ward other parts of the plant, be these either growing organs, such as fruits or young leaves, or special-
ized storage tissues, such as the bark of deciduous trees. In this regard, it is not paradoxical that senes-
cence promotes the rise of both degradative and protective enzymatic activities because the ordered dis-
mantling needed for optimal exploitation of nutrients requires both specific degradation and protection
against uncontrolled agents, an unavoidable by-product of breakdown.
Fruit ripening is another physiological process that is usually associated with senescence because it
shares with truly senescent processes several metabolic features, especially the dismantling of chloro-
plastic components and structures. However, fruit ripening also has many particular metabolic character-
istics, and its final goal as a physiological process is different from that of senescence, being the devel-
opment of physicochemical and organoleptic properties that facilitate the spreading of seeds. This
teleological difference is reflected in the fact that fruits usually continue to act as a sink of nutrients dur-
ing ripening, whereas other senescing organs behave as a source. We will not consider the particular fea-
tures of fruit ripening in this chapter.
Despite the inherent diversity of senescence, three broad phases or stages may be distinguished in a
typical senescent process. First, there is a phase of selective degradation of certain molecules whose ly-
sis does not cause a major impairment of the physiological function of the senescent structure. Therefore,
the mobilized molecules may be thought as nutrient storage materials, and this stage may be termed stor-
age mobilization. In some cases, senescence may be reversed during this phase by suitable changes in the
environmental conditions. The second stage is characterized by the extension and generalization of
breakdown to components that are central in maintaining the physiological function, which is conse-
quently lost. Somewhere along this phase, which might be called generalized breakdown, the senescent
process becomes irreversible and the cells are definitively targeted to death. Finally, once the senescent
structure has been emptied of profitable nutrients, there is a third stage of abscission(i.e., shedding of the
senescent part from the rest of the plant) and death. Abscission, a biochemically and physiologically com-
plex process, is studied in another chapter and will not be discussed further here. Although exceptions or
overlapping of stages may be found in many particular senescent processes, the preceding three-phase
scheme may serve as a developmental outline that emphasizes the strategy of senescence.
Senescence of crop plants is of special interest because it encompasses phenomena of economic im-
portance that occur both in the field and during storage and handling of plant products of commercial
value. Moreover, on the basis of the current knowledge of the genetic control of senescent processes, it is
already possible to manipulate several features of senescence, using recombinant DNA technology, for
improving the quality of the crops. Tomatoes that are bruise resistant [12,13] or that do not overripen
[14,15], soybean plants with increased seed yield [16], and tobacco with higher biomass [17] due to de-
layed senescence are among the first offspring of this approach that will surely bring great benefits to
agriculture in the near future.
The aim of this chapter is to provide an outline of the natural patterns and features of senescence,
with special emphasis on the control and development of the senescence process at the molecular level
and the experimental approaches to its study. It is intended not to be exhaustive but rather representative
of the current trends in the field, and it is specially devoted to crop plants. The reader interested in more
detailed physiological information is referred to the excellent book by Noodén and Leopold [18].
II. PATTERNS OF SENESCENCE IN THE LIFE CYCLE OF PLANTS
Senescence is a very pervasive phenomenon that may be encountered in all plants and at all stages of the
life cycle, related to developmental as well as adaptive functions. It shows a variety of patterns ranging
from death of specific cells to the decline of the entire plant. Patterns of plant senescence may be illus-
trated with two extreme behaviors. There are plants, such as trees, in which survival of the individual over
a long period (including several reproductive phases) is the most important commitment. These plants
(termedpolycarpic) usually undergo a periodic senescence, which is limited to older organs and com-
bined with the growth of young ones. Other plants (e.g., the annuals) sustain only one reproductive phase
and die with the development of fruits. The latter species (called monocarpic), in which individuals are
subordinated to survival of the population, develop whole plant senescence directed to mobilization of
nutrients to the growing fruits. Between these extremes, there is a broad spectrum of life cycles display-
ing intermediate senescence strategies. Nevertheless, the major food crops are typically monocarpic, stor-
ing a high proportion of the plant biomass in the harvestable fraction (fruits or seeds).
182 PEÑARRUBIA AND MORENO