Defining Senescence and Death in Photosynthetic Tissues

Summary

Having clear concepts of senescence and aging, as well as the diverse related terminology, will foster these and related fields by facilitating communication within and between the academic constituencies that study them. Moreover, it will enable those working on photosynthetic tissues to contribute more to and draw more help from other fields ranging from gerontology to evolution. Senescence is a very important developmental process in the life cycle of organisms, especially photosynthetic organisms where it is often very dramatic. Senescence is commonly viewed as an increase in the probability that an organism will die as it ages chronologically (demographic view); however, plant physiologists/molecular biologists go a step further to define senescence as internal processes that actively cause death (physiological view). During senescence, a very wide range of changes (senescence syndrome) occur at the cell, organ and organismic levels; however, many are not causal and therefore not senescence per se. Although chloroplast degradation during senescence may not cause death, the decline in photosynthesis is very important and may limit agricultural/biomass productivity. Senescence is actively driven by the nucleus (necrogenic processes), but it also involves shutdown of life-supporting (biostatic) processes. Death, the endpoint of senescence, can be defined generally as the collapse of homeostasis. At the cell level, death appears to be the loss of the plasma membranes’ ability to retain/exclude molecules. Exactly how senescence brings the cells that comprise organs/organisms to death is not clear; however, programmed cell death processes participate in the end stage.