Senescence

Senescence refers to all of the changes that take place in a plant that will finally lead to the death of cells, tissues, and, eventually, the whole plant body. These changes can be seen to occur in some cells even in very young, vigorously growing plants. For example the contents of those cells that make up the xylem tissue must senesce and die very early in development. The hollow cells, their cell walls arranged in a pipeline, can then allow water to flow up the plant in a process called transpiration.

Apart from the very precisely controlled death of specific cells during early development, senescence can also be seen in large, multicellular plant organs including leaves and fruits. Golden fields of ripening grain and the reds and yellows of the fall landscape in forests are both due to the pigment changes occurring during the early stages of senescence in millions and millions of leaves. The green chlorophyll pigments are removed, some yellow carotenes remain, and some species synthesize the red anthocyanin pigments at this time. Similar pigment changes occur in many fruits. These symptoms of organ senescence are often accompanied by changes in the levels of plant hormones in the cells, with shifts in the absolute amount and sensitivity towards the gaseous hormone, ethylene, playing a pivotal role.

Annual herbaceous plants live only a single growing season, with senescence occurring in all of the structures as the next generation, represented by the seeds, is shed. In perennial plants like trees, the leaves may be shed every year in a process called abscission , but the main part of the plant will continue to survive. Abscission in these deciduous plants is often considered as a part of their senescence, although the process of leaf abscission and the formation of a leaf scar is an active process. Some perennials are evergreen, and their leaves may be retained and function in the processes of photosynthesis for several years.

Although some individual trees and some seeds can survive for many decades or even centuries, eventually disease and other environmental challenges will lead them to their death. At that time the plant body, along with those thousands of tons of plant material returned to the soil every year, will be recycled by microbes and other soil organisms and feed a new generation of living plants.

In a world increasingly dominated by global markets for fruits, vegetables, and horticultural products, including cut flowers, people's ability to control the rate of senescence in plant tissues has become one of the most important technologies. Harvesting, transport, storage, and distribution facilities are now focused on attempts to delay the natural senescence of a huge range of living commodities. Scientists will continue to develop their understanding of the biochemistry and molecular biology of plant senescence and refine the environmental controls in storage and transport facilities so that the world's harvest can feed everyone.

see also Hormones, Plant; Water Movement in Plants

Roger F. Horton

Bibliography

Buckner B., D. Janick-Buckner, J. Gray, and G. S. Johal. "Cell-Death Mechanisms in Maize." Trends in Plant Science 3 (1998): 218–223.

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

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

senescence The changes that occur in an organism (or a part of an organism) between maturity and death, i.e. ageing. Characteristically there is a deterioration in functioning as the cells become less efficient in maintaining and replacing vital cell components. In animals this results in a decline in physical ability and, in humans, there is also often a reduction in mental ability. Not all the parts of the body necessarily become senescent at the same time or age at the same rate. For example, in deciduous trees the shedding of senescent leaves in the autumn is a normal physiological process. Various theories have been advanced to explain why ageing occurs. The telomere theory of ageing stems from the finding that the tips (telomeres) of chromosomes in body cells shorten with each round of chromosomal replication and cell division. It is suggested that eventually essential genetic information at the chromosomal extremities is lost, thus disrupting vital cellular functions and causing cell death. An alternative account is the mitochondrial theory of ageing, which proposes that over an individual's lifetime, mutations accumulate in mitochondrial DNA to the extent that there is an age-related decline in the formation of ATP by oxidative phosphorylation.

senescence The complex deteriorative processes that terminate naturally the functional life of an organ or organism. In plants, this may be associated with flowering and fruiting, and an important contributing factor is the decrease in chlorophyll content and in the ability of the plant to photosynthesize. Plant senescence is controlled by hormones.

senescence (si-nes-ĕns) n. the condition of ageing, which is often marked by a decrease in physical and mental abilities.
—senescent adj.