THE RELATIONSHIP BETWEEN YIELD AND THE ANTIOXIDANT DEFENSE SYSTEM IN TOMATOES GROWN UNDER HEAT-STRESS

Four putative heat-tolerant tomato (Lycopersicum esculentum) cultivars (Tamasabro, Heat Wave, LHT-24, and Solar Set) and one putative heat-s ensitive tomato cultivar (Floradade) were grown in the field under non stress (average daily temperature of 26 degrees C) and heat-stress (av erage daily temperature of 34 degrees C) conditions. At anthesis, appr oximately five weeks after being transplanted to the field, leaf sampl es were collected for antioxidant analyses. Yield was determined by ha rvesting ripe fruit seven weeks after the collection of leaf samples. Heat stress resulted in a 79.1% decrease in yield for the heat-sensiti ve Floradade, while the fruit yield in the heat-tolerant cultivars Hea t Wave, LHT-24, Solar Set, and Tamasabro was reduced 51.5%, 22.1%, 43. 8%, and 34.8% respectively. When grown under heat stress, antioxidant activities were also greater in the heat-tolerant cultivars. Superoxid e dismutase (SOD) activity increased up to 9-fold in the heat-tolerant cultivars but decreased 83.1% in the heat-sensitive Floradade. Catala se, peroxidase, and ascorbate peroxidase activity increased significan tly in all cultivars. Only Heat Wave showed a significant increase in glutathione reductase in response to heat stress but all heat-tolerant cultivars exhibited significantly lower oxidized ascorbate/reduced as corbate ratios, greater reduced glutathione/oxidized glutathione ratio s, and greater alpha-tocopherol concentrations compared to the heat-se nsitive cultivar Floridade. These data indicate that the more heat-tol erant cultivars had an enhanced capacity for scavenging active oxygen species and a more active ascorbate-glutathione cycle and suggest a st rong correlation between the ability to up-regulate the antioxidant de fense system and the ability of tomatoes to produce greater yields whe n grown under heat stress.