PLANT PHYSIOLOGY AND BIOCHEMISTRY

Lecture Notes

6. PLANT RESPIRATION

Regulation of Respiratory Processes

Endogenous mechanisms of respiratory regulation in plants

The Regulation of Respiratory processes occurs at various levels.

This includes substrate-level control of Respiration (availability, quantity, and composition of respiratory substrates); Introduction/15.html">Regulation of enzyme Activity (oxidoreductases); the interplay between different respiratory pathways (respiratory cycles, mitochondrial ETC, oxidoreductases of Cell/35.html">Mitochondria, Cytoplasm, and other Organelles); Changes in the physicochemical environment within The Cell; hormonal influence; and genomic control, among others.

The Pasteur Effect

O₂ levels in Tissues influence The rate of respiration and the consumption of respiratory substrates for synthetic processes.

The inhibition of sugar breakdown and their more efficient utilization in the presence of oxygen is known as the Pasteur effect.

The Mechanism of this phenomenon is as follows:

- high levels of O₂ → increased respiration → higher ATP production → decreased Phosphofructokinase activity and slowed Glycolysis;

- excess ATP → enhanced glucose resynthesis;

- increased O₂ levels → activation of the Krebs cycle and the Pentose Phosphate Pathway → formation of intermediates required for Biosynthesis.

In the presence of O₂, all cycles (including synthetic ones) are highly active. Furthermore, the increase in ATP resulting from respiration further promotes synthesis.

Respiratory Control

Plants possess a mechanism of respiratory control (the acceptor control mechanism). Respiratory control refers to the dependence of the mitochondrial oxygen consumption rate on the concentration of ADP.

The dependence of the respiratory rate (RR) on the mass action ratio of The ATP system can be expressed as follows:

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where the concentration of ADP acts as the limiting factor.

As cellular functional activity increases, ATP is consumed and ADP levels rise, which in turn leads to an increase in the respiration rate, electron (e^-) transport, and the intensity of Oxidative Phosphorylation.

Regulation of glycolysis

The intensity of glycolysis is controlled at several checkpoints.

Glucose uptake is regulated at the level of the enzyme hexokinase via feedback inhibition: an excess of the reaction product (glucose-6-phosphate) allosterically inhibits the enzyme's activity.

The second regulatory checkpoint for the rate of glycolysis is at the level of phosphofructokinase. This enzyme is allosterically inhibited by high concentrations of ATP and citrate, and is activated by inorganic phosphate, ADP, and fructose-1,6-diphosphate.

ATP and acetyl-CoA reduce The activity of Pyruvate kinase and the pyruvate dehydrogenase complex; the latter is also inhibited by an excess of NADH.

Regulation of the Krebs cycle (TCA cycle)

The utilization of cellular acetyl-CoA depends on the energy status of the cell. When energy demand is low, the Krebs cycle is inhibited, and acetyl-CoA is increasingly diverted toward biosynthetic processes. A key feature of TCA cycle regulation is the dependence of all four dehydrogenases on the [NADH]/[NAD⁺] ratio.

The activity of citrate synthase is inhibited by high concentrations of ATP and citrate.

Isocitrate dehydrogenase is inhibited by NADH and activated by citrate.

α-Ketoglutarate dehydrogenase is inhibited by its reaction product, succinyl-CoA, and activated by adenylates (ADP, ATP, AMP).

Succinate oxidation by succinate dehydrogenase is inhibited by oxaloacetate and accelerated by ATP, ADP, and reduced ubiquinone (QH₂). Malate dehydrogenase is inhibited by oxaloacetate and, possibly, by ATP.

An alternative pathway for e^- transport in mitochondria may also play a regulatory role.

Regulation of the PPP and The Glyoxylate cycle

PPP oxidation is regulated by the concentration of NADP⁺, as it involves two NADP-dependent dehydrogenases (glucose-6-phosphate and 6-phosphogluconate dehydrogenases). It is also regulated by the level of BIOSYNTHETIC PROCESSES IN the cell that consume NADPH.

A deficiency of inorganic phosphate inhibits glycolysis and activates the PPP. 6-phosphogluconic acid and erythrose-4-phosphate inhibit glycolysis and promote the PPP.

The activity of the glyoxylate cycle decreases as the concentrations of oxaloacetate and phosphoenolpyruvate rise.

Ecological and ontogenetic aspects of respiration

Influence of Environmental factors

Oxygen concentration. Respiration is inherently linked to oxygen consumption. However, a decrease in the partial pressure of oxygen from 21% to 9% has virtually no effect on respiratory intensity. At 5% oxygen content, uptake in young plants decreases slightly, while CO₂ release remains largely unchanged.

