Influence of environmental factors on the plant respiration process

The process of respiration is linked to the continuous consumption of oxygen by plants. Oxygen is necessary for the aerobic phase of respiration to occur; it serves as the final electron acceptor moving along the respiratory chain. In the absence of oxygen, glycolysis occurs (a process of anaerobic breakdown of glucose that proceeds with energy release, with pyruvic acid as the final product), followed by alcoholic fermentation; the accumulation of the resulting alcohol damages cell membranes, increasing their permeability. The absence of the Krebs cycle and the pentose phosphate cycle under these conditions leads to a deficiency of intermediate products necessary for synthetic processes. The synthesis of adenosine triphosphate is sharply reduced, as oxygen is required for the functioning of the electron transport chain. In the event of an oxygen deficiency, carbon dioxide continues to be released at almost the same rate as under aerobic conditions, which leads to an increase in the respiratory quotient (the ratio of the number of moles of CO2 released during respiration to the number of moles of O2 consumed). As the oxygen content increases, anaerobic respiration intensifies, while fermentation weakens. The effect of oxygen, which inhibits fermentation and the formation of anaerobic metabolism products and reduces the expenditure of carbohydrates on respiration, is known as the Pasteur effect. Above-ground plant organs rarely experience an oxygen deficiency because its concentration in the atmosphere is relatively stable. The situation is different with roots. The root system of plants growing in structureless and waterlogged soil suffers from an oxygen deficiency. When oxygen is scarce, the intensity of respiration in root cells drops sharply, which leads to a decrease in the uptake of nutrients by plants. Carbon dioxide concentration also has a significant influence on respiration. When its content in the air increases, the intensity of respiration falls significantly. This is caused by: 1) a general anesthetic effect of high carbon dioxide concentrations on the plant organism; 2) inhibition of the activity of respiratory enzymes; 3) closure of stomata, which limits oxygen access.

Light

When leaves are illuminated, respiration is activated. Shade leaves are particularly sensitive to light, and their respiration can increase sharply under short-term exposure to direct light. This process is most strongly affected by the short-wave part of the visible spectrum, especially ultraviolet rays. It follows that the influence of light on the respiration process is due not to thermal energy, but to the specific effect of certain spectral ranges. Respiration in individual plant species also changes significantly under the influence of lunar cycles, and maximum respiratory intensity is observed during the full moon. There are indications that cosmic rays have a significant influence on plant respiration.

Mineral Nutrition

The role of mineral elements in plant respiration

The level of nutrients in the soil, as well as the application of mineral fertilizers, affects the intensity of respiration. The degree of influence is determined by the physiological role of the element. For instance, metal ions are an essential component of a number of oxidative enzymes:

• iron is the active center of heme, which plays the role of a prosthetic group in catalase, peroxidase, cytochrome oxidase, and cytochromes;
• manganese ions take an active part in the oxidation of carboxylic acid products and, as a consequence, in the plant respiration process;
• copper is a direct component of the respiratory enzymes ascorbate oxidase and polyphenol oxidase;
• molybdenum is a cofactor for the enzymes nitrate reductase, xanthine dehydrogenase, and formate dehydrogenase.

Zinc is a component of carbonic anhydrase and carboxylase, which are involved in respiration processes. The action of the above elements can also affect respiration indirectly by altering the permeability of protoplasm. Phosphorus is essential for the formation and functioning of all enzymes involved in the respiration process without exception. Iron-sulfur proteins serve as components of the respiratory electron transport chain. As their name suggests, they contain sulfur in addition to iron.

Influence of nitrogen metabolism on respiratory processes

Respiration intensity correlates closely with the content of nitrogen in vegetative organs, which is linked to the high expenditure required for protein formation, especially enzymatic protein. As protein content increases, the respiration intensity of vegetative organs rises due to their high metabolic activity. At the same time, intensive growth leads to the import of additional nitrogenous compounds and, consequently, to the turnover of a greater amount of protein, accompanied by increased expenditure of respiratory energy.

The proportionality between protein content in plants and maintenance respiration is preserved as long as there is a close correlation between protein content and its turnover rate. This is because about 70% of the maintenance respiration energy is spent on ensuring protein turnover.

This is most noticeable in dense crops under conditions of excessive nitrogen fertilizer application, where a deficit of carbohydrates and a relative excess of protein are observed in the lower-tier leaves. On one hand, an increased protein content in plants increases maintenance respiration costs, and on the other hand, it reduces the biomass respiration intensity and, consequently, diminishes their adaptive capabilities. Furthermore, the forms of nitrogen applied to the nutrient medium influence the nature of the respiratory process:

• With nitrate nutrition of plants, in contrast to ammonium nutrition, the intensity of their respiration decreases noticeably, and the formation of organic acids increases significantly.
• When applying ammonium forms of nitrogen fertilizer, essential oils, rubber, and other reduced substances accumulate to a greater extent in the plants.

Practically all nutrients have an indirect effect on plant respiration.