Alleviating effect of exogenous nano-silica on the saline-alkali stress of apple rootstock Qingzhen No.1

【Objective】Soil salinization is a severe environmental factor that inhibits plant growth and productivity. Nanomaterials hold potential in enhancing plant stress resistance. Among them, nano - silica (SiO2 NPs) has been proven to promote plant growth and alleviate environmental stress. SiO2 NPs can significantly enhance plant resistance to both biotic and abiotic stresses. They improve plant growth under stress conditions by maintaining water balance, protecting cell membrane integrity, and promoting nutrient absorption. However, as a representative of nanobiotechnology, the application of silica nanoparticles (SiO2 NPs) under saline-alkali stress conditions and their effects on apple seedlings have not been sufficiently studied.【Methods】Under the rain shelter, apple rootstock seedlings of Qingzhen No.1 were subjected to saline - alkali stress. Then, they were irrigated with different concentrations of exogenous SiO2 NPs (25, 50, 100, and 200 mg ·L-1 ). The CK group was irrigated daily with pure water, whereas the SA- T4 group was irrigated daily with 500 mL of saline - alkali solution. Saline - alkali stress was established after three consecutive days of irrigation. Subsequently, groups T1 to T4 were irrigated daily with suspensions of silicon dioxide nanoparticles at corresponding concentrations, while the CK and SA groups were irrigated with clear water (approximately 100 mL). Starting from the day after the application of exogenous substances, leaves and roots were collected from the upper - middle part of the plants on the 0th, 6th, 12th, and 18th days of treatment. These leaves and roots were quickly frozen in liquid nitrogen and stored at -80 ℃ for subsequent related index detection. By analyzing seedling development, ion homeostasis, antioxidant activity, and osmotic regulation, the regulatory mechanisms of applying exogenous silicon dioxide nanoparticles at different concentrations under uniform saline - alkali stress conditions were explored.【Results】Saline-alkali stress significantly inhibited seedling growth, resulting in a reduction of photosynthetic pigments, leaf chlorosis, the accumulation of hydrogen peroxide (H2O2) and superoxide anion (O2 - ), an increase in malondialdehyde (MDA) content and relative electrical conductivity (REC), the disruption of osmotic regulation substances (soluble sugar (SS), soluble protein (SP), proline (Pro)), and an abnormal elevation of antioxidant enzyme systems (CAT, POD, SOD activity). It also disrupts the ion balance. Exogenous SiO2 NPs treatment effectively reversed the stress effects, and the 50 mg·L-1 (T2) treatment yielded the best results. The application of SiO2 NPs significantly alleviated the negative impacts of saline-alkali stress on the apple rootstock Qingzhen No.1, including an increase in leaf area, plant height, stem diameter, and photosynthetic pigments. Furthermore, SiO2 NPs could reduce REC, MDA, H2O2, and O2 - , and enhance the activity of major antioxidant enzymes such as POD and CAT. In terms of ion balance regulation, SiO2 NPs could mitigate the toxic effects of excessive Na+ on cells by inhibiting the absorption and transport of Na+ in roots and reducing the loss of K+ , thereby maintaining a stable Na+ /K+ ratio and ensuring normal cellular physiological functions. To further elucidate the alleviation mechanism of SiO2 NPs on salt-alkali stress in the Qingzhen No.1 apple rootstock, this study employed the paraffin sectioning technique to observe and quantify the microanatomical characteristics of leaves, aiming to evaluate the mitigation effects under different concentrations of SiO2 NPs treatments. The leaf microstructure revealed that under control (CK) conditions without salt-alkali stress, the palisade tissue cells of Qingzhen No.1 leaves were fully differentiated, with regular morphology. They were tightly arranged in elongated columns beneath the upper epidermis, exhibiting minimal intercellular spaces. In contrast, the spongy tissue cells were loosely arranged, with well - developed intercellular spaces, forming a continuous ventilation system that ensured efficient gas exchange. However, under SA treatment, the normal anatomical structure of the leaves was significantly disrupted. The thickness of the palisade tissue decreased by 42.05% compared with the control. Cell elongation along the longitudinal axis was inhibited, the arrangement became disordered, with evident cellular deformation and enlarged intercellular spaces. The volume of spongy tissue cells was simultaneously reduced by 30.41%, accompanied by abnormal expansion of intercellular spaces, which resulted in a loose internal spatial structure and compromised tissue integrity in the leaves. The application of exogenous SiO2 NPs could significantly alleviate the damage to the leaf anatomical structure induced by salt-alkali stress. Among them, the T2 treatment showed the most pronounced mitigation effect. Based on the comprehensive analysis of various physiological and molecular indicators, the alleviation effects of different treatment groups on saline - alkali stress were ranked as follows: CK (1.115 7) ＞ T2 (0.833 9) ＞ T1 (0.041 4) ＞ T3 (-0.293 6) ＞ T4 (-0.574 6) ＞ SA (-1.122 9). This confirmed that 50 mg·L-1 SiO2 NPs mitigated saline-alkali stress damage through synergistic multi-pathway actions: enhancing photosynthetic performance, optimizing antioxidant defense, and regulating the osmotic adjustment system.【Conclusion】It can be concluded that under the experimental conditions, exogenous SiO2 NPs could alleviate saline-alkali stress in apple rootstocks. The 50 mg · L- 1 SiO2 NPs treatment showed the most significant effect by restoring the growth inhibition caused by saline-alkali stress. This concentration exerts its synergistic effects through multiple pathways: reducing the content of the oxidative stress indicator MDA by 59.27% and REC by 40.11%; regulating osmotic adjustment substances such as proline to increase their content by more than 56.55%; and lowering the Na+ /K+ ratio to 0.079, thereby mitigating ion toxicity. In conclusion, 50 mg ·L- 1 SiO2 NPs could effectively mitigate the damage of saline-alkali stress on apple seedlings. This study would provide new insights for the prevention and control of saline- alkali stress in apples and also demonstrate the application potential of nanomaterials in enhancing crop stress resistance.