Spatial co-application of soil-applied ZnO nanoparticles and foliar 24-epibrassinolide: A complementary strategy to improve plant–soil interactions and rice yield under salt stress

Salinity stress disrupts aboveground physiological performance and the microecological balance of the rhizosphere. Although zinc oxide nanoparticles (ZnO NPs) and 24-epibrassinolide (EBR) are known mitigators of plant stress, the effects of their combined application remain unknown. Here, we coupled soil-applied ZnO NPs with foliar-applied EBR and evaluated their effects on rice productivity under salt stress in a two-year field experiment. Combined treatment with ZnO NPs and EBR increased grain yield by 19.5%–21.7% relative to the control, primarily due to an increased grain-filling rate. Further analysis suggested differential contributions of EBR and ZnO NPs to above- and belowground traits across developmental stages. At the heading stage, EBR explained the largest proportion of the variation in photosynthetic rate, osmolyte content, and antioxidant activities. ZnO NPs were associated with enhanced rhizosphere enzyme activities, improved nutrient availability, and restricted root sodium accumulation. This belowground improvement was associated with enhanced leaf physiological performance during grain filling. The combined application also enriched dominant bacterial phyla ( Proteobacteria, Acidobacteriota , and Chloroflexi ) and increased the levels of deoxyguanosine and deoxyadenosine in the rhizosphere. An exploratory structural equation model identified associations among rhizosphere properties, leaf physiology, and grain yield. This combined application strategy shows promise for improving rice productivity in saline-alkali fields by simultaneously enhancing aboveground physiological performance and belowground ecological stability.