<P> Chlorophyll has a vital function: it captures solar rays and uses the resulting energy in the manufacture of the plant's food--simple sugars which are produced from water and carbon dioxide . These sugars are the basis of the plant's nourishment--the sole source of the carbohydrates needed for growth and development . In their food - manufacturing process, the chlorophylls break down, thus are being continually "used up". During the growing season, however, the plant replenishes the chlorophyll so that the supply remains high and the leaves stay green . </P> <P> In late summer, as daylight hours shorten and temperatures cool, the veins that carry fluids into and out of the leaf are gradually closed off as a layer of special cork cells forms at the base of each leaf . As this cork layer develops, water and mineral intake into the leaf is reduced, slowly at first, and then more rapidly . During this time, the chlorophyll begins to decrease . Often, the veins are still green after the tissues between them have almost completely changed color . </P> <P> Much chlorophyll is in photosystem II (light - harvesting complex II or LHC II), the most abundant membrane protein on earth . LHC II captures light in photosynthesis . It is located in the thylakoid membrane of the chloroplast and it is composed of an apoprotein along with several ligands, the most important of which are chlorophylls a and b . In the fall, this complex is broken down . Chlorophyll degradation is thought to occur first . Recent research suggests that the beginning of chlorophyll degradation is catalyzed by chlorophyll b reductase, which reduces chlorophyll b to 7 ‐ hydroxymethyl chlorophyll a, which is then reduced to chlorophyll a . This is believed to destabilize the complex, at which point breakdown of the apoprotein occurs . An important enzyme in the breakdown of the apoprotein is FtsH6, which belongs to the FtsH family of proteases . </P> <P> Chlorophylls degrade into colorless tetrapyrroles known as nonfluorescent chlorophyll catabolites . As the chlorophylls degrade, the hidden pigments of yellow xanthophylls and orange beta - carotene are revealed . These pigments are present throughout the year, but the red pigments, the anthocyanins, are synthesized de novo once roughly half of chlorophyll has been degraded . The amino acids released from degradation of light harvesting complexes are stored all winter in the tree's roots, branches, stems, and trunk until next spring, when they are recycled to releaf the tree . </P>

When do the leaves change colors in the fall