Silicon mitigates copper toxicity of Citrus sinensis (L.) by improving nutrient status, root development, photosynthetic performance and antioxidant response

• Si decreases excessive Cu translocation from roots to leaves of C. sinensis . • Si regulates the nutrient homeostasis of C. sinensis under copper toxicity. • Si increases the electron transport efficiency under Cu toxicity. • Si activates the antioxidant defense of C. sinensis under Cu toxicity. China is the world's largest producer of citrus fruits. However, soil acidification and irrational copper (Cu) application in citrus orchards frequently induce Cu toxicity, restricting the sustainable development of the citrus industry. Studies have shown that silicon (Si) fertilization effectively mitigates the toxicity of heavy metals. However, research on Si-mediated alleviation of Cu toxicity in citrus species remains limited. In the present study, sand-cultured seedlings of sweet orange [ Citrus sinensis (L) Osbeck Cv. Xuegan] were subjected to eight treatments containing 2 Cu concentrations (0.5 μM and 300 μM) and 4 Si concentrations (0, 1, 3, and 5 mM). The effects of those treatments on biomass accumulation, Cu and Si distribution, root architecture, nutrient uptake, leaf photosynthetic performance and antioxidant systems were investigated to reveal the physiological and biochemical mechanisms underlying Si-mediated alleviation of Cu toxicity in C. sinensis seedlings. The present results verified that supplementation with Si at an optimal level promoted C. sinensis Cu tolerance by decreasing Cu translocation, maintaining nutrient homeostasis, improving photosynthetic performance, and activating the antioxidant defense in C. sinensis . Overall, the present results indicated that 3 mM Si is more beneficial against 300 μM Cu stress in C. sinensis seedlings. The results identified the benefits of Si in woody plants and will provide a necessary reference for Cu detoxification in acidified citrus orchards of South China.