PIGMENT DEGRADATION, LIPID-PEROXIDATION, AND FREE-RADICALS IN LEAVES DURING AUTUMN SENESCENCE

The intensity of free-radical processes was investigated in leaves of Acer platanoides L., Aesculus hippocastanum L., and Betula pendula L. during autumn degradation of pigments. During synchronous senescence, chlorophyll a is degraded before chlorophyll b; the levels of caroteno ids changes insignificantly at the terminal stages. The leaf senescenc e is accompanied by a 4-fold increase in generation of superoxide radi cals in the chloroplast (gerontoplast) membranes with a concomitant in crease in the signal from an unidentified stable organic radical. The level of lipid peroxidation products assayed by the 2-thiobarbituric a cid test constantly decreases. However, high-temperature chlorophyll c hemiluminescence indicates the formation of labile peroxides during th e period corresponding to an approximately 60% loss of chlorophyll. No consistent changes in superoxide dismutase and peroxidase activities and no changes in alpha- and gamma-tocopherol levels were found in the autumn senescent leaves. The data suggest that during the natural lea f senescence, regardless of the increased generation of oxygen radical s and the possible presence of a limited amount of products of lipid p eroxidation in chloroplasts, the rate of free-radical oxidation appear s to be strictly controlled.