Foliar application of zinc oxide nanoparticles improved yield and 2-acetyl-1-pyrroline content in fragrant rice under salt stress

Exogenous application of zinc oxide nanoparticles (ZnO NPs) has been shown to increase the stress resistance of crops, however, its effects on the yield and 2-acetyl-1-pyrroline (2-AP) content of fragrant rice under salt stress remain unclear. The present study investigated the effects of foliar application of ZnO NPs on the yield and 2-AP content of fragrant rice under salt stress. The experiment involved two fragrant rice varieties, i.e. Ruanhuayou1179 and Ruanhuayoujinsi, and four levels of ZnO NPs, i.e. spraying water (CK), 100 ​mg ​L −1 (NP1), 200 ​mg ​L −1 (NP2), and 400 ​mg ​L −1 (NP3), with 0.3​% (5 ​dS ​m −1 ) saltwater (a mixture of freshwater and seawater). The results demonstrated that the NP1 treatment exhibited the highest yield for both fragrant rice varieties among all treatments, primarily due to an increase in grain number per panicle and grain filling rate. Furthermore, compared with CK, the NP1 treatment significantly enhanced aboveground biomass, chlorophyll content, and potassium ion content in the leaves while reducing malondialdehyde content. Compared with CK, the NP1, NP2, and NP3 treatments significantly increased 2-AP content of both fragrant rice varieties by 46.23–46.67​%, 31.66–43.99​%, and 21.72–39.31​%, respectively, mainly due to increased levels of leaf proline, Δ1-pyrroline-5-carboxylate, 1-pyridine, and methylglyoxal contents. The NP1 treatment also upregulated the 2-AP synthesis enzymes and their gene expression (such as ornithine aminotransferase, diamine oxidase, and Δ1-pyrroline-5-carboxylate synthetase) in comparison with CK. In conclusion, foliar application of 100 ​mg ​L −1 ZnO NPs proved to be the most effective in enhancing both yield and 2-AP content in fragrant rice under salt stress. • Foliar application (FA) of ZnO NPs enhances yield in fragrant rice under salt stress. • FA of ZnO NPs enhances salt tolerance in fragrant rice due to K + and K + /Na ​+ ​ratio. • FA of ZnO NPs improves 2-AP by increasing enzyme activity and gene expression. • Proline accumulation synergistically increases 2-AP and salt tolerance in fragrant rice.