Abstract: Cadmium (Cd) is one of the most toxic pollutants. In order to investigate the regulatory effect of the exogenous signaling molecule hydrogen peroxide (H₂O₂) on physiological responses of naked oat (Avena nuda) under Cd stress, a pot experiment was conducted in a greenhouse at the Longdong University Experimental Base from April to July 2017. Perlite was used as the culture substrate; the naked oat cultivar ‘Dingyou No.6’ was selected as the variety for testing. Four treatments were included: 1) Naked oat leaves were sprayed with distilled water, and roots fed with Hoagland’s nutrient solution (CK); 2) Naked oat leaves were sprayed with 5 mmol·L^-1 H₂O₂ solution, and the roots fed with Hoagland’s nutrient solution; 3) Leaves were sprayed with distilled water, and the roots were fed with Hoagland’s nutrient solution containing 50 mg·L^-1 Cd²⁺; 4) Leaves were sprayed with 5 mmol·L^-1 H₂O₂ solution, and the roots were fed with Hoagland’s nutrient solution containing 50 mg·L^-1 Cd²⁺. The effects of exogenous H₂O₂ on growth, reactive oxygen species (ROS) metabolism, photosynthetic parameters and activities of key enzymes involved in carbon assimilation were assessed. The results showed that foliar spraying of H₂O₂ significantly alleviated the decrease of root length, plant height, fresh weight and dry weight of naked oats seedlings under Cd stress. Spraying the Cd²⁺ stressed seedlings with H₂O₂ significantly decreased the contents of superoxide anion, H₂O₂, malondialdehyde and ascorbic acid and the activities of catalase, and it increased the activities of superoxide dismutase and ascorbate peroxidase and the glutathione, flavonoid, total phenol and proanthocyanidin levels in seedling leaves, but had little influence on peroxidase activity, chlorophyll a and chlorophyll b contents, chlorophyll a:b ratio, stomatal conductivity, and transpiration rate of leaves. Also, spraying H₂O₂ significantly alleviated the Cd inhibitory effects on carotenoid content, net photosynthetic rate, the activities of ribulose 1, 5-biphosphate carboxylase, sedoheptulose-1, 7-bisphosphatase, fructose-1, 6-bisphosphate aldolase and transketolase, and it decreased the intercellular CO₂ concentration in leaves. The above results indicate that exogenous H₂O₂ can reduce the oxidative damage induced by Cd stress through up-regulation of the ROS scavenging system, and increased activity of key enzymes of carbon assimilation, thus mitigating the inhibition of photosynthesis under Cd stress. In summary, H₂O₂ alleviates the inhibitory effects of Cd stress on the growth of naked oat seedlings and enhances the tolerance of naked oats to Cd stress.

Key words: Avena nuda, H₂O₂, Cd stress, reactive oxygen species, photosynthetic parameter, carbon assimilation enzyme