Plant Physiology - Lecture Notes - O. M. Tarnopilska 2019

5. Plant Respiration
5.8 Influence of External and Internal Factors on Respiration

Temperature. Respiration in some plants can occur even at temperatures below 0 °С. For instance, spruce needles respire at 25 ^оС. Like any enzymatic reaction, The rate of respiration increases with rising temperature up to a certain limit (35-40 ^оС).

Oxygen is essential for respiration because it serves as the final electron acceptor in the Respiratory Electron Transport chain. An increase in atmospheric oxygen to 8–10% leads to a higher rate of respiration. Further increases in oxygen concentration do not significantly affect respiration. However, in an atmosphere of pure oxygen, Plant Respiration decreases, and prolonged exposure results in plant death. This fatality is caused by the intensification of free-radical reactions within Cells and membrane damage resulting from Lipid Peroxidation.

Carbon dioxide is the end product of respiration. At high concentrations of this gas, plant respiration decreases due to the following factors: 1) respiratory Enzymes are inhibited; 2) Stomata close, which impedes the diffusion of oxygen into the cells.

Water content. A water deficit in growing Tissues increases the rate of respiration by triggering The breakdown of complex CARBOHYDRATES (such as starch) into simpler sugars, which act as respiratory substrates. However, this disrupts the Coupling of oxidation and phosphorylation, rendering respiration a wasteful expenditure of matter. A different pattern is observed in dormant Organs: increasing the water content in seeds leads to a sharp increase in respiration intensity.

Light. It is difficult to isolate The Effect of light on the respiration of green plants because respiration occurs simultaneously with the opposing process of Photosynthesis. The light intensity at which the rate of photosynthesis equals the rate of respiration, in terms of carbon dioxide absorbed and released, is called the compensation point. Respiration in non-green tissues is activated by short-wave light, as the absorption maxima of flavins and Cytochromes lie within the 380–600 nm range.

Mineral nutrients. Elements such as phosphorus, sulfur, iron, copper, and manganese are essential for respiration, as they either form part of enzymes or, like phosphorus, serve as intermediates. When the salt concentration in the nutrient solution used to grow seedlings is increased, their respiration is stimulated (the "salt respiration" effect).

Mechanical injury accelerates respiration due to the rapid oxidation of phenolic and Other Compounds that escape from ruptured vacuoles and become accessible to oxidases.

Changes in respiration rate during ontogenesis. Light-demanding plants exhibit a higher respiration rate compared to shade-tolerant ones. Plants of northern latitudes respire more intensively than southern ones, especially at low temperatures. The highest respiration rate is characteristic of young, actively growing tissues and organs. Upon cessation of growth, leaf respiration drops to about half of its maximum level and then remains relatively stable for a long time. As leaves yellow and during the period preceding complete fruit ripening, these organs experience an activation of Ethylene synthesis followed by a brief surge in respiration known as the Climacteric rise. Ethylene increases membrane permeability and Protein Hydrolysis, leading to an elevated level of respiratory substrates. However, this burst of respiration is not coupled with ATP synthesis.