Physiological, Canopy, and Yield Responses of Quinoa to Irrigation and Nitrogen Management in the U.S. Midwest

Abstract

Quinoa (Chenopodium quinoa 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 (AEₙ) 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, AEₙ, 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-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 AEₙ 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.