Plant-Based Diagnostic Tools for Evaluating Wheat Nitrogen Status

The nitrogen nutrition index (NNI), based on critical plant N dilution curves, was developed to determine the in-season N status of many species including wheat (Triticum aestivum L.). We assessed the relationship between wheat NNI and two simpler diagnostic tools; namely, leaf nitrogen (N(L)) concentration and chlorophyll meter (CM) readings. The study was conducted at six site-years (2004-2006) in Quebec, Canada, using four to eight N fertilizer rates (0-200 kg N ha(-1)). Leaf N concentrations and CM readings were determined from the uppermost collared leaf during the growing season along with NNI determinations. Generally, NNI, N(L) concentrations, and CM readings increased with increasing N rates. Leaf N concentrations and CM readings were significantly related to NNI during the growing season. Normalization of the CM values, relative to high N plots (relative chlorophyll meter [RCM] readings), improved the relationship with NNI by reducing site-year differences. However, variation among sampling dates was observed in all relationships. By restricting the sampling dates to essentially the elongation stage, the relationship between NNI and N(L) (NNI = -0.43 + 0.035 N(L); R(2) = 0.52), CM (NNI = -0.64 + 0.039 CM; R(2) = 0.68), or RCM (NNI = -1.31 + 2.45 RCM; R(2) = 0.82) was generally improved. Nitrogen concentration, CM reading, or RCM reading of the uppermost collared leaf, preferably at the elongation stage, can therefore be used to assess the nutritional status of spring wheat.