Chitosan Concentration Modulates Selenium Biofortification and Rice Grain Quality via Chitosan–Nano-Selenium Interactions

Chemically synthesized selenium (Se) nanoparticles are often of low stability due to their surface charge. Therefore, it is crucial to employ dispersants to improve their stability. However, it remains largely unknown how the dispersant concentration affects the efficacy of Se nanoparticle complexes. To determine how the chitosan concentration used to prepare chitosan–nano-Se complexes affects grain Se biofortification, heavy metal accumulation, and aroma quality in different rice varieties, in this study, we prepared chitosan–nano-Se complexes (CS1, CS2, and CS3) with chitosan concentrations of 0.1, 0.3 and 0.5 mg/mL, applied them to four rice varieties via foliar spraying at the heading stage, and analyzed their effects on the levels of Se, heavy metals, and volatile organic compounds (VOCs) in rice grains. The results showed that chitosan concentration influenced the uptake of nano-Se, accumulation of heavy metals, and content of VOCs in a rice variety-dependent manner. CS1 treatment resulted in the highest total Se content in rice grains, and CS3 treatment maximized the content of VOCs including 2-AP, while no consistent trend was observed in heavy metal accumulation. These findings indicate that the preparation protocol of chitosan–nano-Se complexes should be selected according to the rice varieties and specific objectives for the targeted regulation of Se enrichment, heavy metal reduction, and aroma improvement in rice production.