Throughout much of the world, plants are subjected to freezing temperatures. The leaves of most plants are unable to withstand the cold weather and face certain death during the winter. If the leaves did not prepare for the onset of cold weather by undergoing senescence, the first freeze would kill the leaves before materials within them
Throughout much of the world trees and other plants are subjected to freezing temperatures. Leaves age and fall prior to winter. Without the abscission process to cause the plant to prepare for winter by shedding its leaves, the senescent tissues would shade the new spring growth that appears the following growing season on what are shown here as bare branches.
could be salvaged. Without the abscission process to remove the dead leaves, the senescent tissues would shade the new spring growth that appears the following growing season. Hence, senescence and abscission provide a means by which perennials can recycle a major portion of leaf materials as the plants prepare for both the cold weather and the following growing season.
Competition for nutrients from other parts of the plant may initiate the senescence process. The pull of nutrients to another part of the plant such as roots, flowers, or fruit would reduce the amount of these materials bound for the leaves. The reduced supply of nutrients could very well decrease synthetic rates, and the overall result would be a decline in major leaf macromolecules such as proteins, chlorophyll, and nucleic acids.
Competition alone, however, cannot account for the senescence and abscission phenomena, because even in plants that do not produce fruit, the leaves experience aging and the loss of leaves. In addition, numerous studies have shown that leaf senescence will still occur when flowers are removed from the plant soon after being formed. Although competition for nutrients may not be the sole cause of the phenomenon, the mobilization of substances such as amino acids and carbohydrates from the leaves to other metabolic sinks, such as the fruit, is definitely linked to the initiation of senescence. Several of the plant hormones or other factors that stimulate mobilization also hasten senescence. Hence, it is possible that the competition for nutrients triggers the production of some unknown senescence hormone by the fruit or some other competing plant part. This theoretical substance would be transported to the leaves, where it would initiate mobilization of leaf contents. This mobilization might enhance senescence, which, in turn, might trigger the metabolic reactions that lead to abscission of the leaves.
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