Senescence

Senescence refers to all of the many changes that inevitably lead to the death of part or all of a plant. Two related terms, aging and longevity, are also often used when senescence and eventual death are being discussed. Aging means all of the changes that occur over time, whether or not these changes lead to death, and longevity refers to how long a seed, or plant, or part of a plant, survives.

Senescence occurs in all plants and at all stages of the life cycle. Even in a young bean seedling, senescence processes have begun. The cotyledonary leaves, which were present in the dry seed, will rapidly give up their nutritive reserves to the growing plant, undergo senescence, and fall off.

At the same time, root hairs and root cap cells are dying and being continuously replaced as the root grows. Elsewhere in the plant, cells are dividing, expanding, and differentiating. In the final stages of formation of vessels and tracheids , the living cell contents will undergo senescence and be removed. The remaining hollow tubes, consisting of just the cell walls, become the water-conducting pipes of the xylem tissues used in transpiration . Thus both whole-organ senescence (for example, a leaf, petal, or fruit) as well as specific-cell senescence, occur as a normal part of plant development.

One of the visible symptoms of leaf senescence is the loss of the green chlorophyll pigments allowing the yellow carotenoid pigments to show through. In some maples (Acer spp.) in North America, red pigments, the anthocyanins, are made at this time. The senescence of leaves is triggered by environmental shifts (i.e., temperature and the relative lengths of night and day) and is also dramatically altered by applications of plant hormones. Cytokinins, auxins, and gibberellins can often delay senescence, while abscisic acid, ethylene, and jasmonates will accelerate these processes in some species. Some plants (for example, annuals like wheat, or perennial, deciduous maple trees that drop their leaves) have highly synchronous leaf senescence. In contrast, the bristlecone pine (Pinus aristata ) may keep individual needles for up to thirty years. Among other plant structures, petals often undergo rapid senescence after the flower has been pollinated, and the final part of the ripening and softening processes in fruits is also a form of senescence. The manipulation of these processes—from the vase-life of cut flowers to the shelf-life of tomatoes—will remain an important economic target.

Underlying many, and perhaps all, of these diverse, visible patterns of senescence is an ordered sequence of gene expression, called programmed cell death, which is triggered either by environmental shifts or by internal mechanisms in the plant. By mutating, eliminating, or altering the expression of these genes, the pattern of senescence can be changed. A full understanding of these genetic processes, and their linkage to the environmental, hormonal, and time-dependent expressions of senescence in specific cells, tissues, and organs, will provide a new view of death in the plant kingdom.

see also Deciduous Plants; Genetic Mechanisms and Development; Hormonal Control and Development; Hormones.

Roger F. Horton

Bibliography

Buckner, Brent, Diane Janick-Buckner, John Gray, and Guri S. Johal. "Cell Death Mechanisms in Maize." Trends in Plant Science 3 (1992): 218-23.

Nooden, Larry D., and A. Carl Leopold, eds. Senescence and Aging in Plants. London: Academic Press, 1988.

Thimann, Kenneth Vivian, ed. Senescence in Plants. Boca Baton, FL: CRC Press, 1980