Factors Affecting Respiration

The rate at which a plant or tissue respires is not constant; it responds continuously to conditions both inside the organism and in its environment. Internal factors include the sheer amount of metabolically active protoplasm present and how physiologically active it is (more active protoplasm respires faster); the concentration of the available respiratory substrate, to which respiration rate is directly proportional; and mechanical wounding of a plant organ, which stimulates a local burst of increased respiration in the injured region. External factors include temperature, for which the optimum for respiration is around 30 degrees Celsius, with the rate falling off at both lower and (beyond a point) higher temperatures; the surrounding CO2 concentration, where a high concentration actively suppresses respiration rate; light, which affects respiration only indirectly (mainly through its effect on photosynthesis and substrate availability, not by acting on respiratory enzymes directly); water availability, since respiration rate falls as tissue hydration decreases, meaning proper hydration is essential for normal respiratory activity; certain chemical inhibitors such as cyanide, which directly poison respiratory enzymes; and osmotic/ionic stress -- when a plant or tissue is transferred from plain water into a salt solution its respiration rate rises, a phenomenon specifically called salt respiration. Two related threshold concepts are worth noting: when oxygen is abundant, aerobic respiration proceeds at its optimum rate and anaerobic respiration is completely suppressed, a state called the extinction point; unusual, temporary spikes in respiration rate also occur naturally during fruit ripening (called a climacteric, seen in fruits such as apple, banana, mango, papaya and pear), a phenomenon linked to cyanide-resistant respiration, which i …