Wheat grain yield can be decomposed in yield components (number of spikes / m2, number of grains / spike, number of grains / m2, thousand grain weight (TGW)) correlated with each other due to the compensations that occur between the different scales. Many studies identified the number of grains per m2 as the main determinant of yield. However, with higher occurrence of abiotic stresses during the post-flowering period, TGW may become an important source of genetic variability for postflowering abiotic stresses tolerance. TGW is usually analysed, as the other yield components, through its average value at the plot scale. Yet, there is a large variance of the final sizes of individual grains. The aim of this study is to understand if grains with various potential size at anthesis react differently to post-flowering stresses and if this differential response is a source of abiotic stresses tolerance.

The work presented here focuses on the characterization of the variance of individual grain sizes through the study of grains bulks originating from 2 large genetic panels grown for 2 years under optimal conditions and under water stress, i.e. four environments. Average yield components and the individual grain masses estimated by their sizes (about 500 grains) were measured. Our results show that the genetic variability is stronger for the variance than for the average of grain sizes. This individual grain sizes variance was positively correlated with TGW and negatively with the number of spikes per m2 for all environments, showing that this variance is in part the result of pre-flowering development. Nevertheless, the correlations of individual grain sizes variances between environments were high (R = 0.68, R = 0.62 in 1st and 2nd year respectively), which therefore suggests a specific genetic determinism for this trait. This determinism was then studied by genome-wide association focusing on the presence, or absence, of colocalizations with traits related to yield components. The variance of grain sizes is therefore defined partially by specific genetic determinism and partially in response to different strategies to set potential yield at anthesis. Future studies will determine whether grain sizes variances have differential effects on post-flowering abiotic stress tolerance during grain filling.