Selenium aggravates carbon-nitrogen metabolic imbalance by promoting copper uptake and translocation in apple (Malus domestica Borkh.) plants under copper stress

Abstract

Copper (Cu) is an essential trace element for plant growth that has concentration-dependent effects, and selenium (Se) is a beneficial element that regulates plant physiological and biochemical processes. However, the effect of Se on Cu absorption and availability remains unclear. Hydroponic experiments were conducted to determine the effects of optimal exogenous Se application on the physiological traits, Cu uptake/transport, and Cu forms of different apple organs under normal (NCu) and high Cu concentrations (HCu). Se upregulated Cu uptake-related genes (COPT1, ZIP2, and ZIP4) in leaves and roots under both Cu levels, promoting Cu uptake. Cu accumulation under NCu + Se and HCu + Se treatments increased by 72.7% and 41.2% compared to NCu and HCu treatments, respectively. Under NCu, Se upregulated Cu detoxification genes (MT2, CCS, and CSD1), downregulated transport genes (HMA5 and YSL3), decreased cellular inorganic/organic Cu, and increased bound Cu, carbon and nitrogen metabolism, and the antioxidant capacity, leading to a 52.3% biomass increase. Under HCu, Se downregulated detoxification genes (CCS and MT2), increasing all Cu forms and inducing H₂O₂, O₂^∙−, and malondialdehyde accumulation. Although plants enhanced the activity of antioxidant enzymes (superoxide dismutase, catalase and peroxidase) activity and osmotic regulators (sugars, free amino acids, soluble proteins) to alleviate stress, growth-related energy was diverted, inhibiting growth. These results reveal new insights into Cu absorption and bioavailability in apple in response to Se application and provides a basis for the safe application of Se fertiliser under different soil Cu levels.