ROLE OF THE ASCORBATE-GLUTATHIONE ANTIOXIDANT SYSTEM IN CHILLING RESISTANCE OF TOMATO

The role of lipid and water soluble antioxidants in providing tolerance to chilling stress in tomato leaves was investigated using two species, Lycopersicon esculentum and L. hirsutum, which differed in their sensitivity to chilling. The levels of alpha-tocopherol, ascorbate and glutathione, and their reduced/oxidized ratios in non-chilled leaves were similar in the two species indicating that the tolerant L. hirsutum did not have an inherently higher antioxidant potential than the sensitive L. esculentum. The plants were chilled at 2-degrees-C at 250 mumol m-2 s-1 PPF for 72 h, which caused differential but sublethal injury in both species. The levels of alpha-tocopherol, and the reduced/oxidized ascorbate and glutathione ratios, declined after the chilling stress more in L. esculentum than in L. hirsutum, which is consistent with greater exposure to activated oxygen. The activities of superoxide dismutase, ascorbate peroxidase, and monodehydroascorbate reductase were very similar between the two species and did not increase in response to chilling. A higher glutathione reductase activity was observed in L. hirsutum than in L. esculentum. An active dehydroascorbate reductase was not isolated from L. hirsutum but could be quantified in L. esculentum. L. hirsutum responded to the low temperatures by increasing total ascorbate, carotenoids and glutathione during the chilling treatment. This synthesis of antioxidants may be part of a complex mechanism of chilling tolerance involving both avoidance and protective elements of tolerance to activated oxygen.