The yellow in plant leaves during the fall is caused by the presence of xanthophyll. Xantophylls are present throughout the growing season, though their yellow is masked by the green of chlorophyll. Only when chlorophyll is destroyed through environmental changes, including autumn's cool temperature and lower light intensities, does the yellow appear.
Xanthophyll intensity is based on the plant's genetics as well environmental conditions. Bright, sunny, cool autumn days will promote vivid yellow colors.
See: Anthocyanin; Carotene; Chlorophyll; Tannin
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