Leaf senescence and related processes

GUIAMÉT JUAN J.; LUQUEZ VIRGINIA M.C.

Crops: growth, quality and Biotechnology

WFL Publisher

Lugar: Helsinki, Finlandia; Año: 2006; p. 1238 - 1253

1. An Overview of Senescence in PlantsThe yellow and reddish colors of the foliage of deciduous trees in autumn, or the yellowing of crops approaching maturity are examples of conspicuous senescence phenomena. Yellowing of outer leaves of lettuce heads or of broccoli florets during storage are similar, though perhaps not so striking phenomena. These de-greening, senescing leaves are actively dismantling their photosynthetic apparatus and other cellular structures, and this process will eventually result in the death of the leaf. Senescence is defined as a series of genetically programmed deteriorative changes that culminate in the death of cells, tissues, organs or organisms, and it represents the terminal phase in their development. Senescence is also an important process that limits postharvest storage of green vegetables. Although senescence is a programmed process inthe life cycle of leaves and, as such, it is not a disease or disorder, untimely senescence caused by disease or stress factors may reduce the yield of most crops, e.g. by terminating fruit filling prematurely, and it can adversely affect the appearance and nutritional value of vegetables. Conversely, delaying senescence by cultural, hormonal or genetic manipulations might increase the grain yield of some crops and extend the storage life of leafy vegetables.The timing of leaf senescence in the life cycle of plants is quite variable. Older leaves senesce progressively from the base of the stem upwards as new leaves develop at the top of the plant and even in evergreens, where the whole foliage does not turn yellow at the same time, older leaves lose chlorophyll and senesce as newer leaves develop at the tipof each branch. However, the whole foliage senesces more or less simultaneously at the end of the reproductive phase in monocarpic species (i.e., those fruiting once in their life cycle), or at the end of the growing season in deciduous trees, and in bulbous and rhizomatous perennials. In the context of postharvest physiology, it is quite relevant that detachment of leaves, or cutting off the aboveground parts of the plant, combined with light deprivation accelerates senescence in almostall species. The overall physiological and biochemical changes that take place in senescing leaves are very similar, regardless of the timing of senescence and the factors inducing it.Given the modular organization of the shoot, senescence may have evolved as a salvage mechanism whereby mineral nutrients are recycled from leaves in unfavorable positions within the canopy (e.g., shaded) or during adverse environmental conditions, to contribute to the nutrition of more productive parts of the canopy or to stores of nutrients (e.g., in seeds,bark, bulbs) that can be utilized later during more favorable times. Thus, leaf senescence may be viewed as a trade-off where photosynthetic capacity is sacrificed to increase the efficiency of nutrient utilization by the plant. Likewise, the senescence of detached leaves in darkness may represent a response to the combined stress imposed by wounding, cutting off the leaf or shoot from its supply of nutrients and cytokinins, and light deprivation, and, therefore, it may haveevolved as a response to dispose of damaged or severely shaded leaves.In this chapter, we will first cover the physiological and biochemical changes that take place in senescing leaves. Since manipulation of senescence to increase crop yields or extend the postharvest life of vegetables may require a detailed knowledge of the regulation of senescence, we will next concentrate on the hormonal, environmental and genetic controlsof senescence. Finally, we will focus on the impact of senescence on postharvest deterioration of vegetables, and on prospects for senescence manipulation through cultural and genetic approaches.