|MadSci Network: Botany|
In general terms, leaf senescence is a way for a deciduous plant to prepare for winter and recycle some of the valuable and often scarce mineral nutrients, such as nitrogen and phosphorus. Leaf senescence is also a way to to get rid of old and photosynthetically less efficient leaves in both deciduous and evergreen plants. Leaf senescence is considered a genetically programmed process and is an area of considerable recent research. The hormones ethylene and cytokinin are involved in regulation of senescence. Cytokinin applied to leaves delays senescence. Various enzymes are also involved. One important process in leaf senescence is the formation of an abscission zone which seals off the vascular connections and allows the leaf to be shed. This is important to prevent disease-causing microbes from entering and the vascular connections from leaking. The development of fall leaf colors is a very noticable change that occurs because chlorophyll synthesis stops and the current chlorophyll degrades thereby revealing yellow carotenoids in chloroplasts that were there all along. Anthocyanins are formed in vacuoles in some leaves giving reds and purple shades. Anthocyanin formation in fall leaves is promoted by dry, sunny days and cool, dry nights. Spectacular fall leaf colors are a multi-billion dollar tourist attraction that may help prevent some forests from being logged. There are several ideas for the development of anthocyanins including their acting as a sunscreen, to attract animals that disperse seeds, protect from freeze damage, or prevent water stress. References Leaf sensecence: Physiology and molecular biology Leaf Senescence Web Search Leaf Abscission Leaf Abscission Web Search Chemistry of Autumn Colors Possible Functions of Red Fall Leaf Color Fall Leaf Colors Web Search
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