Exogenous Nano-Silicon Treatment Improved the Low-Temperature Tolerance of Rice Seedlings

Silicon plays an important role in enhancing plant tolerance to abiotic stress. However, the differential regulatory effects of ionic silicon (Ion-Si) and silicon nanoparticles (SiNPs) on rice seedlings under low temperature (LT) stress have been less studied. This study aimed to investigate the effects of ionic silicon and silicon nanoparticles on rice growth, photosynthetic performance, carbon metabolism, antioxidant defense, and yield formation under low-temperature stress. The results indicated that low-temperature stress significantly inhibited the growth of rice seedlings. Exogenous application of Ion-Si and SiNPs effectively alleviated LT-induced growth inhibition and promoted the recovery of rice. SiNPs demonstrated a stronger effect than Ion-Si in maintaining seedling growth, particularly in enhancing plant height, root length, leaf area, dry weight, and root activity. Low-temperature stress significantly reduced chlorophyll content and photosynthetic capacity, including net photosynthetic rate, stomatal conductance, transpiration rate, intercellular CO₂ concentration, and Rubisco activity. However, under LT stress, both Ion-Si and SiNPs increased chlorophyll content, improved photosynthesis, and enhanced Rubisco activity, with SiNPs showing greater improvement in photosynthetic performance compared to Ion-Si. Additionally, silicon application regulated carbohydrate metabolism by increasing soluble sugar content and enhancing the activities of sucrose phosphate synthase and sucrose synthase, thereby promoting osmotic regulation and energy supply. SiNPs had a stronger effect on carbohydrate metabolism and photosynthate transport than Ion-Si. Furthermore, LT stress increased oxidative damage, manifested as elevated levels of H₂O₂ and malondialdehyde. Exogenous Ion-Si and SiNPs reduced ROS accumulation and lipid peroxidation by increasing the activities of antioxidant enzymes, including superoxide dismutase, peroxidase, and catalase. Compared with Ion-Si, SiNPs showed a stronger ability to enhance antioxidant defense and alleviate oxidative damage. Application of silicon mitigated yield loss under low temperature stress, and SiNPs was more effective than Ion-Si in maintaining rice yield, mainly by increasing the number of effective panicles, grains per panicle, and seed setting rate. This study revealed the distinct physiological roles of Ion-Si and SiNPs in rice cold tolerance and provided a theoretical foundation for the application of silicon-based fertilizers in rice production under low-temperature conditions.