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Through stimulation of root growth, increasing atmospheric CO 2  concentration ([CO 2 ]) may facilitate access of crops to sub-soil water, which could potentially prolong physiological activity in dryland environments, particularly because crops are more water use efficient under elevated [CO 2 ] (e[CO 2 ]). This study investigated the effect of drought in shallow soil versus sub-soil on agronomic and physiological responses of wheat to e[CO 2 ] in a glasshouse experiment. Wheat ( Triticum aestivum  L. cv. Yitpi) was grown in split-columns with the top (0–30 cm) and bottom (31–60 cm; ‘sub-soil’) soil layer hydraulically separated by a wax-coated, root-penetrable layer under ambient [CO 2 ] (a[CO 2 ], ∼400 μmol mol -1 ) or e[CO 2 ] (∼700 μmol mol -1 ) [CO 2 ]. Drought was imposed from stem-elongation in either the top or bottom soil layer or both by withholding 33% of the irrigation, resulting in four water treatments (WW, WD, DW, DD; D = drought, W = well-watered, letters denote water treatment in top and bottom soil layer, respectively). Leaf gas exchange was measured weekly from stem-elongation until anthesis. Above-and belowground biomass, grain yield and yield components were evaluated at three developmental stages (stem-elongation, anthesis and maturity). Compared with a[CO 2 ], net assimilation rate was higher and stomatal conductance was lower under e[CO 2 ], resulting in greater intrinsic water use efficiency. Elevated [CO 2 ] stimulated both above- and belowground biomass as well as grain yield, however, this stimulation was greater under well-watered (WW) than drought (DD) throughout the whole soil profile. Imposition of drought in either or both soil layers decreased aboveground biomass and grain yield under both [CO 2 ] compared to the well-watered treatment. However, the greatest ‘CO 2  fertilisation effect’ was observed when drought was imposed in the top soil layer only (DW), and this was associated with e[CO 2 ]-stimulation of root growth especially in the well-watered bottom layer. We suggest that stimulation of belowground biomass under e[CO 2 ] will allow better access to sub-soil water during grain filling period, when additional water is converted into additional yield with high efficiency in Mediterranean-type dryland agro-ecosystems. If sufficient water is available in the sub-soil, e[CO 2 ] may help mitigating the effect of drying surface soil.

Elevated [CO2] mitigates the effect of surface drought by stimulating root growth to access sub-soil water