Why is there loss of pigments & leaves during certain seasons of the year and how does this relate to the rate of photosynthesis?
When the leaves change color there are reasons; the length of the night and the weather. The longer the nights cause the leaf fall and the color changes in the leaves.
A color of the leaves are the pigments and reflect specific compounds are present in the leaves. For example, the regular green color shows us that chlorophyll is present. This is most critical for photosynthesis, how plants use light to make sugar. The trees in the colder areas store chlorophyll during the winter(dormant) period.Another color is yellow, which shows that carotenoids are present in the leaves. Another color is red which shows that anthocyanens are present. The chemical compounds that cause the beautiful colors are in the leaves at all times; however during chlorophyll production the green is only seen. For example, when the light is short during winter and the night is longer the chlorophyll production slows down and stops so the colors show up.
They fall down from the tree because the veins of the leaves close off. This is also cause by the shorter light and longer nights.