Leopold [1] distinguishes four possible patterns of senescence in different plants:- Senescence of the whole plant at the end of the reproductive phase, which is typical of mono-
carpic plants. - Senescence of the aerial parts of the plant maintaining the underground structures. Bulbs belong
to this group. - Senescence of the leaves only, the stems and roots remaining alive. This is the case with decid-
uous plants. - Progressive senescence of the leaves and other organs along the stem beginning at the base. An-
nual plants usually senesce in this way.
Another way to categorize senescence patterns is on the basis of the structural level (i.e., cellular, tis-
sue, organ, or organism level) at which they act, as Noodén [3] has proposed. In this regard, senescence
is sometimes restricted to a single cell or a small number of cells within a tissue as, for example, in the
formation of the root aerenchyma or the xylem tracheids [3]. Cellular senescence could serve either to
clear away the cell in order to create a hollow structure (as in the former cases) or to nurture the neigh-
boring cells with the breakdown products. The most spectacular examples of the latter strategy take place
in pollinated ovaries during the first stages of embryo development, involving senescence of specific cells
in an orderly sequence of events [3,19]. In all these cases, cellular death is clearly a result of an internal
developmental program and is properly termed apoptosis.
Frequently, senescence affects whole organs of a plant. Although the global nutritional balance of
the plant usually determines the onset of senescence of mature organs, these behave afterward as inde-
pendent structures establishing internal (spatial and temporal) patterns of nutrient mobilization before ab-
scission. Leaves, flowers, and fruits are the most studied and better known instances.
Leaves may follow different patterns of senescence. For example, in trees the oldest leaves may de-
cline when the new leaves are growing as part of a progressive senescence; or all the leaves may senesce
together seasonally. In the first case, the breakdown products serve directly as a nitrogen source for the
new organs [20]; in the second case, the nutrients are stored in the branches waiting for the next growing
period [21]. In any case, yellowing (due to destruction of photosynthetic pigments) is a conspicuous fea-
ture of leaf senescence, evidencing that chloroplasts are early and sensitive targets of the dismantling ac-
tivities that take place during the senescence of green (i.e., photosynthetic) tissues.
Flowers usually have the shortest longevity among plant organs [22]. Flower parts such as calyx, pe-
rianth, androecium, gynoecium, and peduncle are interrelated but differ from each other in both structure
and physiology. Senescence of flowers is a representative example of a kind where several components
decay tightly enchained to growth and development of other structures. Pollination is a central event in
flower development. Some parts of flowers, such as perianth, senesce after pollination, whereas others,
such as ovaries, develop. Changes originated by pollination, collectively termed the pollination syn-
drome, include a number of developmental processes, such as perianth pigmentation changes, ovary mat-
uration, and ovule differentiation, which are crucial to ensure fertilization and embryogenesis [23]. The
signal brought forth by pollination may be a requisite for perianth senescence. The most spectacular cases
are orchids, whose unpollinated flowers can stay fresh for 6 months waiting for a specific insect to be fe-
cundated. However, factors other than pollination could cause flower senescence in other species, possi-
bly as a result of regulation by an endogenous clock [24].
Ovaries senesce naturally if not stimulated either by pollination or by hormonal induction of
parthenocarpic fruits. The most spectacular modifications during fruit ripening probably occur at the plas-
tids and cell wall, where important changes in structure and composition take place [25]. Fruit softening
is a consequence of the induction of specific cell wall hydrolases [26]. Besides, the ripening-associated
color changes of fruits are a result of the transition from chloroplast to chromoplasts that are rich in red
or yellow carotenoid pigments [27]. In most fruits, there is also a decrease in acidity during ripening, as
well as an increase in sweetness in addition to changes in aroma produced by volatile odorant compounds
[28].
Whole plant senescence, a typical feature of monocarpic plants, is characterized by a general mobi-
lization of nutrients that are transported to the fruits, and especially to the seeds, which act as a strong
sink. The molecular nature of the hormonal effectors that are responsible for whole plant senescence is
controversial. Even if senescence and death are internally programmed, elimination of flowers and fruits
SENESCENCE IN PLANTS AND CROPS 183