The water use dynamics of canola cultivars grown under elevated CO₂ are linked to their leaf area development

The 'CO₂ fertilisation effect' is often predicted to be greater under drier than wetter conditions, mainly due to hypothesised early season water savings under elevated [CO₂] (e[CO₂]). However, water savings largely depend on the balance between CO₂-induced improvement of leaf-level water use efficiency and CO₂-stimulation of transpiring leaf area. The dynamics of water use during the growing season can therefore vary depending on leaf area development. Two canola (Brassica napus L.) cultivars of contrasting growth and vigour (vigorous hybrid cv. Hyola 50 and non-hybrid cv. Thumper) were grown under ambient [CO₂] (a[CO₂], ∼400 μmol mol^-1) or e[CO₂] (∼700 μmol mol^-1) with two water treatments (well-watered and mild drought) in a glasshouse to investigate the interdependence of leaf area development and water use. Dynamics of water use during the growing season varied depending on [CO₂] and cultivars. Early stimulation of leaf growth under e[CO₂], which also depended on cultivar, overcompensated for the effect of increased leaf-level water use efficiency, so that weekly water use was greater and water depletion from soil greater under e[CO₂] than a[CO₂]. This result shows that the balance between leaf area and water use efficiency stimulation by e[CO₂] can tip towards early depletion of available soil water, so that e[CO₂] does not lead to water savings, and the 'CO₂ fertilisation effect' is not greater under drier conditions.