Evidently, plant respiratory systems evolved under low-oxygen conditions, as evidenced by the high affinity of cytochrome oxidase for oxygen. Furthermore, this allows plants to possess a broad adaptation to low O₂ concentrations (a reliability feature). This plant characteristic is crucial because the gas composition within tissues differs significantly from that of the atmosphere. The O₂ content in the parenchyma can fluctuate between 7% and 17% throughout the day. When the O₂ concentration drops to 1-2%, the respiratory quotient rises sharply, and respiration shifts to an anaerobic Fermentation process.

Excess O₂ occurs only locally. It is associated with an intensification of free-radical reactions within the cell, which can cause significant metabolic disturbances.

Carbon dioxide. An increase in the concentration of CO₂, as the end product of respiration, causes a decrease in the intensity of the process. Decarboxylation reactions and succinate dehydrogenase activity are inhibited, and the RQ decreases. This leads to tissue acidification—acidosis—which results in harmful consequences. Increased CO₂ concentration triggers stomatal closure and inhibits seed germination.

Temperature. Respiration, as an enzymatic process, is temperature-dependent. In the temperature range from 0 °C to 20 °C, the Q₁₀ of respiration is approximately 2-3. At higher temperatures, Q₁₀ decreases. One reason for this is the reduced solubility of O₂ in fluids as temperature rises.

Respiration in overwintering plants is observed at very low temperatures (-20 °C to -25 °C). The temperature optimum for temperate species lies between 35-40 °C. Maximum temperatures (45-55 °C) are determined by the Protein Denaturation threshold. The duration of thermal stress is also significant.

Water regime. Changes in plant tissue Hydration are reflected in the dynamics of respiratory intensity. Rapid, temporary water loss intensifies respiration as a stress response. Prolonged water deficit causes a gradual decrease in respiration, which occurs more slowly than the decline in photosynthetic intensity. The respiration of air-dry seeds (10-11% H₂O) is very low. As seed moisture increases to 15%, respiration increases 4-5 fold, and with a further increase in hydration to 35%, respiration increases by a factor of 1000 or more.

The sharp increase in respiration during seed Swelling is accompanied by heat release, which can lead to overheating (spontaneous heating) during storage.

Changes in respiratory intensity during water loss also depend on the age and physiological state of the plant.

Mineral Substances. Salt solutions typically enhance the respiration of seedling roots—the "salt respiration" effect. It is believed that The Effect of salts on respiration is indirect. Certain metal cations (magnesium, potassium) increase the activity of oxidative Enzymes, while their deficiency reduces respiration. Heavy metal cations inhibit respiration.

Light. The effect of light on the respiration of green plant Organs has not been sufficiently studied. The illumination level at which the intensity of Photosynthesis equals the intensity of respiration is called the compensation point. Respiration does not cease in the light; under normal conditions, its intensity and the rate of organic matter breakdown should be significantly lower than the intensity of photosynthesis.

Respiration (oxygen uptake and carbon dioxide release) in non-green tissues is stimulated by short-wavelength light, specifically ultraviolet (380 nm), as well as blue and green light (400-500 nm).

Injury. Mechanical stress triggers a transient increase in oxygen uptake. While pressure and bending have a minor effect, cutting and wounding induce a very strong response. In the latter case, various substrates become exposed to oxidases, and repair processes are activated.

Changes in respiratory intensity during ontogeny

Young, actively growing plant organs and tissues exhibit the highest rates of respiration. As a young leaf grows and develops, its respiratory intensity increases. Subsequently, respiration declines to half its maximum level, with a brief surge observed just before senescence. Flowering and fruiting are accompanied by increased respiration in flowers and fruits. Before the full ripening of fleshy fruits, a temporary rise in respiration occurs, known as the Climacteric rise. This is preceded by an increase in Ethylene production within the tissues. Climacteric phenomena can be suppressed by storing fruits in cold environments enriched with CO₂ and nitrogen.

Cellular respiration is the sum of biochemical reactions in which CARBOHYDRATES, Lipids, and Amino Acids are oxidized into carbon dioxide and water.

The released Energy is stored in the chemical bonds of ATP molecules.

Fig. 16. Electron Transport Chain of plant mitochondrial respiration.

Fig. 17. Anaerobic carbohydrate breakdown.

Fig. 18. The Krebs Tricarboxylic Acid Cycle.