Senescence

Senescence is the orderly, age‐induced breakdown of cells and their components, leading to the decline and ultimate death of a plant or plant part. The timing of senescence is species‐specific and varies among the organs of individual plants. Some species of plants produce short‐lived flowers whose petals last for only a few hours before shriveling and dropping off, while the leaves of deciduous plants last through long growing seasons before senescing.

Senescence is a metabolic process; therefore, it requires energy. It is not simply the ending of growth. Leaves, for example, move the products of photosynthesis—and their own structural substances—out of leaf tissue into stem and root tissue during senescence and before their vascular connections are severed at abscission. One of the first materials to degrade is the energy‐converting pigment chlorophyll. As the bright green color of chlorophyll fades, the yellow‐orange colors of the carotenoids become prominent and combine with the red‐blue anthocyanins to produce the vivid colors of autumn in the trees and shrubs of the northern deciduous forest.

The role of hormones in senescence is not clear. Not only the kinds, but the proportions of each are important. Ethylene promotes abscission of leaves, flowers, and fruits, while IAA retards senescence and abscission. When days shorten in autumn, IAA production decreases, and ethylene production increases, hastening changes in the cells of the abscission zone. When the degradation of the cell wall materials is complete, nothing remains to hold the leaf to the stem, and with any slight disturbance the leaf falls. Some evidence indicates that a senescence factor, presumably an unknown hormone, exists in some plants (like soybeans), but it has yet to be isolated or synthesized.