Leaf senescence and lipid peroxidation: Effects of some phytohormones, and scavengers of free radicals and singlet oxygen

During in vitro senescence (chlorophyll loss) of oat ( Avena sativa L. cv. Victory) leaf segments and of leaf discs of Rumex obtusifolius L, the activity of catalase decreases and lipid peroxidation increases. The activity of superoxide dismutase (SOD) decreases in Rumex leaf discs but changes little in oat leaf segments. Kinetin treatment of oat leaf segments, and GA₃ treatment of Rumex leaf discs, inhibit decline in the enzyme activities and increase in the level of lipid peroxidation and strongly inhibit senescence. In either leaf tissue a treatment with ethanol or vitamin E (scavengers of free radicals) or with diphenylisobenzofuran (scavenger of singlet oxygen) results in a strong inhibition of lipid peroxidation and senescence, but does not affect much the decline in the SOD and catalase activities. It is concluded that, i) senscence-associated lipid peroxidation is induced by free radicals and singlet oxygen; and, ii) kinetin and GA₃ inhibit senescence mainly by a modulation of lipid peroxidation through maintaining high levels of such cellular scavengers as SOD and catalase.