Regreening of Yellow Leaves

Abstract

Summary Yellowing of leaves is the most widely observed manifestation of senescence in plants. Various aspects of this type of programmed cell death are extensively covered elsewhere in this book. One largely overlooked, and perhaps peculiar, feature of plant senescence is regreening of leaves which are already in advanced stages of yellowing. This rejuvenation of main energetic and nutrition tissues is governed by molecular processes and signals which are mostly uncharacterized and poorly understood. Most of the data describing regreening of leaves comes from ultrastructural studies of plastids found in various stages of this process. Reversal of gerontoplast formation has been studied at the level of plastoglobuli formation and disappearance, reassembly of thylakoid structures, lipid remobilization, photosynthetic pigment retention and synthesis. Several investigations have been also focused on naturally occurring regreening models, either induced by insect habitation on leaves, or by physical damage or removal of meristematic tissues. It is now clear that plant hormones, mostly cytokinins, play very important roles in promoting reassembly of the photosynthetic apparatus and the consequent re-establishment of energy supply to the plant. Furthermore, a special kind of regreening was observed in some aurea mutants of deciduous woody species. Aurea mutants are able to regreen their bleached leaves several times during the vegetation season. It seems that remnants of the thylakoid membrane system are necessary for re-assembly of functional thylakoids in plants.