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 bidirectional15N tracer (NH4+assimilation efficiency and source utilization), this study investigated these mechanisms in 35-day-oldOrychophragmus violaceusplantlets 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 two15NO3−-labels (δ15N-NO3−= 8.08‰, low15N,L) and (δ15N-NO3−= 22.67‰, high15N,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 associated15N fractionation during assimilation. Our results demonstrate that NO3−alleviates NH4+toxicity primarily by enhancing photosynthetic performance and optimizing NH4+utilization efficiency.