Physiological, canopy, and yield responses of quinoa to irrigation and nitrogen management in the U.S. Midwest

Quinoa (<i>Chenopodium quinoa</i> Willd.) exhibits substantial physiological plasticity under water and nitrogen (N) stress; however, the mechanistic integration of stomatal regulation, canopy development, and agronomic efficiency of nitrogen (AE_n) under humid temperate field conditions remains poorly understood. This study mechanistically evaluated the effects of irrigation regime and N rate on leaf gas exchange, canopy vigor, AE_n, and grain yield during the 2024 and 2025 growing seasons in the U.S. Midwest. Two genotypes were tested under four irrigation regimes (full, progressive, deficit, and extreme deficit) and three N rates (0, 75, and 150 kg N ha^-1) using a split-split plot design. The results indicated that the high-yielding genotype consistently produced higher grain yield across treatments, whereas the low-yielding genotype consistently produced lower grain yield; however, both genotypes responded similarly to irrigation and N application, with no significant genotype × management interactions. Irrigation primarily regulated gas exchange during early growth and flowering, whereas nitrogen status and irrigation × N interactions governed physiological performance during grain filling. Increasing N enhanced net photosynthesis, stomatal conductance, and NDVI at peak canopy development, but these increases did not translate into proportional yield gains at the highest N rate. Grain yield at 150 kg N ha^-1 under extreme deficit irrigation was comparable to that at 75 kg N ha^-1 under progressive irrigation, indicating strong water-nitrogen trade-offs. Agronomic efficiency of nitrogen differed significantly among irrigation regimes, whereas N rate and genotype had no significant effects, and AE_n declined with increasing N input. Overall, moderate irrigation combined with 75 kg N ha^-1 was associated with improved physiological performance, canopy vigor, and favorable grain-yield and agronomic efficiency nitrogen responses, highlighting grain filling as the primary physiological control point under humid temperate conditions.