Rising ambient CO₂ is associated with global warming and is known to alter rainfall patterns with increased risk of crop failures. Plant species differ in their response to elevated atmospheric CO₂ concentration. We determined leaf water potential (LWP), water use efficiency and biomass yields in Napier grass (Pennisetum purpureum) a C₄grass and Reed (Phragmites australis)a C₃grass exposed to ambient and 560ppm CO₂concentrations in glasshouse bays maintained at 15/25^oC for 5 weeks. Napier grass is an important forage in tropics while Reed is occasionally used for forage. The aim was to determine how the two grasses differ in the measured attributes when subjected to water stress under the different CO₂concentrations.

Elevated CO₂ did not have significant effect on either predawn or midday LWP in both grasses in the first two weeks following withdrawal of watering. Between the grasses, C₃ had more negative LWP than C₄at predawn or midday under both CO₂concentrations. By the third week after water withdrawal, both grasses maintained similar predawn and midday LWP under both CO₂ concentrations, but the midday LWP was less negative in P. purpureum under elevated CO₂ than ambient CO₂. Compared with ambient concentration, elevated CO₂concentration increased biomass accumulation by 72% in Napier grassand 9% in Reed. The photosynthetic water use efficiency increased by 97% in P. purpureumand by 64% in P. australis. It was concluded that elevated CO₂ is more likely to enhance biomass accumulation in the Napier grass than in the Reed.