Under normal conditions, in presence of sunlight a plant has higher rate of photosynthesis than respiration. In the night, the same plant has zero photosynthesis and high rate of respiration. If the process of photosynthesis and respiration alternates during day and night respectively, the plant remains healthy. In adverse conditions, where plant exhibit much higher rate of respiration (a catabolic process- breakdown of glucose) than photosynthesis (anabolic process- synthesis of glucose), the plant is most likely to shed its dead/ senescent leaves. The senescent leaves correspond to the leaves with high respiration and zero/ very low photosynthesis.
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