Why do trees lose their leaves?

In some countries, like in Canada, some deciduous tree species have developed an adaptation to survive during the winter cold. They lose their leaves. Indeed, the leaves are susceptible soft tissue organs. The surface exposure to cold air is very large and they are thin. In the middle of August, depending on the region,young branches have hardened their bark. The  larger branches are well protected. The roots in the ground are well isolated by the snow cover and  are  protected from the cold. How do trees know when they should get rid of their leaves before they freeze? The leaves have sensors, the phytochromes, they are  photoreceptors that are sensitive to red light. In the fall, when the days shorten phytochromes perceive this change. They will activate a process in the leaves making them to age very quickly.

autumn without leaves

With the days getting colder and the photoperiod reducing, the trees know that  they have to get rid of their leaves. In plants, growth processes are controlled by a plant hormone called auxin. This phytohormone is involved in the leaves formation . It helps  keep the  leaves young. This is a  growth phytohormone. In the fall, the production of auxin is reduced. When the photoperiod becomes shorter, the tree produces more abscisic acid. This phytohormone is produced during stress periods. A third plant hormone, ethylene intervene in the aging process of the leaf. This gas involve in the destruction of chlorophyll. The presence of a high concentration of ethylene in the place where the leaf petiole will detache from the branch are swelling the cells of the  abscission  zone causing a lengthening of these on a few rows which produces a pressure difference. At the same time, the cells of the  abscission zone produce two  enzymes,the cellulase and the pectinase. Cellulase is an enzyme that attacks the cellulose cell's walls. Pectinase another enzyme that attacks the pectin who is responsible for sticking the cells together.

autumn dead leaves

The destruction of the cellulose cell's walls by cellulase, pectin by pectinase and the pressure exerted by the cells are going to break the cells of the abscission zone from outside to inside the petiole. To protect against fungi, bacteria and insects that could benefit from this gateway in the tree, the cells on the edge of the  abscission zone will secrete suberin. Suberin is a waxy substance that is the main constituent of cork. Once the leaf has fallen, the  tree will be well protected from external attacks by the cork.