The evolution of senescence in annual plants: The importance of phenology and the potential for plasticity

Senescence, the decline in survival or fecundity with age, is a predicted outcome of the declining force of selection at late ages. In this chapter we suggest that in semelparous plants, where reproduction is fatal, seasonal limits on the opportunity for reproduction may also influence the timing of senescence. Further, the timing, or phenology, of germination and flowering can influence these seasonal limits. To support this hypothesis, first we use an integrated life-cycle model of the semelparous annual Arabidopsis thaliana to show that under reasonable assumptions about seasonal drivers of mortality, the window for reproduction (and therefore senescence) depends on (1) where in Europe the plant lives and (2) genetic variation that determines life-cycle phenology. In particular, we find that genetic variation that influences the timing of an early life-stage transition (germination timing) can have ramifying effects on the time available for reproduction within each environment. Next, we review recent research on the molecular and genetic basis of semelparity and senescence. This work emphasises the environmental dependence and tight molecular regulation of many of the component processes that ultimately lead to whole-plant senescence. In combination, both the modelling and the review emphasise the possibility of phenotypic plasticity in the regulation of the component processes that underlie survival and fecundity and therefore of senescence. Future work should focus on dissecting the genetic basis of the environmental dependence of the survival/fecundity relationship and creating a better understanding of how the senescence of individual tissues relate to senescence on the whole-plant level.