Context: A better understanding of the plasticity of grain number (GN) components may be essential to design better management and breeding strategies. Alterations in resource availability during pre-anthesis may affect the interplay between the main GN components, spike number (SN) and grains per spike (GS). Quantifying the magnitude, and understanding the physiological bases, of differences in plasticity of GN components within elite material would be valuable for breeding. Objective: The aim of this study was to investigate the physiology of GN determination, the plasticity of, and any possible trade-offs between its components. Methods: Two modern genotypes (Ascott and Sy Moisson), expected to have contrasting GS response to changes in SN, were subjected to 5 levels of resource availability (two levels of shading, two levels of thinning and an unmanipulated control) from the onset of stem elongation to anthesis in two locations (France and Spain). Results and conclusions: Yield in the control was relatively high in both locations and strongly related to GN, which was more plastic in Ascott than in Sy Moisson; and the difference in plasticity was related to different responses of the two main GN components. SN acted as a coarse regulator of yield contributing with crop level plasticity while GS acted as a fine-tuning mechanism expressing the smaller responses to resource at genotypic level. The higher GS plasticity of Ascott-like genotypes may provide more stability in early stress scenarios where the SN is reduced as long as no later limitations occur. Implications: The acceptance of a hypothesis on genotypic variation in plasticity of GS derived from multilocation and multiyear datasets, and the consistency of the genotypic difference in response to changes in resources across the two locations, suggests that the genotypic difference in plasticity may be constitutive and would therefore be useful in designing strategic crosses aiming to combine plasticity of GS with other yield-related attributes.