The changes in membrane permeability (soluble leakage), lipid peroxidation, and activities of superoxide dismutase (SOD) and catalase have been studied during in situ senescence of leaves of Nicotiana tabacum L., cv. Wisconsin 38. After full leaf expansion was reached there was a rapid, almost linear increase in the rate of 86Rb leakage from the preloaded leaf discs, with leaf age. Parallel with this increase in membrane permeability was a cumulative increase in the level of lipid peroxidation. At the same leaf age there were changes in the activities of SOD and catalase. SOD activity decreased on the basis of fresh weight but did not change when measured on the basis of protein content probably due to relative stability of SOD during the senescence-associated general decline in protein content. Catalase activity first increased parallel with the chlorophyll content of the leaf and then, after full leaf expansion, declined on the basis of both fresh weight and protein content. These changes in membrane permeability, lipid peroxidation, and the enzyme activities coincide in leaf age with the decline in protein and chlorophyll contents and in chlorophyll a: b ratio. When the senescence of the bottom-most leaves was reversed by removing the stem from immediately above them, the senescence-associated changes in protein and chlorophyll contents, lipid peroxidation, and the enzyme activities were also reversed. It is suggested that leaf senescence may be a consequence of cumulative membrane deterioration due to increasing level of lipid peroxidation probably controlled by, among other factors, the activities of SOD and catalase.

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