Salt Stress is Aggravated by NH4+ and Attenuated by NO3− in Parallel to Impairment in Photosynthesis and Oxidative Damages in Jatropha curcas Plants

Abstract This study aimed to investigate how different nitrate/ammonium (NO 3 − /NH 4 + ) ratios modulate the physiological, biochemical, and photochemical responses of Jatropha curcas under saline conditions. Specifically, we examine the intricate interplay between nitrogen metabolism, photosynthetic performance, and oxidative stress mitigation. Whole plants were exposed to salinity under varying NO 3 − /NH 4 + ratios to evaluate biomass, gas exchange, photosystem II (PSII) photochemistry, and the activities of nitrogen-assimilating enzymes. Additionally, a detached leaf-disc assay using progressive NH 4 + concentrations was conducted to isolate direct ammonium toxicity from salinity-induced effects. Increasing the proportion of NH 4 + progressively aggravated salt stress, severely suppressing both shoot and root biomass. This decline correlated with reduced net CO 2 assimilation driven by concurrent stomatal and biochemical limitations, alongside an uncoupling of electron transport from carbon fixation. High NH 4 + levels also compromised PSII photochemistry, suppressed nitrate reductase activity, and induced severe oxidative damage, as evidenced by elevated H 2 O 2 and lipid peroxidation. Conversely, NO 3 − containing treatments sustained nitrogen assimilation, preserved PSII structural integrity, and mitigated oxidative stress under salinity. Finally, a leaf-disc experiment confirmed that NH 4 + accumulation directly triggers concentration-dependent reactive oxygen species (ROS) accumulation and membrane degradation, independent of salt stress. Ammonium nutrition exacerbates salt sensitivity in Jatropha curcas by disrupting nitrogen metabolism, impairing photosynthesis, and inducing oxidative stress. Conversely, nitrate nutrition mitigates salt damage by sustaining nutrient assimilation and maintaining cellular redox homeostasis. These findings underscore balanced NO 3 − /NH 4 + management as a viable strategy to enhance plant tolerance in saline environments.