ECOLOGICAL BIOCHEMISTRY - Study Guide - V. M. Isaienko 2005

Chapter 3. ALLELOPATHIC INTERACTION OF HIGHER PLANTS

3.2. Mechanisms of Action of Allelopathically Active Substances

Allelopathically active substances perform A wide variety of Functions. For instance, scopoletin inhibits The rate of Photosynthesis in several species of sunflower, tobacco, and pigweed. Quinones such as 2,3-dichloro-1,4-naphthoquinone, 2-amino-3-chloro-1,4-naphthoquinone, and 2,3,5,6-tetrachloro-1,4-benzoquinone affect The activity of the photosynthetic apparatus in plants.

As already discussed above, juglone from the black walnut Juglans regia inhibits oxygen uptake by corn roots as well as by the Cell/35.html">Mitochondria of Cells in this plant. Certain quinones, aldehydes, benzoic and cinnamic acid derivatives, and Coumarins affect the rate of oxygen consumption by the Yeast Saccharomyces cerevisiae. The Terpenes cineole and dipentene from the leaves of Salvia leucophylla inhibit oxygen uptake by the mitochondria of Avena fatua and Cucumis sativus, interfering with tricarboxylic acid transformations in the Krebs cycle. Salicylic and cinnamic acid aldehydes, 2-methyl-1,4-naphthoquinone, cinnamic, β-resorcylic and o-coumaric acids, and scopoletin lower the ATP/O ratio in mitochondria, which indicates the suppression of Oxidative Phosphorylation in the mitochondria of various plant and yeast cells. Most simple phenols, benzoic acid derivatives, dehydrogallic and ellagic acids, cinnamic acid derivatives, esculetin, and Flavonoids affect The oxidation of succinate by plant cell mitochondria. This list of effects exerted by allelopathically active substances on the functional activity of energy processes in plant cells by no means exhausts the capabilities of these compounds. The fact that many allelopathic substances adversely affect Plant Respiration points to the potential for significant disruption of cellular METABOLISM processes.

It has been established that potatoes grown between rows of young apple trees release toxic substances that inhibit tree growth, reduce the nitrogen content in branches and roots, and alter the protein composition of branch bark.

Quinones reduce the carbon content in plant cell Lipids and inactivate coenzyme A, thereby causing a deficiency of NADPH. Cinnamic and ferulic acids affect glucose metabolism and inhibit The Biosynthesis of A number of Proteins and organic acids. A variety of other allelopathically active substances exhibit similar properties.

Salicylic, benzoic, cinnamic, 2-naphthoic acids and their derivatives affect the permeability of Introduction/36.html">Biological Membranes to inorganic ions. Glyco-derived toxins of Corynebacterium, marasmin of Fusarium, 2-picolinic and dehydrofurycarinic acids also influence biological membrane permeability. Consequently, altering the permeability of biological membranes represents a key MECHANISM OF ACTION for certain allelopathically active substances.

There are numerous plant toxins that inhibit a range of Enzymes. For example, acacia tannin inhibits cellulase activity, ROOT exudates of Lupinus albus and Zea mays suppress catalase and peroxidase, and potato tuber polyphenols inhibit phosphorylase, among others.

A number of allelopathically active substances affect plant Cell Division processes. Thus, coumarin is capable of blocking mitosis in onion and lily root cells. Cinnamic acid causes an enlargement of metaphase nuclei in Pisum sativum root cells. Volatile terpenes from the leaves of Salvia leucophylla inhibit mitosis in Cucumis sativus seedling root cells, while scopoletin does so in Phleum pratense root cells. Weed toxins from Sonchus arvensis, Chenopodium album, and Cirsium arvense suppress the mitotic activity of wheat and rye roots.

It has been established that scopoletin prevents the oxidation of indole-3-acetic acid. A number of polyphenols (chlorogenic, isochlorogenic, and neochlorogenic acids) inhibit the activity of indole-3-acetic acid oxidase, whereas dihydro-ρ-coumaric, ρ-coumaric, and 2,4-dihydroxybenzoic acids activate this enzyme. Polyphenols and indole-3-acetic acid act synergistically in inducing plant growth by preventing the decarboxylation of indole-3-acetic acid. Ferulic and sinapic acids act in a manner similar to polyphenols.

Phloridzin and structurally related flavonoid Glycosides, as well as naringenin and 2',4,4'-trihydroxychalcone, activate indole-3-acetic acid oxidase, which may be related to the inhibitory effect of many flavonoids on plant growth.

In addition to suppressing plant growth induced by indole-3-acetic acid, a number of plant toxins (particularly Tannins) also inhibit growth stimulated by gibberellic acid.

Allelopathically active substances affect the uptake of mineral salts by plants. For instance, sugar beets significantly reduce the zinc content in the soil. Consequently, zinc-sensitive plants such as corn and beans are almost incapable of growing in these soils. Corn sown in fields infested with quackgrass (Elytrigia repens) suffers from nitrogen and potassium deficiencies. Salicylic acid inhibits phosphorus uptake in plants. Ferulic acid also inhibits phosphorus absorption while slightly enhancing the assimilation of potassium, magnesium, calcium, iron, molybdenum, and manganese. Ion absorption is of paramount importance for plant GROWTH AND REPRODUCTION. Many allelopathically active substances participate in regulating this function.

Certain substances are capable of affecting turgor pressure. Thus, in addition to its effect on photosynthesis, scopoletin can alter the degree of stomatal opening, particularly in tobacco and sunflowers. Treatment of oats with victorin leads to a narrowing of the stomatal aperture and, consequently, a reduction in the rate of Transpiration. Quackgrass rhizome extract, tannic acid, and many other allelopathically active substances act in a similar manner.

Concluding this Brief Overview of the mechanisms through which allelopathically active substances exert their effects, It is worth noting that their modes of action are highly diverse, and most of these compounds require further detailed research.