RESUMO
The present investigation was conducted so as to unravel the various underlying antioxidant enzyme and non-enzyme defence mechanisms in some selected Boro rice cultivars that differ in temperature stress tolerance. Oxidative injury under heat and cold stress, H2O2 level showed a decline in roots and shoots of Boro in stressed condition whilst significant rise in the susceptible varieties was observed under both the stresses. However, susceptible varieties, such as Disang (shoots), Moricha (shoots) and China Boro (roots), showed a decreased H2O2 content at recovery. Under cold stress, roots and shoots of Boro and Laal Bihari showed a decreased level of lipid peroxidises and Boro and Kolong under heat stress. In contrast, significant enhancement of lipid peroxidase was revealed in the susceptible varieties. Remarkable increase in non-enzymatic antioxidants like proline, glutathione and ascorbate content was seen in the shoots of Boro in the treated and the recovery conditions. On the other hand, in enzymatic antioxidants like ascorbate peroxidase, guaiacol peroxidase, superoxide dismutase, catalase, and glutathione reductase activity, marked enhancement in ascorbate peroxidase activity was seen in the roots and the shoots of Boro and Kolong in treated and recovery samples and decreased in Swarnabh under heat stress. The guaiacol peroxidase activity of roots and shoots increased in Boro and Kolong under heat stress, and decreased in China boro and Swarnabh. The superoxide dismutase activity in the roots and shoot of Boro increased significantly under both the stress conditions in treated and recovery. Root and shoots of Swarnabh and Moricha showed decline in SOD activity in stressed conditions. The catalase activity in the case of Boro, showed a significant increase in both its roots and shoots under cold and heat stresses in the treated and the recovery samples. Moreover, under heat stress, the root and the shoots of Boro and Kolong showed the maximum glutathione activity, whilst Swarnabh and China Boro showed reduced glutathione activity at 96 h and recovery. On the other hand, the gene expression pattern of the cold-responsive genes (OsHAN1/OsCYP9B4 and FeSOD1) showed significant upregulation in the tolerant than the sensitive cultivars. Similarly, heat-responsive genes (OsTT1/OsPAB1 and OsHsfC1b) are also highly upregulated in the tolerant than the susceptible ones. Thus, the findings would provide a thorough insight into various non-enzymatic and enzymatic antioxidants and stress-responsive genes of Boro rice that could help in the future rice breeding programmes for cold and heat stresses.
