Introduction

There have been two distinct definitions of "plant senescence" which have developed within the literature. First, physiologists and cell biologists use the term senescence to describe the continual turnover of cells and plant parts that occurs within an individual as part of an internally controlled program of development. In cases of monocarpy (semelparity), this program can be responsible for the death of the whole organism. The details of this program of "physiological senescence" within individuals are addressed in the other chapters of this book. The second, alternative approach to senescence is termed "evolutionary senescence" and it addresses theories and experimental evidence explaining variation in mortality patterns among individuals within populations and between species. Senescence, as viewed by most animal and evolutionary biologists (see Chapter 1), is a decline in age-specific survival and reproduction with advancing age. The evolutionary theories of senescence are designed to explain why this senescence occurs in most species, and to explain the variation in the rates of evolutionary senescence between different species. It is this, evolutionary, population-level, approach to senescence that will be considered in this chapter. This chapter will apply a demographic approach to senescence that has traditionally been used exclusively by animal biologists and theoreticians, to the study of plants and the determination of mortality patterns and longevity.

From an evolutionary perspective, the phenomenon of senescence presents a paradox: Why should a trait that causes an individual to have an increased probability of dying with age persist in a population? The theory of evolution by natural selection suggests that heritable traits that improve the survival and reproduction of individuals should spread through a species because of their higher rates of transmission. We know that there is genetic variation in mortality patterns and longevity in populations. In classic studies with the fruit fly, Drosophila melanogaster, for example, researchers have successfully used artificial selection experiments to extend the life span and reduce rates of senescence (cf. Rose, 1984). This large degree of genetic variation in life span is expected in most species, and these results suggest that natural selection could potentially act on this genetic variation to change the time of onset and rate of senescence. Moreover, we also know that there are large differences between species in the rates of senescence. Birds, for example, generally live longer and have a lower rate of increase in mortality with age, in other words a lower rate of senescence, than mammals of comparative size. Some species are thus more effective at preventing or repairing damage than others are. The presence of genetic variation for senescence both within and between species needs to be explained from an evolutionary perspective. The objective of this chapter is to present the study of whole plant senescence within an evolutionary and demographic context. In the first part of this chapter the theories which have been proposed to explain the evolution and persistence of senescence will be discussed, and experimental tests of the theories will be evaluated. To study senescence at the level of the whole plant, demographic evidence for a decline in mortality and reproduction with age is essential. In the second part of this chapter, demographic evidence for senescence in plants will be evaluated and the techniques and problems that are unique to demographic studies of whole plant senescence will be discussed.

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