If this happens over a prolonged period of time the plant shall loose its biomass (dry mass). This is because sugars are continuously respired lost as carbon dioxide to the air.Plants face this problem at the canopy level. In a cluster of leaves only the top layers photosynthesize actively. The lower most thickly shaded leaves do more of respiration and a little photosynthesis over a extended period of time. Such shaded foliage becomes a net liability on the plant. That is why old shaded leaves are designed to turn senescent and fall off.
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