Leaves photosynthesize during the spring, summer, and fall until they attain their maximum size. ThAftenr that environmental and hormones auxin and ethylene inside the tree and in the abscission zone cells. With these hormonal changes, photosynthesis and respiration become more slowly, and enzymes break down proteins in the leaves. Instead of nourishing the tree, even so, the senescent leaves become inefficient. Their green pigment, chlorophyll, gets wrecked.