Toxic effect of hexavalent chromium on seed germination and seedling growth of red kidney bean and the alleviation effect of exogenous NO

To study the stress effect of chromium (Cr) on seed germination and early seedling growth of red kidney beans and the alleviation effect of sodium nitroprusside (SNP, NO donor), the red kidney bean variety ‘Hongyun 3’ was used as the test material. The effects of Cr and exogenous NO on seed germination, antioxidant system of embryos and seedlings, photosynthetic characteristics of seedlings, and absorption and transport of nutrient elements in red kidney bean were studied using paper germination and hydroponic methods. Treatment with 20 μmol·L⁻¹ K₂Cr₂O₇ stress induced oxidative damage in red kidney bean embryos and seedlings, inhibited the absorption and transport of nutrients by seedlings, reduced the photosynthesis of seedlings, and inhibited the germination of red kidney bean seeds and the growth of early seedlings. The addition of 100 μmol·L⁻¹ SNP (exogenous NO) could alleviate the oxidative stress damage of Cr to embryos and seedlings, reduce the accumulation of Cr in seedlings, promote the absorption and transport of nutrients in seedlings, and enhance the photosynthesis of seedlings. At 72 h of seed germination and growth, the contents of H₂O₂, \mathrmO_2^- , and malondialdehyde (MAD) in embryo decreased by 36.15%, 30.96%, and 34.97%, respectively, compared with Cr stress treatment. Under Cr stress, the contents of H₂O₂, \mathrmO_2^- , and MDA in roots of seedlings treated with SNP decreased by 55.25%, 48.60%, and 33.54%, respectively; and in leaves of seedling growth for 10 d decreased by 27.71%, 36.40%, and 53.85%, respectively, compared to Cr stress treatment. The activities of SOD, POD and CAT in roots and leaves of seedlings significantly decreased by 14.49% and 31.76%, 51.72% and 19.15%, and 28.18% and 42.59%, respectively, compared to Cr stress treatment. The root activity was 59.16% higher than that in Cr stress treatment. The content of Cr in roots and leaves of seedlings decreased by 69.14% and 24.08% compared with Cr stress treatment. The contents of Mg, Fe, Cu, and Zn in roots of seedlings increased by 37.10%, 17.96%, 5.17%, and 13.60%, respectively, compared with Cr stress treatment. The contents of Mg, Fe, Cu, and Zn in leaves of seedlings increased by 32.57%, 29.49%, 22.45%, and 36.07%, respectively, compared with Cr stress treatment. The net photosynthetic rate and PSⅡ maximum photosynthetic efficiency of seedlings were 58.60% and 11.27% higher than those of Cr stress treatment. Exogenous NO alleviates the stress damage caused by Cr on seed germination and early seedling growth of red kidney beans by inducing an increase in antioxidant enzyme activity in the embryo, reducing the absorption of Cr by seedlings, promoting the absorption and transport of nutrients by seedlings, and enhancing photosynthesis in seedlings.