The breakdown of cell components and membranes that eventually leads to the death of the cell is called senescence. As mentioned in Chapter 7, the leaves of deciduous trees and shrubs senesce and drop through a process of abscission every year. Even evergreen species often retain their leaves for only 2 or 3 years (with the notable exception of bristle-cone pines, which hold their leaves for up to 30 years), and the above-ground parts of many herbaceous perennials senesce and die at the close of each growing season.
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Why do plant parts senesce? Some studies have suggested that certain plants produce a senescence "factor" that behaves like, or is actually, a hormone, but we are not yet certain of the precise mechanisms involved. We do know, however, that both ABA and ethylene promote senescence. On the other hand, auxins, gibberellins, and cytokinins delay senescence in a number of plants that have been studied. Other factors, such as nitrogen deficiency and drought, also hasten senescence.
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