|MadSci Network: Botany|
The photoperiod, or length of the daily light period, is considered the environmental trigger for the processes of fall leaf senescence and leaf abscission. The pigment phytochrome senses the photoperiod, or actually nightlength. Hormone levels change in response to the photoperiod trigger and an abscission layer forms which allows the leaf to fall off. The main "abscission hormone" is now considered to be ethylene. Sprays of a chemical, Ethephon, that releases ethylene, are used commercially to promote leaf abscission prior to cotton harvest. Reduction in auxin and cytokinin levels also occurs and contributes to the process. Applying auxins or cytokinins to leaves can delay leaf abscission. The hormone abscisic acid also plays a role by triggering ethylene synthesis. Deciduous leaves are a way for the plant to save energy by making "cheap" new leaves each year rather than invest more in "expensive" evergreen leaves and maintaining them yearround. However, the plant also has mechanisms to recycle a lot of the scarce mineral nutrients, such as nitrogen and phosphorus, prior to leaf drop. Sometimes an early fall cold spell will kill deciduous leaves before the abscission layer has formed, and the dead leaves will hang on into winter. Some species, such as oaks, beeches and hornbean naturally retain some dead leaves because their abscission layers do not complete fromation until early spring. Retention of dead leaves, termed marcescent leaves, may be a defense mechanism against herbivores. The webpages cited have more details. References Why and How Do Deciduous Species Shed Leaves? Falling Tree Leaves: Leaf Abscission Abscission Zone Photo Problem: Leaves on Trees Don't Drop in Fall Effects of Marcescent Leaves on Winter Forage Quality of Oak, Hornbeam and Beech and the Their Influence on Food Selection by Cervidae
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