Dual Mechanisms of Nitrate in Alleviating Ammonium Toxicity: Enhanced Photosynthesis and Optimized Ammonium Utilization in Orychophragmus violaceus

Ammonium (NH4+) toxicity impairs plant growth, but nitrate (NO3−) can mitigate this effect through unresolved mechanisms. Using leaf δ13C values (photosynthetic capacity) and a bidirectional 15N tracer (NH4+ assimilation efficiency and source utilization), this study investigated these mechanisms in 35-day-old Orychophragmus violaceus plantlets grown in modified Murashige and Skoog media under varying NH4+:NO3− ratios. 15N isotope fractionation during NH4+ (same fixed 20 mM NH4Cl) assimilation decreased with increasing NO3− supply (10, 20, and 40 mM NaNO3). Under 20 mM NH4+ (δ15N = −2.64‰) at two 15NO3−-labels (δ15N-NO3− = 8.08‰, low 15N, L) and (δ15N-NO3− = 22.67‰, high 15N, H), increasing NO3− concentrations enhanced NO3− assimilation, alleviating acidic stress from NH4+ and improving photosynthesis. Higher NO3− levels also increased NH4+ utilization efficiency, reducing futile NH4+ cycling and decreasing associated 15N fractionation during assimilation. Our results demonstrate that NO3− alleviates NH4+ toxicity primarily by enhancing photosynthetic performance and optimizing NH4+ utilization efficiency.