Objective  To explore the effects of phosphorus application on photosynthesis and antioxidant enzyme system of cucumber seedlings under cadmium (Cd) stress in calcareous soil , and to provide a scientific basis for rational application of phosphate fertilizers and alleviation of Cd toxicity.

  Method  Under different Cd addition levels (0, 1, 3, 5 mg kg⁻¹), the effects of different phosphorus concentrations (0, 50, 100, 150, 200 mg P₂O₅ kg⁻¹) on cucumber seedling biomass, leaf photosynthesis and antioxidant enzyme system were studied by using the pot complete random design scheme.

  Result  Under the single Cd treatment, with the increase of Cd application, the biomass of cucumber increased first and then decreased, the net photosynthetic rate (Pn), stomatal conductance (Gs) and transpiration rate (Tr) of cucumber leaves were significantly reduced, and the intercellular CO₂ concentration (Ci) increased. The addition of appropriate phosphorus ( ≤ 150 mg P₂O₅ kg⁻¹) were significantly increased the cucumber biomass, Pn, Gs, Tr, total chlorophyll and carotenoid content (Car), and reduced Ci in cucumber leaves. For the antioxidant enzyme system, with the increase of Cd application, the activities of peroxidase (POD) and catalase (CAT) in cucumber leaves decreased significantly, and the content of malondialdehyde (MDA) increased significantly. After applying P, the antioxidant enzyme system can be significantly enhanced. When the application of Cd was 1 mg kg⁻¹ and phosphorus application of 50-100 mg P₂O₅ kg⁻¹, the CAT activity increased by 1.9%-9.6%%. When the amounts of Cd applied were 3 and 5 mg kg⁻¹, the POD and CAT activities increased to varying degrees with the increase of P application. Compared with no P application, the POD activities increased by 149.1%-187.8% in 3 mg Cd kg⁻¹ treatment, 150.4-252.0% in 5 mg Cd kg⁻¹ treatment, and the CAT activities increased by 5.8 %-16.6 %, 0.55%-19.4%, respectively. The POD and CAT activities of cucumber leaves reached the maximum value when the phosphorus application of 100 mg P₂O₅ kg⁻¹. At the same time, P application could reduce the MDA content in leaves, and gradually decreased with the increase of P application.

  Conclusion  In calcareous soil, the phosphorus application rate of 100 or 150 mg P₂O₅ kg⁻¹, it can significantly alleviate the Cd toxicity and enhance the photosynthetic characteristics and antioxidant enzyme system of cucumber leaves, while phosphorus applications up to 200 mg P₂O₅ kg⁻¹ inhibited the growth of cucumber.