Biomass Partitioning to the Grains As Determinant of Grain Yield In Wheat and Barley.

The main objective of the work was to assess, in a comparative way, variations in grain yield (GY) generation between wheat and malting barley cultivars under contrasting soil nitrogen levels (30 to 200 kg N ha^-1). Changes in GY, numerical components, biomass production and partitioning were evaluated. GY ranged from 196 to 746 g m^-2. Variations in GY by effect of N were associated with variations in total biomass (p< 0.001), while variations in GY by effect of cultivars were associated with differences between species in the partitioning of biomass to the grains (i.e. harvest index, HI; p< 0.01). Independently of the N level, barley cultivars showed higher or similar GY than wheat cultivars; however, wheat showed higher yield responsiveness than barley with improvement in the N condition. The proportion of chaff in the spike at maturity was the best estimator of the differences in HI between species (R²= 0.92 p< 0.001), associated with a low growth of the vegetative structures of the spike in post-anthesis (R²= 0.72 p< 0.001). There was no difference between wheat and barley in total biomass at maturity (p> 0.10); however, the species differed in the efficiency with which converted in biomass the PAR absorbed (radiation use efficiency, RUE) as wheat was more efficient than barley (2.73 and 2.28 g MJ^-1, respectively). Wheat and barley showed lower RUE during post-anthesis than during pre-anthesis, but the differences tended to be higher in barley than in wheat: while in barley the RUE in post-anthesis represented a 48% of the RUE in pre-anthesis, in wheat it represented a 63%. Biomass accumulated during both pre- and post-anthesis, as well as during the whole cycle, was strongly associated with the crop growth rate during the corresponding phase (p< 0.05), without differences between species.