Leaf senescence: an overview

Senescence constitutes the last phase of plant growth and is characterized by a series of degenerative events that decrease metabolic activities and cause the death of cells, tissues and organs. Yellowing of leaves is a morphological indicator of senescence, which results due to loss of green pigments, i.e., chlorophylls. Alteration in mitochondrial and thylakoid structure, shrinkage of chloroplast and nucleus are prominent effects observed during senescence. Essential macromolecules like carbohydrates, proteins, lipids, nucleic acids, and photosynthetic pigments such as chlorophyll and carotenoids are degraded during senescence and these degraded constituents play important role in nutrient recycling mechanisms, which are mediated by senescence associated genes (SAGs). Sugar senescing mechanism, down regulation of photosynthetic genes, phytohormones, reactive oxygen species and SAGs play important role in signal transduction pathways to initiate progress and terminate the senescence process. The levels of heavy metals have increased in the environment due to various anthropogenic activities, which produce phytotoxic effects and reduce productivities. This review paper describes various biochemical/metabolic changes associated with leaf senescence.