Many plants die after first reproduction, as if committing suicide, a phenomenon known as ‘monocarpic senescence’. The process is different from the ageing seen in most animals. A ‘death hormone’has been proposed to explain the phenomenon, both for plants and also separately for animals. This paper considers for plants whether and how such a death hormone could have evolved.
Monocarpic senescence has sometimes been attributed to starvation of the vegetative parts for photosynthate or mineral nutrients, by nutrient diversion to developing fruit. In this case, no evolutionary problem arises. In other cases, the phenomenon seems to be one of programmed senescence, since details of the process conflict with an explanation in terms of nutrient diversion, since there is apparently a hormonal senescence signal, and since, e.g., the same type of senescence occurs even in male plants that do not bear fruit.
A range of evolutionary mechanisms are considered, by which a death hormone could have evolved: species selection, kin selection, direct Darwinian selection, pleiotropy, functional correlation, orthogenesis, genetic drift and founder effects. The two most likely are identified as: (1) kin selection, in which the mother plant dies, and thus facilitates establishment of her offspring, and (2) selection for efficiency of assimilate mobilisation. Neither of these explanations seems completely satisfactory.