Senescence: Types

Senescence is the old-age stage of a plant's life cycle, following germination, the juvenile stage and maturation - it covers every collective, progressive, deteriorative process that eventually destroys an organ's or organism's organisation and function altogether. A key difference from animals is that plants continue producing entirely new organs throughout life even as older organs are deliberately allowed to senesce under a tightly controlled programme, mainly to recover and redirect their nutrients elsewhere in the plant. Leopold's 1961 classification recognises four distinct patterns of senescence, based on which parts of the plant are affected, shown in Figure 15.28. Overall senescence affects the entire plant at once, which then dies - typical of annuals like wheat and soybean, but also seen in some perennials like agave and bamboo. Top senescence affects only the aerial (above-ground) parts, while the underground root/storage system stays alive - common in perennials such as banana and gladiolus. Deciduous senescence is restricted to the leaves alone, with the bulk of the stem and root system staying alive - typical of deciduous trees lik …