Photosynthesis is the most important biological process on earth. Plants require photosynthesis for their survival, and our existence depends on that process. The main function of photosynthesis is to convert solar energy into chemical energy and then store that chemical energy for future use. That chemical energy is stored as glucose (a form of sugar). Chlorophyll, the green pigment in leaves, absorbs sunlight which is then used to convert carbon dioxide from the air and water into glucose (sugar) and oxygen.
In autumn, as the days shorten and temperatures fall, the seasonal change signals for the plant to produce less chlorophyll, eventually producing none at all. The plant starts to mobilize resources from the leaves and withdraw them to the woody, more permanent parts to be available next season. This process, called photoperiodism, of going into dormancy starts before the leaves begin to change colors, and begins the processes leading up to their fall from the trees. Seasonal cooler temperatures at night cause the veins in leaves that carry fluids into and out of the leaf to gradually close off as a layer of cells forms at the base of each leaf. These clogged veins trap sugars in the leaf and reward us with brilliant autumn leaf colors.