There is a lack of studies that have investigated grain yield, its components and photosynthesis in late stages of wheat growth, giving us insufficient understanding of how these factors interact to contribute to yield during this period. As a result, three field experiments were carried out examining 20 winter wheat genotypes of diverse origins under irrigated, terminal drought and dryland conditions in the southern Idaho. Our objective was to evaluate the interaction between post-anthesis physiological traits, especially leaf-level photosynthetic capacity, senescence and yield components on grain yield in different moisture regimes. Genotype differences were found in leaf-level photosynthesis and senescence, canopy temperature depression, grain yield and yield components in each water regime. Grain yield was closely associated with traits related to grain numbers. In all three moisture regimes, positive correlations were observed between grain yield and photosynthesis that were dependent on the timing or physiological growth stage of the photosynthetic measurement: highly significant correlations were found in the mid-and late grain filling stages, but no correlations at anthesis. Consistent with these findings, flag leaf senescence at the late grain filling stage was negatively correlated with grain yield and photosynthetic rate (under terminal drought and dryland conditions). These findings provided evidence that grain yield was sink-limited until the final stages of growth, at which time sustained photosynthesis and delayed senescence were critical in filling grain. Because the trends were consistent in moisture sufficient and deficient conditions, the results suggest that late-season photosynthesis and delayed leaf senescence are driven by the size of the reproductive carbon sink, which was largely governed by factors affecting grain numbers.

Nitrogen and water are two key factors for wheat production due to their major roles in plant growth and development, photosynthesis, yield, and grain protein content. Plant uptake of water and N is fundamentally interactive. Our objectives were: (a) to analyze the effects of different irrigation (IR) and N rates on spring wheat (Triticum aestivum L.) yield and grain protein, and spectral indices (relative greenness [SPAD] and Normalized Difference Vegetation Index [NDVI]), and (b) to identify the optimum IR and N requirements for wheat grain production in semi‐arid conditions of Montana and Idaho. This article details the results from field experiments conducted at three locations for two growing seasons (6 site‐years). Relative greenness measured by SPAD chlorophyll meter was used to assess plant N status, whereas NDVI was used for both plant N status and estimation of wheat yield. Both SPAD and NDVI values increased as N and IR application rates increased. The SPAD and NDVI values explained 80 and 84% of the variation in wheat yield, respectively. We found that IR at 75% of evapotranspiration (ET) throughout the growing season is adequate to optimize wheat yield and grain protein. Nitrogen rate was not correlated with wheat yield at any of the site‐years. Based on this study's results, approximately 150 kg N ha−1 (total, soil residual N plus N added as fertilizer) may be sufficient to optimize yield and grain protein content of irrigated spring wheat in semi‐arid cropping systems.

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