Oxidative stress and antioxidant activity as the basis of senescence in maize leaves

Leaf senescence was studied in an early (X 3342) and late (Deccan 103) cultivar of maize. Observations were recorded in the flag leaves with the initiation of tasseling at an interval of 5 days upto the senescent leaf stage. Hydrogen peroxide content and lipid peroxidation increased from young leaf stage to the senescent leaf stage. Chlorophyll content initially increased up to 10th day after tasseling (DAT) and decreased thereafter, while carotenoid content increased up to 30 DAT and then decreased. Activities of superoxide dismutase (SOD), ascorbate peroxidase (APOX), peroxidase (POX) and catalase (CAT) increased up to 20-25 DAT and thereafter, decreased in both the cultivars. Among the SOD isoforms, Mn-SOD activity was highest in the two cultivars and was responsible for the peak in total SOD activity observed at 20-25 DAT. Fe-SOD and Cu/Zn-SOD activities did not vary much at different stages. Late cv. Deccan 103 maintained lower hydrogen peroxide content and lipid peroxidation, and higher activities of superoxide dismutase, specifically Mn-SOD, APOX and CAT and contents of chlorophyll and carotenoids at 40 DAT than early cv. X 3342. The results suggest that leaf senescence in general and early senescence of early cv. X 3342 is associated with higher oxidative stress and a decline in antioxidant activity towards maturity, and Mn-SOD has a major role in the scavenging of superoxide radicals during maize leaf senescence. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.