Tolerance to paraquat-mediated oxidative and environmental stresses in squash (Cucurbita spp.) leaves of various ages

Paraquat is labeled for row-middle application on cucurbits, but drift to crop foliage is inevitable. Experiments were conducted to determine whether differential tolerance to paraquat existed among leaves of various ages in Cucurbita spp. (squash) and other plants, and to examine whether leaves tolerant to paraquat are also tolerant to other herbicides and abiotic stresses. Physiological responses to paraquat, including antioxidant activity, were investigated in squash leaves to identify mechanisms of paraquat tolerance. Although the level of paraquat tolerance differed by leaf age, cultivar, and species, the level of paraquat injury was lower in younger leaves than in older leaves in 14 of 18 squash cultivars and 5 of 12 other species tested. Cellular leakage and lipid peroxidation were consistently lower in the youngest leaf (leaf 4) than in the older leaves. Quantum yield and relative chlorophyll content were the same in all leaves of nontreated plants. Epicuticular wax content was higher in the youngest leaf than in leaves 1, 2, and 3 of cv. ‘Joongangaehobak’ and ‘Wonbiaehobak’. However, leaf cuticle content was not consistent with leaf ages. Differential leaf response to paraquat was partially correlated with the change in catalase, peroxidase, ascorbate peroxidase (APX), and glutathione reductase activities in nontreated and treated leaves. The APX activity in the youngest leaf was generally 2 times higher than in leaves 1-3 in both nontreated and treated plants. Ascorbate antioxidant levels were also higher in the youngest leaf than those in leaves 1-3. Leaves tolerant to paraquat were also tolerant to diquat and to abiotic stresses, low temperature and drought. However, tolerance to oxyfluorfen, which has a different mode of action than paraquat and diquat, was higher in older than in younger leaves. Higher tolerance to paraquat-mediated oxidative and abiotic stresses in young leaves of most squash cultivars might contribute to the differential prevention of oxidative damage in leaves of various ages.