Physiological and biochemical responses to oxidative stress induced by paraquat or water deficit were examined in this study with respect to photoinhibition, stomatal conductance, photosynthetic pigments, organic solutes, and antioxidant enzymes, aiming at a better understanding of the defense networks connected with antioxidant agents in sugarcane and also to identify cultivars more tolerant to such stress conditions. In the droughtinduced experiment, the plants were exposed to 15 days without irrigation, and in the study with paraquat (PQ), 150 lM of the herbicide was applied to the plants 24 h before the evaluations, conducted simultaneously with the drought experiment. In the second part of the experiment leaf disks were cut from cultivars and immersed in a paraquat solution in Petri dishes and analyzed for chlorophyll degradation. In cultivars SP83-2847 and IAC91- 5155, smaller reductions were observed in the maximum photochemical efficiency of photosystem II (Fv/Fm) and photosynthetic pigments, when submitted to water deficit and treated with paraquat, and greater reductions were found in cultivars RB92579 and SP81-3250 which had lower dry matter under drought stress. Paraquat produced a toxic effect because of degradation of proline and carbohydrates in cultivars. Both stressors led to an increase in the activity of superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX) enzymes in most of the cultivars, confirming the importance of interaction of these enzymes against oxidative stress damages.
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