Foliar Titanium Dioxide Nanoparticles Enhance Rice Yield by Improving Photosynthesis, Ion Balance, and Antioxidant Defense Under Salt Stress

Salinity stress severely limits rice productivity and grain quality worldwide. Although exogenous foliar application of titanium dioxide nanoparticles (nano-TiO₂) has been reported to enhance crop stress tolerance, its regulatory roles in yield formation and grain quality in rice varieties with differing salt tolerance are not well understood. In the present study, two contrasting rice varieties, <i>viz.</i>, Jingliangyou 3261 (JLY3261; salt-tolerant) and Yuxiangyouzhan (YXYZ; salt-sensitive), were applied with five nano-TiO₂ foliar application treatments-<i>viz</i>., CK: water spray; Ti1: 15 mg L^-1; Ti2: 30 mg L^-1; Ti3: 45 mg L^-1; and Ti4: 60 mg L^-1-at the jointing and panicle initiation stages. Plants were irrigated with 0.3% saltwater to simulate salt stress. The results showed that Ti2 and Ti3 treatments led to 8.59% and 14.80% increases in grain yield in JLY3261 and YXYZ, respectively, compared with CK. Ti2 and Ti3 treatments significantly increased the leaf area index, net photosynthetic rate, and aboveground biomass of both varieties at the heading stage. Meanwhile, the activities of antioxidant enzymes such as superoxide dismutase and peroxidase, as well as nitrogen metabolism enzymes including nitrate reductase and glutamine synthetase, were improved with a substantial reduction in malondialdehyde contents. Application of nano-TiO₂ upregulated the expression of ion transport-related genes such as <i>OsSOSs, OsNHXs</i> and <i>OsHKTs</i>, thus improving leaf K⁺ accumulation and reducing Na⁺ content to optimize the K⁺/Na⁺ ratio. In addition, Ti2 and Ti3 treatments improved the milled rice rate, head rice rate, and protein content, while they decreased the chalkiness degree of both rice cultivars. Principal component analysis showed that the aboveground biomass at the heading stage was a core evaluation index for both varieties. Overall, foliar application of 30-45 mg L^-1 nano-TiO₂ was found to be effective regarding growth and yield improvement in rice under saline conditions. This study provides a theoretical basis for agro-management strategies for rice cultivation in saline-alkaline soils.