High vapour pressure deficit and low soil water availability enhance shoot growth responses of a C₄ grass (Panicum coloratum cv. Bambatsi) to CO₂ enrichment

The hypothesis that shoot growth responses of C₄ grasses to elevated CO₂ are dependent on shoot water relations was tested using a C₄ grass, Panicum coloratum (NAD-ME subtype). Plants were grown for 35 days at CO₂ concentrations of 350 or 1000 μL CO₂ L^-1. Shoot water relations were altered by growing plants in soil which was brought dally to 65, 80 or 100% field capacity (FC) and by maintaining the vapour pressure deficit (VPD) at 0.9 or 2.1 kPa. At 350 μL CO₂ L^-1, high VPD and lower soil water content depressed shoot dry mass, which declined in parallel at each VPD with decreasing soil water content. The growth depression at high VPD was associated with increased shoot transpiration, whereas at low soil water, leaf water potential was reduced. Elevated CO₂ ameliorated the impact of both stresses by decreasing transpiration rates and raising leaf water potential. Consequently, high CO₂ approximately doubled shoot mass and leaf length at a VPD of 2.1 kPa and soil water contents of 65 and 80% FC but had no effect on unstressed plants. Water use efficiency was enhanced by elevated CO₂ under conditions of stress but this was primarily due to increases in shoot mass. High CO₂ had a greater effect on leaf growth parameters than on stem mass. Elevated CO₂ increased specific leaf area and leaf area ratio, the latter at high VPD only. We conclude that high CO₂ increases shoot growth of C₄ grasses by ameliorating the effects of stress induced by either high VPD or low soil moisture. Since these factors limit growth of field-grown C₄ grasses, it is likely that their biomass will be enhanced by rising atmospheric CO₂ concentrations.