Shoot biomass in wheat is the driver for nitrogen uptake under low nitrogen supply, but not under high nitrogen supply

This study was aimed to determine whether the accumulation of shoot biomass is the driver of greater nitrogen (N) uptake in genotypes with higher vigorous growth, or whether greater N uptake leads to the greater growth. Two glasshouse experiments were conducted to answer this question. In experiment 1 (Expt 1), N uptake was manipulated by growing wheat plants in vertically divided pots which allowed 100, 50 and 30% of the root system to be supplied with N. Two cultivars were included that contrasted in vigorous growth (rate of shoot biomass accumulation). In experiment 2 (Expt 2), shoot biomass accumulation was manipulated by removing tiller primordial. Two commercial cultivars were grown which differed in their tillering capacity. For each cultivar, one treatment had biomass accumulation constrained by the surgical removal of young tillers as they were exserted. Exposure of 100, 50 and 30% of the root system to N supply generated differences in N uptake at stem elongation and N uptake was positively correlated to accumulation of shoot biomass (R2 = 0.97). N uptake per unit of root biomass with access to N increased to meet shoot requirements. Removal of young tillers generated differences in accumulation of shoot biomass at flag leaf stage. In the high N treatment in Expt 2, the root:shoot ratio increased in both genotypes in response to tiller removal; the reduction in N uptake in the cultivar Janz was proportional to the reduction in shoot biomass whereas in the cultivar Wyalkatchem, the reduction in N uptake was less than the reduction in biomass. Under low N supply, differences in shoot biomass appeared to be the driver for the differences in the N-uptake rather than the differences in N-uptake generating differences in biomass, while in Expt 2 a poor correlation between shoot N uptake and shoot biomass was found under high N supply. This has implications for selection of genotypes for great N-uptake efficiency.