Optimized nitrogen supply enhances nitrogen homeostasis, ATP-coupled energy metabolism, and morpho-physiological...

Optimizing nitrogen (N) supply is critical to sustaining rice productivity while reducing fertilizer inputs and ecological costs. We examined the aromatic rice landrace Oryza sativa L. cv. Keteki Joha under a nitrate (NO₃^-) gradient (0-20 mM) in a hydroponic system to define the optimum N regime and its mechanistic basis. Moderate NO₃^- supply (4-10 mM, optimum 6 mM) significantly improved shoot and root biomass, chlorophyll content, photosynthetic efficiency, and water uptake, whereas both deficiency (≤0.25 mM) and excess (≥15 mM) suppressed growth. Enhanced performance at the optimum level was closely associated with higher vegetative N use efficiency (vNUE), improved membrane stability, balanced reactive oxygen species (ROS) metabolism, and more effective antioxidant defense. At the molecular level, key genes involved in N and energy metabolism (OsYL1, OsATP synthase1, OsATP-NAD kinase, OsLFNR1) were upregulated under optimal supply, thereby sustaining ATP generation and redox homeostasis, while their expression was suppressed under N deficiency or excess. This study provides the first mechanistic evidence that Keteki Joha exhibits a narrow N responsive window that safeguards N homeostasis, ATP-coupled energy metabolism, and morpho-physiological adaptation. Although validated in hydroponics, these findings offer a physiological framework for precision nutrient management and guide breeding strategies to enhance NUE, resilience, and sustainability in traditional aromatic rice.