Expression of key antioxidant enzymes under combined effect of heat and cadmium toxicity in growing rice seedlings

Abstract

Effect of Cd2+ toxicity and heat stress in sensitive rice cv. DR-92 and tolerant rice cv. Bh-1 grown in North East region of India were studied in sand cultures. Increasing levels of 0–500 μM Cd2+ alone and/or heat stress showed increased activities of superoxide dismutase, guaiacol peroxidase, ascorbate peroxidase and glutathione reductase enzymes which were associated with induced oxidative stress and altered enzyme activities. The values for SOD and POD activities were always more in cv. DR-92 whereas CAT and GR activities were higher in cv. Bh-1 in roots and shoots under Cd2+ or heat stress alone in sensitive cv. DR-92. Upon imposition of a combination of Cd2+ + heat the activities of SOD and POD decreased significantly in root/shoot of both the sensitive and tolerant rice varieties. A nine fold increase in GR activity under combination of heat + 100 μM Cd2+ stress in shoots of cv. Bh-1 at day 15 was noted when compared to controls. The dual stress combination of Cd2+ + heat did not alter catalase activity in vivo in both the rice varieties. Results suggest a time-specific and varietal distribution of the antioxidant enzymes in rice plants subjected to Cd2+ and/or heat stress. Tolerant cv. Bh-1 has better survival to combined stressors like Cd2+ and heat than sensitive rice cv. DR-92 and heat stress when given in combination with Cd2+ toxicity seem to mitigate the effect of Cd2+ stress alone in rice. The study indicates individual Cd2+ toxicity and heat stress and a combination of the two stresses to have separate implications on antioxidative defense mechanism in rice plants. Among enzymes of the defense apparatus ascorbate peroxidase and glutathione reductase appear to serve as an important component for better survival of rice plants under combination of Cd2+ + heat stress.