Antioxidative defense system in an upland rice cultivar subjected to increasing intensity of water stress followed by recovery

Rice ( Oryza sativa L.) cv. Tulsi is recommended for Eastern India, for upland ecological cultivation systems where a crop experiences natural cycles of water deficit and water sufficiency, depending upon the monsoon rains. In this experiment, this cultivar was subjected to three cycles of water stress of increasing stress intensity. Each stress cycle was terminated by rewatering the plants for a 48-h period. The level of stress was measured by quantification of H₂O₂. The response of antioxidant metabolites such as ascorbate and glutathione, and enzymes such as superoxide dismutase (SOD, EC 1.15.1.1), catalase (CAT, EC 1.11.1.6), ascorbate peroxidase (APX, EC 1.11.1.11), glutathione reductase (GR, EC 1.6.4.2) and guaiacol peroxidase (POX, EC 1.11.1.7) was analysed in terms of activity and isozyme pattern for each cycle of stress and recovery. The differential response of the antioxidant enzymes with increasing stress intensity followed by recovery, highlight the different role of each in the drought acclimation process of upland rice. SOD and POX activity in stressed plants was higher than the controls in all the three cycles. The second level of stress saw an increase in all the enzymes with APX and GR showing its maximum activity and there was a better management of H₂O₂ levels. There was an induction of a new CAT isoform in stressed plants in the third cycle of water stress. The co-ordinated defense helped the plants to recover in terms of growth on rewatering after stress cycles.