Water use and growth responses of dryland wheat grown under elevated [CO2] are associated with root length in deeper, but not upper soil layer

Elevated [CO2] stimulated root length of wheat in both upper and deeper soil layers, and this stimulation was modified by cultivars and irrigation regimes. Across cultivars and all treatments, water use, biomass and grain yield were positively associated with root length in the deeper soil layer but not with root length in the upper soil layer. The 'CO2 fertilisation effect' on biomass and grain yield was of similar magnitude under both irrigated and rainfed conditions. Although e[CO2] did not increase canopy cover in these experiments, the CO2 effect on water use depended on cultivars and irrigation regimes. Despite greater e[CO2]-induced stimulation of tillers and spikes, the cv. Scout did not receive more biomass or grain yield benefit from the 'CO2 fertilisation effect' compared to cv. Yitpi.