Effects of Bacillus cereus on Photosynthesis and Antioxidant Metabolism of Cucumber Seedlings under Salt Stress

Soil salinization is the leading environmental factor that restricts crop growth. This study studied the effects of Bacillus cereus (B. cereus) on growth, photosynthesis, and antioxidant metabolism in salt stressed-cucumber seedlings. The results showed that B. cereus could maintain high activity in the high salt environment (4% NaCl). B. cereus significantly increased plant height, stem diameter, fresh weight, and dry weight of cucumber seedlings under salt stress, and increased root vitality, net photosynthetic rate (Pn), stomatal conductance (Gs), intercellular CO2 concentration (Ci), and transpiration rate (Tr) of cucumber seedlings under salt stress. B. cereus significantly increased the maximum photochemical quantum yield of photosystem II (Fv/Fm), the actual photochemical quantum yield (phi PSII), and the quantum yield of regulatory energy dissipation Y (NPQ) under salt stress, which were 9.31%, 20.44%, and 5.22% higher than those under salt stress, respectively. The quantum yield of non-regulatory energy dissipation Y (NO) was reduced by 19.81%. Superoxidase (SOD), peroxidase (POD), and catalase (CAT) activities in leaves and roots of cucumber seedlings were significantly increased by B. cereus under salt stress. Compared with salt stress, SOD activities in leaves were significantly increased by 1.70% and 6.32% on the first and third days after treatment. At 1 d, 3 d, and 5 d after treatment, SOD activity in roots increased by 3.06%, 11.24%, and 3.00%, POD activity in leaves increased by 113.38%, 38.81%, and 52.89%, respectively. The POD activity in roots increased by 56.79% and 10.92% on the third and fifth days after treatment, the CAT activity in leaves increased by 8.50% and 25.55%, and the CAT activity in roots increased by 30.59% and 84.45%. Under salt stress, the H2O2 and MDA contents of seedlings treated with B. cereus decreased significantly. Compared with salt stress, the proline content in leaves decreased by 12.69%, 3.90%, and 13.12% at 1 d, 3 d, and 5 d, respectively, while the proline content in roots decreased by 44.94% and 60.08% at 3 d and 5 d, respectively. These results indicated that B. cereus could alleviate salt-induced inhibition of growth and photosynthesis by regulating antioxidant metabolism of cucumber seedlings and thus enhancing salt tolerance of cucumber seedlings.