A functional explanation for the regulation of grain nitrogen (N) accumulation in cereal by environmental and genetic factors remains elusive. Here, new mechanistic hypotheses of grain N accumulation are proposed and tested for wheat (Triticum aestivum). First, we tested experimentally the hypothesis that grain N accumulation is mostly source regulated. Four contrasting cultivars, in terms of their grain N concentrations and yield potentials, were grown with non-limiting N supply. Grain number per ear was reduced by removing the top part of the ear at anthesis. Reduction in grain number gave a significant increase in N content per grain for all cultivars, showing that grain N accumulation was source regulated. However, on a per ear basis, cultivars with a high grain number fully compensated their N accumulation for reduced grain number at anthesis. Cultivars with a lower grain number did not compensate completely, and grain N per ear was decreased by 16%. Second, new mechanistic hypotheses of the origins of grain N source regulation and its response to environment were tested by simulation. The hypotheses were: (a) The regulation by N sources of grain N accumulation applies only for the storage proteins (i.e. gliadin and glutenin fractions); (b) accumulation of structural and metabolic proteins (i.e. albumin-globulin and amphiphilic fractions) is sink-regulated; and (c) N partitioning between gliadins and glutenins is constant during grain development and unmodified by growing conditions. Comparison of experimental and simulation results of the accumulation of grain protein fractions under wide ranges of N fertilization, temperatures, and irrigation supported these hypotheses.One challenge for global nutrition in the next decade is to increase food yield per unit ground area in a sustainable manner while maintaining its end use value (Cassman, 1999; Tilman, 1999; Tilman et al., 2002). Grain protein concentration and composition are major determinants of grain nutritional value (Feil, 1997). The concentration of Lys in grain, the most limiting amino acid in cereals for human and monogastric animals, increases with increasing grain protein concentration (Feil, 1997) despite the decrease of its concentration in total protein (Mossé et al., 1985). Grain protein concentration and composition are also the major determinants of flour functional properties (Weegels et al., 1996;Shewry and Halford, 2002). However, the inverse relationship between grain yield and protein concentration, reported for several species, may prevent breeders from improving these two traits simultaneously (Stewart and Dwyer, 1990; Delzer et al., 1995; Feil, 1997; Brancourt-Hulmel et al., 2003). To break this inverse relationship, genetic increments in grain protein yield must keep pace with those in grain yield.Therefore, efforts to overcome the inverse relationship between grain yield and protein concentration must concentrate on improving grain protein accumulation per square meter and per grain (Feil, 1997; Triboï and Triboï-Blondel, 2002...
