO₂ can be detected in root exudates within just a few hours. Water cultures of mustard and wheat can release from 20 to 50% of their total photosynthetic carbon through their roots. Over the growing season, up to 30% or more of the total nitrogen contained in the plant can be released into the environment via the root system. The rates of K, Na, Ca, Co, and I excretion and absorption are so high that over a vegetative period, a plant can absorb and release tens of times more of these elements than it contains at any single point in time. It has been shown that about 12% of the Ca⁴⁵ absorbed by the plant is excreted by the root system.

The logical question arises: What is the adaptive purpose of excreting assimilated carbon and previously absorbed mineral nutrients?

Undoubtedly, plant root exudates serve as a source of energy in the rhizosphere. First, exudation can be an adaptive response to altered environmental conditions. It has been observed that soil drying—short of causing permanent wilting—along with subsequent re-wetting, intensifies the release of Amino Acids and reduced compounds by plants.

Second, the secretion of extracellular Hydrolases into the rhizosphere indicates that plant forms have evolved to utilize certain mineral nutrient elements. For instance, phosphatase activity is manifested primarily in the zone of direct contact between the root system and phosphorylated compounds.

The amount of nutrients released into the soil significantly exceeds their content within the root itself. This phenomenon indicates that excretory processes are governed by the physiological activity of the entire organism, with the root functioning as an excretory organ. One of the most convincing proofs of intensive root excretion is the documented negative nutrient balance during the final stages of plant ontogeny. Significant amounts of ash elements absorbed by roots are returned to the soil at the end of the growing season (plants can lose about 38% of K, 22% of Ca, and up to 10% of Mg). A portion of the root exudates is reutilized by the same plant or by neighboring plants within the phytocoenosis. Direct exchange of root metabolites between neighboring plants likely plays a leading role in the interrelationships of phytocoenosis members. Such reciprocal chemical interactions among plant organisms occur, for example, through the release of Antibiotics that inhibit microflora growth:

·marasmins—wilting toxins that affect higher plants;

·phytoncides—substances that suppress microorganisms;

·colins—compounds that negatively affect the GROWTH AND DEVELOPMENT of higher plants.

The chemical interaction of plants within ecosystems and phytocoenoses is known as allelopathy. The term allelopathy was coined in 1937 by H. Molisch, who studied the interrelationships between Higher Plants and microorganisms. Significant contributions to the science of chemical plant interactions were made by H. Molisch, M.G. Kholodny, S.P. Kostychev, H. Grümmer, B.P. Tokin, A.M. Grodzinsky, E. Rice, and others. The Chemical Nature of these physiologically active substances is quite diverse: phenolic compounds, Essential Oils, volatile Terpenes, Glycosides, resins, Alkaloids, Tannins, and others. Sometimes, physiologically active substances are synthesized within the plant from inactive precursors. For instance, garlic contains the inactive compound alliin, which is rapidly converted by the enzyme alliinase into allicin (C₆H₁₀OS₂), renowned for its phytoncidal properties.

According to A.M. Grodzinsky, virtually every plant secretes physiologically active substances. He introduced METABOLISM/2.html">THE CONCEPT OF "allelopathic activity", which is characterized by a plant's ability to accumulate a specific concentration of physiologically active compounds in its surrounding environment.

From the earliest days of their existence, plants—by releasing colins above all else—forge a distinct chemical environment around their own root systems. Every plant species maintains its own allelopathic sphere. For other species, this sphere is usually detrimental, though it may be harmless or even beneficial to certain accustomed neighbors. Allelopathy plays a crucial role in the self-thinning of wild plant populations. The denser the plant canopy, the higher the concentration of colins beneath it. Eventually, allelopathic stress reaches a threshold where the concentration remains optimal and stimulating for some individuals, yet exerts a lethal effect on others. Allelopathy thus intensifies differentiation, accelerates the mortality of weaker plants, and enhances the growth of the survivors.

There is no such thing as "voluntary" self-sacrifice or dying off of individual plants for the benefit of other specimens of the same species. Through evolution, many plants have developed The ability to maintain a specific concentration of colins in their environment. When the concentration becomes excessively high, plants slow down their growth, which in turn reduces colin production; as concentrations drop, growth resumes.

While studying chemical interactions in phytocoenoses, A.M. Grodzinsky described 15 distinct mechanisms of allelopathic influence among plants. It became clear that allelopathic interactions are mediated through the soil. Allelopathic mutual effects induce Structural and functional changes in plant metabolism. This phenomenon must be taken into account in agricultural production when designing agrophytocoenoses structures, preventing soil sickness in monocultures, and managing weeds and plant pathogens.

As noted, exudates accumulate in the rhizosphere, where they can serve as a nutritional substrate for rhizosphere and soil microflora, thereby supplying these microorganisms with readily available nitrogen and carbon.

Root exudates represent a vital source of energy for non-symbiotic Nitrogen Fixation. From 25 to 37% of the carbon photosynthesized by the plant is directly expended on nitrogen fixation via root exudation. Based on these data, it is concluded that through the transformation of root exudate carbon, nitrogen-fixing bacterial populations acquire the energy required for growth and the microbiological Fixation of Atmospheric nitrogen. Furthermore, rhizosphere microflora supply the plant—and particularly its roots—with vitamins, the demand for which cannot be met by weak internal synthesis or insufficient transport from above-ground Organs.

Thus, root exudation is an obligatory "expenditure item" in plant assimilate budgets. It is a vital element of soil biology that drives The Development of specific rhizosphere microflora and plays a crucial role in the symbiotic relationships between higher and lower organisms.