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Get Access Abstract
Plants lack a well-differentiated excretory system and the elimination of substances is performed by morphologically identical cells and tissues. Compared to animals, they do not feed on protein and as a result, the small amounts of ammonia formed in the metabolic processes are eliminated by simple diffusion. Plants remove both inorganic substances (mineral ions) and a big number of primary and secondary organic metabolites (monosaccharides, polysaccharides, amino acids, proteins, waxes, terpenes, alkaloids, volatile organic compounds—propanol, acetaldehyde and so on). Excess water can also be eliminated from plants by means of phenomena like tearing and guttation (in addition to transpiration). The eliminated substances can be final products of metabolism—this process is called excretion, or substances that participate in metabolism, in which case it is considered secretion. In contrast to the former the last is carried with metabolic energy expenditure. The secretion of the components is carried out at the level of:
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the cell wall (deposition of lignin, suber, salts of calcium, silicon dioxide, carbonates, sulfates but also of the cutin and waxes by leaf epidermal cells in the process of cuticle formation, representing at the same time an important aspect of cell and plant organ morphogenesis);
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specialized secretory structures (nectary glands, idioblasts, glandular hairs, osmophores);
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collecting channels that store substances inside the plants which can only be released by mechanical lesions (resins, arabic gum);
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many organic substances and metabolic residues are deposited in dead or detachable plant tissues.
The variety of substances secreted by plants is great. These can be substances involved in trophic relationships within the ecosystem, substances with signaling or regulatory functions, etc. Root exudates consisting of various biologically active substances, solubilizers and nutrients are important for promoting growth of soil bacteria in the rhizosphere or directly on root nodules.
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