Prior to any natural abscission that may take place, leaves (or any other plant organ subject to abscission, such as flowers or fruit) undergo senescence. Senescence can be defined as the deterioration that occurs in conjunction with aging, and it results in the death of an organ or organism. Senescence can occur throughout the entire plant, as it does in annuals, or in only the aboveground portion, as it does in perennial herbs. In woody perennials, however, only the leaves senesce, while the bulk of the stem and roots remain alive.
From the time that leaves begin to grow, biochemical activities such as photosynthesis increase. This increase will continue until the leaves expand to maximum size. Soon after they reach maximum size, senescence begins, and photosynthetic rates begin to decrease. As the photosynthetic ability of the leaves declines, there is an accompanying decrease in other metabolic activities. Respiration rates begin to subside dramatically, and leaf protein levels drop sharply because of increased proteolytic activity (enzymatic breakdown of proteins). Protein synthesis diminishes, and there is an increase in the enzymatic degradation of ribonucleic acid (RNA). There is also an increase in the hydrolytic breakdown of carbohydrates. Finally, destruction of the green pigment, chlorophyll, is accompanied by increased visibility of the yellow or orange pigments called carotenoids, which were previously masked by chlorophyll. Most of the protein, carbohydrates, RNA, and chlorophyll degradation products are rapidly transported out of the senescing leaf. The final result is the production of yellowish, dead leaves.
The senescence process is a natural progression of the normal plant life cycle; however, environmental conditions can influence the process. Lack of water will speed the senescence process in most species. Higher-than-normal temperatures also cause an increase in senescence-related reactions. Darkness dramatically hastens senescence: Most leaves will senesce two or three times faster in darkness than if growing under normal light conditions.
Numerous studies strongly suggest that senescence is under hormonal control. Both ethylene and abscisic acid enhance senescence, but ethylene is the more effective of the two. The gibberellins, cyto-kinins, and auxins (other types of hormones) have all been shown to delay the process in various plant species. The exact role of each of these hormones in senescence has not yet been determined, but it is apparent that the process involves the interaction of several of these growth-regulating substances.
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