Effects of CO2 elevation and irrigation regimes on leaf gas exchange, plant water relations, and water use efficiency of two tomato cultivars

• Reduced irrigation increased WUE at the stomatal and plant levels.
• Elevated CO2 increased WUE at the stomatal and plant levels.
• The two treatments combined increased WUE at the plant but not the stomatal level.
• The method of water delivery did not produce significant differences in plant WUE.
• Elevated CO2 increased flower number but decreased fruit set.

We investigated the effects of elevated CO2 concentration ([CO2]), different irrigation regimes, and their interactions on leaf gas exchange, water relations, biomass production, and water use efficiency in tomato plants. In spring 2014, two tomato cultivars (CV1, which is potentially drought tolerant, and CV2 which is potentially heat tolerant) were grown in two separate greenhouse cells at [CO2] of 380 and 590 μmol L−1 (ppm) located at the experimental farm, Taastrup, Denmark. Plants were either irrigated to 18% of volumetric soil water content (FI, full irrigation), or irrigated with 70% water of the fully-irrigated control, delivered to either the whole pot (DI, deficit irrigation) or alternately to only half of the pot (PRD, partial root-zone drying). The experiment was a completed factorial design with four replications per treatment. The two cultivars showed a similar response to soil water deficits, but their water consumption responded differently to high [CO2]. Intrinsic water use efficiency (WUEi, photosynthetic rate/stomatal conductance) and plant water use efficiency (WUEp, aboveground biomass/plant water use) were both significantly increased by reduced irrigation treatments and elevated [CO2], although no significant reduction of stomatal conductance was detected under high [CO2]. There was a positive interaction between CO2 enrichment and water deficits on plant water use efficiency. Root water potential was negetatively affected by reduced irrigation but positively influenced by elevated [CO2], while leaf water potential was significantly decreased only by reduced irrigation. CO2 enrichment increased flower number without affecting fruit number, thereby reducing fruit set. Reduced irrigation in combination with elevated [CO2] caused a significant improvement in plant water use efficiency in both tomato cultivars.