Aging in Perennials

Compared with our knowledge of senescence processes in annuals and biennials, relatively little is known about age-related changes in perennials. The study of aging in plants is very complex and there is no consensus in general concepts related to this topic. Furthermore, there is also a problem of scaling up, which makes us wonder whether cells, tissues/organs or whole organisms really age in plants. This is particularly interesting in the case of perennials, which have the ability to make new leaves every year and live for several years or even centuries or millennia. Recent studies indicate that physiological burdens, such as demands on water and nutrient supply, are responsible for reduced growth as plants age. Aside from the extrinsic factors, it is also possible that intrinsic changes in the shoot meristems could occur through repeated cell divisions and could be fixed during plant development, thereby affecting the physiology of leaves that originated from these cells. Additionally, the increased size associated with the aging of woody perennials (trees and shrubs) has also been proposed as a determining factor responsible for the age-related reductions in growth and photosynthetic rates in leaves. This review is aimed at compiling our current understanding of aging in perennials. After defining some fundamental questions and concepts, and introducing the model plants presently used in the study of aging in perennials, the major role meristems play in perenniality and how aging is manifested in the physiology of perennials (changes in phytohormones, water relations, photosynthesis and oxidative stress) are described. Finally, the causes underlying age-related changes in perennials are discussed in detail and a model based on plant plasticity to explain the aging phenomenon in perennials is presented.