Photosynthetic response to water stress in Themeda triandra and Eragrostis lehmanniana

The influence of water stress on the photosynthetic rate of the C4 grasses Themeda triandra Forsk. and Eragrostis lehmanniana Nees was determined for the vegetative and reproductive phases. Gas exchange was determined with an infra-red gas analyser, white leal water potential was used to quantify water stress. An open system of gas flow was used. The rate of photosynthesis was 0.3131 mg CO2 m- 2 s- 1 and 0.6287 mg CO2 m- 2 s- 1 respectively for T. triandra and E lehmanniana (P ≤ 0.001) if water was not limiting. The rate of photosynthesis began to decline at a leaf water potential of − 1 927 kPa and − 1 625 kPa lor T. triandra and E. lehmanniana respectively. This decline in rate of photosynthesis was significantly (P ≤ 0.001) correlated with leaf water potential, and linear relationships with correlation coefficients of 0.936 and 0.938 were obtained for T. triandra and E. lehmanniana respectively.