HIGH VAPOR-PRESSURE DEFICIT AND LOW SOIL-WATER AVAILABILITY ENHANCE SHOOT GROWTH-RESPONSES OF A C-4 GRASS (PANICUM-COLORATUM CV BAMBATSI) TO CO2 ENRICHMENT

The hypothesis that shoot growth responses of C-4 grasses to elevated CO2 are dependent on shoot water relations was tested using a C-4 gras s, Panicum coloratum (NAD-ME subtype). Plants were grown for 35 days a t CO2 concentrations of 350 or 1000 mu L CO2 L-1. Shoot water relation s were altered by growing plants in soil which was brought daily to 65 , 80 or 100% field capacity (FC) and by maintaining the vapour pressur e deficit (VPD) at 0.9 or 2.1 kPa. At 350 mu L CO2 L-1, high VPD and l ower soil water content depressed shoot dry mass, which declined in pa rallel at each VPD with decreasing soil water content. The growth depr ession 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 transpiratio n rates and raising leaf water potential. Consequently, high CO2 appro ximately doubled shoot mass and leaf length at a VPD of 2.1 kPa and so il water contents of 65 and 80% FC but had no effect on unstressed pla nts. Water use efficiency was enhanced by elevated CO2 under condition s of stress but this was primarily due to increases in shoot mass. Hig h CO2 had a greater effect on leaf growth parameters than on stem mass . Elevated CO2 increased specific leaf area and leaf area ratio, the l atter at high VPD only. We conclude that high CO2 increases shoot grow th of C-4 grasses by ameliorating the effects of stress induced by eit her high VPD or low soil moisture. Since these factors limit growth of field-grown C-4 grasses, it is likely that their biomass will be enha nced by rising atmospheric CO2 concentrations.