Senescence

Plants senesce by predictable, controlled patterns that lead to death. Senescence is distinct, although not always distinguishable, from aging (longevity). Plant organs also senesce, which is frequently enhanced by detachment. Ripening, as described earlier and defined in Table 1, is a special case of the early phase of senescence. In somewhat oversimplified terms, ripening tends to lead to improve ment of quality attributes of fruits, whereas other senescence processes tend to lead to loss of quality. The primary objective of postharvest handling and storage is to control ripening and slow senescence, thus providing the consumer with a product of optimum quality at a reasonable price.

The most obvious signs of senescence in vegetables are loss of color and tissue softening. Unlike similar changes noted in ripening fruits that are considered quality improvements, yellowing and softening of green vegetables is considered detrimental. Yellowing, as in ripening, involves the unmasking of yellow pigments by chlorophyll depletion as chloroplasts are converted to chromoplasts. Likewise, softening is associated with breakdown of cell wall components, such as pectin and cellulose.

The cellular physiology of senescence is complex. Although it was previously believed that senescence resulted from the release of organic acids and hydrolases within the cell, it is now generally recognized that senescence is controlled by genetics. Senescence affects gene expression, protein and nucleic acid degradation, chemical composition and physical properties of membranes, and the structure and function of plant organelles (7). Although predominantly a degradative process, it is misleading to view senescence solely in the context of degradation. Metabolic processes function as a series of delicate balances between biosynthetic and degradative reactions. During growth and development as well as ripening, accumulation of a compound is the net result of biosynthesis exceeding degradation. Likewise, during senescence, net loss of a compound is the result of increased degradation, decreased synthesis, or a change in the balance between the two.

Early changes observed in the ultrastructure of senesc-ing plant tissue include swelling of thylakoids in chloroplasts. Mitochondria and vacuoles are affected later in the process. Changes in the nucleus are generally observed late in senescence before disruption of the plasma membrane, leading to cell and tissue death. Of particular interest in the study of senescence are the increase in protease enzymes and loss of membrane integrity. Membranes serve as internal barriers within the cell and regulate the flow of ions and metabolites. During senescence and aging, these membranes lose their integrity, which leads to increased permeability. A current theory suggests that membrane lipids are degraded by hydrolytic and peroxidative mechanisms, resulting in increased permeability and decreased function of membrane-bound proteins.

Not all degradation of plant tissue is attributable to senescence. Mechanical damage can cause physical rupture of membranes and lead to mixing of enzymes and substrates, which results in the browning and discoloration called a bruise. Inadequate nutrition of the plant during growth and development can lead to deficiencies within a detached plant organ and result in a physiological disorder. Low temperatures and controlled atmospheres slow the respiration and senescence of many plant organs, but certain species are susceptible to physiological disorders that result from exposure to low temperatures, low 02, high C02, or elevated C2H4. In addition, excess water loss can lead to loss of turgor pressure, resulting in undesirable textural properties.

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