Phosphorus application enhances cadmium tolerance in <i>Salix caprea</i> via physiological and anatomical adjustments

Cadmium (Cd) contamination has pronounced negative effects on plant physiological processes, while phosphorus (P) as an essential macronutrient might mitigate Cd toxicity by modulating plant adaptivity. This study employed Salix caprea as a model to assess the effects of P on physiological and anatomical characteristics under Cd stress. The results demonstrated that plant growth, e.g., height, basal diameter and biomass, photosynthetic pigment content and net photosynthetic rate were all significantly inhibited by Cd stress. However, adequate P could alleviate these inhibitory effects in contrast to the P deficiency treatment under Cd stress. Phosphorus sufficiency significantly reduced the levels of reactive oxygen species (O₂˙- and H₂O₂) and malondialdehyde (MDA) in roots under Cd stress, while enhancing the activities of antioxidant enzymes such as superoxide dismutase and ascorbate peroxidase, and increasing the contents of non-enzymatic antioxidants, including ascorbic acid and glutathione. These findings indicate that P reduces Cd-induced oxidative damage by adjusting the antioxidant defense system. Furthermore, P sufficiency enhanced the accumulation of phytochelatins and non-protein thiols in roots, thereby promoting complexation and sequestration of Cd into vacuoles. Adequate P enhanced root mineral uptake, which resulted in higher concentrations of magnesium and manganese in roots. Anatomical analysis revealed that P sufficiency significantly increased the stele-to-root area ratio, thereby enhancing transport efficiency and promoting Cd accumulation in aboveground tissues. Moreover, adequate P significantly increased the levels of abscisic acid, indole-3-acetic acid and gibberellic acid under Cd stress, suggesting a mediated role of hormones in the improved tolerance capacity to Cd by P. In summary, P sufficiency conditions enhanced Cd tolerance in S. caprea by coordinating antioxidant defense, metal chelation, root development and hormonal regulation.