Intensity and duration of nitrogen deficiency on wheat grain number

In humid temperate climates, N is still a major limiting factor for wheat (Triticum aestivum L.) production. Our objective was to understand and quantify N deficiency effects on the crop grain number to develop optimum N fertilizer management strategies for wheat. In this aim, several experiments were conducted on various soil types and climates with 'Soissons' winter wheat. Rates and dates of N fertilizer application were varied in each experiment. This resulted in highly variable dynamics of N accumulation in plants, leading to various N deficiencies throughout the crop cycle. Deficiencies were characterized by a N nutrition index (NNI). Seven criteria describing the deficiency (beginning of deficiency, ED; end of deficiency, ED; duration of deficiency, DD; intensity of deficiency, ID; the product ID X DD = IDD; the lack of nitrogen accumulation at anthesis, LNA; and the NNI at anthesis, NNI2) were estimated for each treatment. Large ranges were obtained for each criterion. Treatments also resulted in highly variable grain numbers. For a N deficient crop, the grain number decrease relative to the control treatment in the same experiment (RGN) was analyzed according to the deficiency criteria. Whatever the grain number component affected (spike number per per square meter or grain number per spike), the RGN appeared to depend on the history of the deficiency, the main explicative variable being IDD, that is, the product of the duration and the intensity of the deficiency. The equation RGN = 1.00355-0.00110 X IDD (R-2 = 0.929) allows the prediction of grain number for wheat crops subjected to various N fertilization strategies.