Exogenous AsA Coordinates Carbon and Nitrogen Metabolism and Enhances Drought Tolerance in Upland Rice ( Oryza sativa L.) by Activating the AsA–GSH Cycle

Abstract

Prolonged drought leads to excessive accumulation of reactive oxygen species, damage to photosystems and an imbalance in carbon and nitrogen metabolism, thereby limiting the reproductive growth and yield potential of drought‐stressed rice ( Oryza sativa L .). The Ascorbate–Glutathione Cycle is a key metabolic pathway for maintaining redox homeostasis in plant cells; however, there remains a lack of evidence from long‐term field trials to demonstrate whether exogenous ascorbate can, by activating this cycle, translate antioxidant responses at the cellular level into yield‐stabilising effects at the field level. The aim of this study was to elucidate the physiological mechanisms by which exogenous AsA regulates drought adaptation and yield formation in upland rice. Using the upland rice variety ‘Hengfengyou Jinsimiao’ as the experimental material, a 3‐year consecutive field trial was conducted from 2023 to 2025. Four treatments were established: normal irrigation combined with clear water spraying (CK), normal irrigation combined with AsA foliar spraying (CK + A), drought stress combined with clear water spraying (D) and drought stress combined with AsA foliar spraying (D + A). Systematic measurements were taken of leaf reactive oxygen species (ROS) and membrane lipid peroxidation levels, components of the AsA–GSH cycle and related enzyme activities, expression of antioxidant‐related genes, endogenous hormones, osmotic regulators, photosynthetic characteristics, carbon and nitrogen metabolic enzyme activities, as well as grain yield and yield components. Drought treatment significantly promoted the accumulation of H 2 O 2 , O 2 − and malondialdehyde, inhibited photosystem II activity, gas exchange and carbon and nitrogen metabolism and reduced grain yield by 18.75%, 19.60% and 19.09% in 2023–2025, respectively. Exogenous AsA did not exert its primary effect by continuously enhancing traditional antioxidant enzymes such as SOD, POD and CAT; rather, it expanded the AsA and GSH antioxidant pools, increased the AsA/DHA ratio and enhanced the activity of APX, MDHAR, DHAR, GR, GPX and γ‐ECS, whilst simultaneously upregulating genes associated with AsA biosynthesis and regeneration, thereby accelerating the AsA–GSH cycle and reducing ROS and membrane lipid peroxidation levels. The restoration of redox homeostasis further maintained Fv/Fm, Fv/Fo and net photosynthetic rate, alleviated the drought‐induced inhibition of GS, GOGAT, GDH, NR and sucrose metabolic enzyme activities and promoted the accumulation of soluble proteins and proline. Exogenous AsA also selectively restored leaf GA levels and reduced the drought‐induced accumulation of ABA. Ultimately, the D + A treatment increased grain yield in 2023–2025 by 12.93%, 14.19% and 15.35%, respectively, compared with the D treatment, primarily by increasing the number of grains per ear and the grain‐setting rate. Exogenous AsA enhances the adaptability of upland rice during the reproductive growth stage to prolonged drought by establishing a continuous regulatory cascade comprising ‘activation of the AsA–GSH cycle—alleviation of oxidative damage—maintenance of photosynthetic and carbon–nitrogen metabolism—coordination of hormonal and osmotic regulation—improved grain formation’, thereby stabilising and mitigating yield losses. Exogenous AsA is therefore not only an antioxidant that directly scavenges ROS, but also a functional factor that regulates redox metabolic networks and coordinates the growth–defence balance, providing a theoretical basis and potential technical approach for the stable cultivation of upland rice in water‐scarce regions.