The stored nutrients are reused in the spring to support new growth . The yellow and orange carotenoid pigments seen in the fall are revealed when chlorophyll is broken down . The red and purple anthocyanin pigments are made in cool weather when newly synthesized sugars are no longer needed for growth . In the laboratory, leaf senescence can be delayed by application of the growth-promoting plant hormones GA, IAA and cytokinin, while senescence can be promoted by applying abscisic acid or ethylene. Temperature Perennial plants that must remain alive where winter weather is harsh survive by becoming dormant in the fall .
Dormancy is a state of reduced metabolic activity .
Shortening days and cool nights cause a number of changes that not only promote survival, but lay the foundation for renewed growth in the spring. Cells accumulate sugars and amino acids that act as antifreeze to lower the freezing point of the water in the protoplasm. The buds that remain following leaf drop become enclosed in thick, protective scales so they can function as the meristems for spring growth. The lipids or oils that make up the plasma membrane are altered to become more fluid , so they are less like a solid and more like an oil at lower temperatures. These changes take place over several weeks and contribute to the process of cold-hardening .