Oxidative Stress Mechanisms during Flower Senescence

Free radicals and other active derivatives of oxygen are inevitable by-products of biological redox reactions and universal under stress situations. Reactive oxygen species inactivate enzymes and damage important cellular processes and subsequently cell components also. Plants and other organisms have evolved a wide range of mechanisms to contend with this problem by promoting the synthesis of antioxidant molecules and enzymes. The effects of the action of free radicals on membranes include the induction of lipid peroxidation and fatty acid de-esterification. Both ethylene biosynthesis and membrane breakdown, which appear to be closely linked, seem to involve free radicals, although the sequence of events generating these free radicals is still poorly understood. The death of petal cells is preceded by a loss in membrane permeability, partly due to increases in reactive oxygen species that are in turn related to the up-regulation of oxidative enzymes and to a decrease in activity of protective enzymes. It also consists of losses in soluble proteins, nucleic acids caused by the activation of proteinases and nucleases and enzyme-mediated alterations of carbohydrate polymers. Many of the genes for these senescence-associated enzymes have been cloned. In some flowers, the degradative changes of petal cells are initiated by ethylene; in others, abscisic acid may play a role. _____________________________________________________________________________________________________________