FREE-RADICAL METABOLISM, PIGMENT DEGRADATION AND LIPID-PEROXIDATION IN LEAVES DURING SENESCENCE

Various lines of evidence suggest that the controlled increase in the production of oxygen radicals is an important factor involved in phyto hormone metabolism, selective changes in membrane permeability, degrad ation of photosynthetic pigments and unsaturated membrane lipids, oxid ative modification and subsequent proteolytic degradation, and other e vents occurring in ageing and senescing leaves. Increased lipid peroxi dation, appearance of chlorophyll allomerised products, as well as flu orescent 'lipofuscin-like' pigments have been documented in different systems of leaf senescence. On the other hand, the accumulation of per oxidised products may result from the alteration to native structural organisation and energy dissipation reactions of the photosynthetic ap paratus. Although there are contradictory and incomplete data on the a ctivity of the systems involved in the metabolism of oxygen radicals, it seems that senescing leaves retain, at least in part, their defence potential against both activated oxygen species and toxic lipid perox idation products. This provides the plant with the ability to successf ully dismantle its photosynthetic apparatus during senescence in a rel atively safe manner.