Melatonin-mediated regulation of proline and nitrogen metabolism coordinates with nano-titanium dioxide-modulated ion homeostasis to enhance rice yield under salt stress

Salt stress is a primary abiotic factor limiting global rice productivity. Exogenous growth regulators and nanomaterials have shown great potential in enhancing crop salt tolerance, but the combined effects and underlying mechanisms of melatonin (MT) and nano-titanium dioxide (nTiO 2 ) on rice yield under salt stress remain poorly elucidated. This study aimed to systematically investigate the regulatory effects of combined exogenous MT and nTiO 2 application on rice salt tolerance and clarify the physiological and molecular mechanisms. Two rice genotypes with contrasting salt tolerance (Jingliangyou 3261, salt-tolerant; Ruanhuanyoujinsi, salt-sensitive) were used in a pot experiment under 0.20% salinity. Four foliar treatments were set at the jointing stage: distilled water control (CK), single MT (200 μmol L -1 ), single nTiO 2 (50 mg L -1 ), and combined MT + nTiO 2 . The MT + nTiO 2 treatment enhanced rice yield under salt stress, with the effect being more pronounced in the salt-sensitive variety. The increase in grain yield was primarily attributed to significant improvements in the grains per panicle and the seed-setting rate. At the heading and grain-filling stages, MT + nTiO 2 treatment enhanced the net photosynthetic rate of leaves, providing sufficient assimilates for translocation to grains. Variance decomposition analysis indicated that the MT treatment accounted for the largest proportion of the total variance of proline and nitrogen metabolism. In contrast, nTiO 2 treatment accounted for the highest proportion of variance in ion homeostasis-related traits and maintained higher K + /Na + ratio by upregulating the expression of ion transporter genes such as OsSOSs , OsNHXs , and OsHKTs . Moreover, the MT + nTiO 2 treatment significantly upregulated the expression of MT biosynthesis genes ( OsASMT , OsSNAT , and OsTDC ) and increased leaf MT content. In conclusion, MT and nTiO 2 enhance rice salt tolerance and grain yield through functional complementarity. The combined effects are additive for core traits, including grain yield, proline accumulation, and total nitrogen uptake, with synergistic effects on grains per panicle, dry matter translocation, and peroxidase activity. This study provides theoretical and technical support for high-yield rice cultivation in saline-alkali regions.