Physiological characterization of sugarcane varieties under oxidative stress caused by the herbicide paraquat.

Drought is the main factor limiting crop yields. A plant’s response to drought is the production of free radicals and drought-tolerant varieties are those characterized by a reduced production of free radicals under water stress. The mechanism of action of photosystem I inhibitor herbicides also results in the formation of free radicals that promote lipid peroxidation, hence destroying cell membranes. This phenomenon is known as oxidative stress and is similar to the response of plants to drought. The objective of this study is to physiologically characterize the sugarcane varieties SP83-2847, SP83-5073, SP80-3280 and CTC2 under levels of oxidative stress caused by the application of paraquat in different doses and to correlate the paraquat tolerance of these varieties with drought tolerance. The relative chlorophyll content (CC), the potential quantum efficiency of photosystem II (Fv/Fm), the visual rating of phytotoxicity (VR) and the fresh biomass (FB) were used as physiological parameters. Oxidative stress was imposed by the application of paraquat at 0 (D0), 25 (D1), 50 (D2), 100 (D3) and 200 (D4) g ha -1 . The evaluations were performed at 24, 48 and 960 hours (40 days) after treatment. There was a negative correlation of the paraquat doses and chlorophyll content (CC) estimated by SPAD, quantum efficiency of photosystem II  (Fv/Fm), visual phytotoxicity ratings and the fresh bio­mass. The response patterns of the varieties known as tolerant to drought to the oxidative stress caused by paraquat are not clear, suggesting different mechanisms from oxidative stress caused by drought, questioning the hypothesis that tolerant varieties to oxidative stress caused by herbicides would also be tolerant to drought.