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

This study investigated crop water use of wheat grown in a dryland Mediterranean-type environment under elevated atmospheric CO₂ concentrations ([CO₂]). Two related cultivars, contrasting in agronomic features (cvs. Scout and Yitpi; Scout has good early vigour and high transpiration efficiency), were grown under ambient [CO₂] (a[CO₂], ∼400 μmol mol⁻¹) and elevated [CO₂] (e[CO₂], ∼550 μmol mol⁻¹) in the Australian Grains Free Air CO₂ Enrichment (AGFACE) facility for two growing seasons. Each year, an irrigation treatment (rainfed versus irrigated) was imposed within the CO₂-treatments. Normalised difference vegetation index (as surrogate for canopy cover) and root length in the upper (0 cm–32 cm) and deeper (33 cm–64 cm) soil layers were measured at stem-elongation and anthesis. 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.