An introduction to senescence involves the terms monocarpic and polycarpic behavior.
Some plants die at the time when their fruits mature i.e. cereals, peas, soybeans, etc.
This behavior is distinguished from polycarpic species which go through many cycles of growth and reproduction before death. Cereals are characterized by having all vegetative apices transformed into determinate reproductive structures. Others like soybean show a close correlation between seed-filling and senescence and this lead to the Nutrient Diversion Hypothesis. This hypothesis is based largely on the observation that removal of flowers or developing fruits will delay senescence. There are many exceptions to this hypothesis and it is an over-simplification, but useful in thinking about mechanisms.