Chilling and Heating Induced Antioxidant Responses in Tomatoes

The ascorbate-glutathione cycle maintains the level of ascorbic acid in the stroma to scavenge hydrogen peroxide, thus protecting the plant from oxidative damages. The objective of this research was to determine the antioxidantive responses to chilling and heating stresses in tomato fruits. Tomatoes cv. ‘Sanibel’ were harvested at the mature-green stage from in late June, 2008. Fruit were stored at 20 °C until ripe. Fruit were then treated with either chilling, hot water, or left untreated as the control. Fruit samples were taken directly after treatment or after 4 days storage at 20 °C. For each sample, the pericarp was collected from 2-4 fruit, rapidly frozen in liquid nitrogen and stored at -80 °C until analysis. For analysis of enzymatic activities, samples were extracted using Tris-HCl buffer, and for analysis of substrates and products, 6% TCA was used for extraction.  Directly after treatments, heating remarkably increased the activity of lipoxygenase, and chilling increased malondialdehyde content. LOX catalyzes the oxidation of lipids, and MDA is a toxic compound produced from lipids by oxidative damages. The increase of LOX activity and MDA content indicates oxidative stress caused by the reactive oxygen species. The heat treatment also reduced ascorbic and dehydroascorbic acid levels, although it induced the activities of peroxidase, catalase, dehydroascorbate reductase and glutathione reductase. After 4 days storage at 20 ºC, antioxidant reductive capacity was significantly higher in the heat treated fruit than in the control.  Chilling directly increased MDA level and decreased ascorbate level, and led to a remarkable increase in hydrogen peroxide and MDA on day 4 after treatment. The increase of all antioxidants indicates the response of the tissue to the reactive oxygen species by scavenging free radicals. The activities of POD, ascorbate peroxidase, monodehydroascorbate reductase were lower in chilled fruit than in the control.  In conclusion, chilling inhibited the activities of multiple ascorbate-glutathione cycle enzymes, while heating increased the activities. This may explain why chilling causes permanent injury to tomatoes but heating improves the resistance of tomatoes to stress.