Yield of canola (Brassica napus L.) benefits more from elevated CO2 when access to deeper soil water is improved

Abstract This study investigated the interactive effects of atmospheric CO 2 concentration ([CO 2 ]) and water availability on yield, root growth and water use of two canola cultivars with contrasting growth and vigour (vigorous hybrid cv. Hyola 50 and non-hybrid cv. Thumper). Plants were grown under ambient [CO 2 ] (a[CO 2 ], ∼400 μmol mol −1 ) or elevated [CO 2 ] (e[CO 2 ], ∼700 μmol mol −1 ) in a glasshouse. Two water treatments (well-watered and drought) were established in each [CO 2 ] treatment. During the growing season leaf gas exchange parameters were measured. Leaf area was measured at 80 days after sowing. Aboveground biomass, seed yield, yield components and root biomass in four different soil layers (Layer 1: 0–20 cm, Layer 2: 21–40 cm, Layer 3: 41–60 cm and Layer 4: 61–80 cm depth) were measured at maturity. Weekly water use was determined gravimetrically. Elevated [CO 2 ] stimulated seed yield (38%), aboveground biomass (34%), root biomass (42%), leaf area (42%) and leaf biomass (41%). Whilst e[CO 2 ] stimulated root biomass in all soil layers, this stimulation was greater in the deeper than upper soil layers, and was associated with greater extraction of deeper soil water under e[CO 2 ]. The cultivar with greater stimulation of deeper root biomass under e[CO 2 ] showed greater yield benefit from the ‘CO 2 fertilisation effect’. Under well-watered conditions, e[CO 2 ]-induced reductions of stomatal conductance (g s ) balanced the effect of increased leaf area on water use, resulting in similar water use compared to a[CO 2 ]. In contrast, under drought conditions, water use was greater under e[CO 2 ] than a[CO 2 ]. The ‘CO 2 fertilisation effect’ depended on cultivar and water treatment. Under well-watered conditions, aboveground biomass of the hybrid cultivar benefitted more from the ‘CO 2 fertilisation effect’. However, under drought both aboveground biomass and seed yield of the non-hybrid cultivar benefitted more from the ‘CO 2 fertilisation effect’. These findings show that interactions between environmental conditions (here experimental water treatments) and expression of genotypic traits (here differences between cultivars) play a decisive role in determining potential yield and growth benefits from rising [CO 2 ].