Paraquat pre-treatment increases activities of antioxidant enzymes and reduces lipid peroxidation in salt-stressed cucumber leaves

To investigate whether paraquat (PQ) is involved in regulation of antioxidant enzymes and lipid peroxidation under short-term salt stress, and to elucidate the physiological mechanism of salt stress mitigated by PQ, a cucumber cultivar (cv. Chunguang no. 2) was exposed to 100 mM NaCl for 48 h after pre-treatment with 10 μM PQ for 1 h. When compared to the control, salt stress increased the levels of malonaldehyde (MDA), superoxide radical (O₂^·−) and hydrogen peroxide (H₂O₂) and the activities of antioxidant enzymes, such as superoxide dismutase (EC 1.15.1.1), ascorbate peroxidase (EC 1.11.1.11) and glutathione reductase (EC 1.6.4.2) in the cucumber leaves. Under salt conditions, PQ pre-treatment prevented oxidative stress as observed by the decreases in MDA, H₂O₂ and O₂^·− that correlated with the increase in antioxidant defenses. We propose that, at low concentrations, the PQ pre-treatment can reduce the salt-induced oxidative damage by increasing the antioxidative mechanisms in cucumber plants.