| Summary: | Soil salinization is the leading environmental factor that restricts crop growth. This study studied the effects of <i>Bacillus cereus</i> (<i>B. cereus</i>) on growth, photosynthesis, and antioxidant metabolism in salt stressed-cucumber seedlings. The results showed that <i>B. cereus</i> could maintain high activity in the high salt environment (4% NaCl). <i>B. cereus</i> 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 CO<sub>2</sub> concentration (Ci), and transpiration rate (Tr) of cucumber seedlings under salt stress. <i>B. cereus</i> significantly increased the maximum photochemical quantum yield of photosystem II (Fv/Fm), the actual photochemical quantum yield (ΦPSII), 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 <i>B. cereus</i> 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 H<sub>2</sub>O<sub>2</sub> and MDA contents of seedlings treated with <i>B. cereus</i> 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 <i>B. cereus</i> could alleviate salt-induced inhibition of growth and photosynthesis by regulating antioxidant metabolism of cucumber seedlings and thus enhancing salt tolerance of cucumber seedlings.
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