Transpiration and water-use efficiency in response to water stress in Themeda triandra and Eragrostis lehmanniana

The influence of water stress on the rate of transpiration and water-use efficiency (WUE) of the C4 grasses Themeda triandra Forsk. and Eragrostis lehmanniana Nees was determined for both the vegetative and reproductive phases Transpiration was determined by means of an infra-red gas analyser, while the leaf water potential was used to quantify water stress. Water-use efficiency is defined as the ratio of mass of CO2 fixed, to mass of water released into the atmosphere by transpiration. If water was not limiting, the rate of transpiration was 75.8809 mg H2O m−2 s−1 and 101.8384 mg H2O m−2 s−1 for T. triandra and E. lehmanniana (P≤0.001) respectively. The WUE of T. triandra and E lehmanniana if water was not limiting was respectively 4.2061 mg CO2 g−1 H2O and 6.7060 mg CO2 g−1 H2O (p≤0.001). The first signs of a decrease in transpiration rate was observed at a leaf water potential of −2054 kPa for T. triandra and −2433 kPa for E. lehmanniana (p≤0.01). The decrease in transpiration rate and WUE was significantly (P≤0.01) correlated with leaf water potential for both species. The rate of transpiration and WUE with water stress differed significantly (p