Changes in antioxidant defense components associated with senescence of detached rice leaves

Different components of the antioxidant defense system were studied during the course of darkinduced senescence of detached primary leaves of rice (Oryza sativa L. cv. IR36) seedlings. The activities of ascorbate peroxidase, peroxidase (o-dianisidine) and superoxide dismutase were increased in senescing leaves as compared to control leaves (freshly excised), while catalase activity showed drastic loss during senescence. Among the non-enzymic antioxidants, glutathione content was elevated and ascorbate content was decreased in senescing leaves. In contrast to other antioxidants, peroxidase activity showed striking elevation even at the advance stages of senescence. Isozyme analysis of peroxidase on native PAGE also revealed higher synthesis of the enzyme from senescing leaves. Determination of peroxidase activity with function-specific substrates such as syringaldazine (for lignification) and NADH (for H₂O₂ generation) also revealed large increases at the final stages of senescence. The activity of IAA oxidase remained unchanged at this time period. Probable role of antioxidants along with peroxidase(s) during senescence had been discussed.