Respiration and Antioxidant Enzymes Activity in Watermelon Seeds and Seedlings Subjected to Salt and Temperature Stresses

This research aimed to evaluate the effect of salt and temperature stress on water uptake and respiration of watermelon seeds during germination process and to quantify changes in the activity of the antioxidant enzymes ascorbate peroxidase (APX), catalase (CAT) and glutathione-S transferase (GST) involved in protection against reactive oxygen species. The research was performed at the Seed Analysis Laboratory (LASESA) of Embrapa Semi-Arid, Petrolina, Pernambuco State, Brazil, from september to december 2011. The experimental design was completely randomized in a factorial 2x3 (cultivars x stress conditions) for respiration evaluation, 3x4 (cultivars x electrical conductivities) for salt stress assays and 3x3 (cultivars x temperature) for temperature stress assays. The data were submitted to the mean test and evaluated using the standard errors of means. Respiration was measured by CO2 releases by watermelon seeds cv. cv. Crimson Sweet and Charleston Gray evaluated by an infrared gas analyzer, from 0-120 hours of seed imbibition in different environmental conditions (0 dSm-1/25°C, 0 dSm-1/30°C, 4 dSm-1/25°C). The antioxidant enzymes ascorbate peroxidase (APX), catalase (CAT) and glutathione-S transferase (GST) were evaluated in cvs. Crimson Sweet, Charleston Gray and Fairfax seeds and seedlings after five days imbibition in different electrical conductivities (0, 4 and 6 dSm-1) or temperatures (20, 25, 30°C). Crimson Sweet seed respiration rate was increased with increasing temperature, salinity however did not influence the respiration of seeds until the radicle protrusion. The activities of APX and CAT enzymes were antagonistically influenced stresses. The activity of GST was not altered with increased electrical conductivity, however high temperatures led to increase of its activity in watermelon seedlings. The antioxidant detoxification system was activated when imposing temperature and salt stress in all studied watermelon cultivars. Different cultivars of watermelon show different adaptation to salt and temperature stress.