Determination of a critical nitrogen dilution curve for spring wheat
Plant-based diagnostic tools of N deficiency can be based on the concept of critical N dilution curves describing whole-plant critical N concentration (Nc; g kg–1 of dry matter [DM]) as a function of shoot biomass (W; Mg DM ha–1). This has been tested for several crops, including winter wheat (Triticum aestivum L.) but has not been tested for spring wheat. Our objectives were to determine a critical N dilution curve specific to spring wheat, to compare this curve with existing critical N dilution curves for winter wheat, and to assess the plausibility of using it to estimate the level of N nutrition. The study was conducted at six site-years (2004–2006) in Québec, Canada, with four to eight N fertilization rates (0–200 kg N ha–1). Shoot biomass and N concentration were determined on five to eight sampling dates during the growing season, and grain yield was measured at harvest. A critical N dilution curve (Nc = 38.5 W–0.57) was determined for spring wheat and was different from those reported for winter wheat. The N nutrition index (NNI = Nobserved/Nc) calculated from this spring wheat critical N dilution curve was significantly related (R2 = 0.70; P < 0.001) to relative grain yield. This critical N dilution curve and the resulting NNI adequately identified situations of limiting and nonlimiting N nutrition and could be used to establish the N nutrition status.