Selenium-induced avoidance mechanism, ionic interactions, and antioxidant system to mitigate manganese toxicity in apple rootstock 'Qingzhen 1′

• Se reduces Mn accumulation and relieves growth inhibition in ' Qingzhen 1 '. • Se mitigates Mn stress via modulating amino acid and citric/tartaric acid. • Se adjusts GS/PCS/MT levels for enhanced Mn detoxification. • Se improves micronutrient uptake to restore nutritional balance under Mn stress. • Se alleviates Mn oxidative damage by strengthening antioxidant defense. Manganese toxicity poses a significant challenge to apple tree growth. Although selenium has been shown to mitigate heavy metal toxicity, the underlying mechanisms by which Se reduces Mn accumulation and toxicity remain poorly understood, particularly in the apple rootstock ‘ Qingzhen 1 ’. This study investigates the role of Se in Mn detoxification, focusing on avoidance mechanisms (such as amino acid and organic acid regulation, phytochelatin and metallothionein expression), ion competition, and modulation of the antioxidant defense system. The application of selenium not only reduced the Mn content by 14.64% but also improved key growth parameters compared to the Mn only treatment. The amino acid level was increased by 2.86%-32.55%, the concentration of citric acid and tartaric acid was increased by 9.01% and 47.30%, the activity and expression of glutathione synthase and plant chelating protein synthase were increased, and the expression of metallothionein was regulated. Furthermore, Se facilitated the uptake of essential nutrients, including Ca, Zn, Cu, and S, which helped restore the plant's nutritional balance. Additionally, Se bolstered the plant's oxidative stress tolerance by enhancing the activities of catalase and superoxide dismutase, while increasing levels of ascorbic acid, glutathione, and non-protein sulphhydryls. Consequently, less hydrogen peroxide and nitric oxide accumulation was observed. Overall, selenium mitigated Mn toxicity in apple rootstock ‘ Qingzhen 1 ’ by activating avoidance strategies, optimizing ion competition, and enhancing antioxidant responses. These findings provide new insights into how Se can reduce Mn uptake and alleviate Mn-induced toxicity in plants.