Genetic variability in biomass allocation to roots in wheat is mainly related to crop tillering 1 dynamics and nitrogen status 2 3

18 Improving arable crops Nitrogen Use Efficiency (NUE) is a major target of plant breeding. In 19 wheat, a large part of the Nitrogen (N) harvested in the grain originates from N remobilization 20 from vegetative organs during the post-anthesis period. While N remobilization from above-21 ground organs such as leaves and stems has been extensively studied, studies about N 22 remobilization from roots are scarce. The existence of genetic variability for biomass 23 allocation to the root pool as well as for root N concentration and remobilization may strongly 24 affect the N economy of the crop. By studying the root system of 16 elite wheat genotypes 25 under two contrasted N treatments, we showed that the biomass allocation to roots was 26 strongly related to crop tillering dynamics and N nutritional status. Therefore, the apparent 27 genetic variability for biomass allocation to roots is not intrinsic, but rather a consequence of 28 genetic variability for crop growth and N utilization taken as a whole. In addition, we showed 29 here that the N amount potentially remobilized from roots to the grain is extremely small. 30 Existing genetic variability for root N content and remobilization efficiency cannot explain 31 observed differences in genotypes grain N concentration or N yield. These results indicate 32 that there is little prospect for breeding strategies specifically aiming at optimizing wheat root 33 biomass allocation and N remobilization for improving NUE and GPC for elite genotypes at 34 least in highly productive conditions. These results nevertheless do not imply that the root 35 compartment should be totally discarded in all breeding programs since architectural traits 36 such as root length or distribution may impact strongly crop performance.