Photosynthesis and Water Relations of Two Savanna Grasses as Influenced by Shade and Water Stress

Net CO2 assimilation and plant water relations of the African savanna grasses Panicum maximum Jacq. and Tliemeda triandra Porrsk. were measured under controlled conditions. We hypothesised that 1; triandra would be better adapted to higher light and temperature conditions and could withstand greater moisture stress than could P. maximum, as T. tria11dra is commonly found in open areas, whereas P. maximum dominates in the shade of trees. Plants of both species were transplanted into a common soil in large pots and placed under shade (approximately 150 µrnoVm1/s photon flux density (PFD)) or high light (approximately 1200 µmoVm1/s PFD) conditions, with either no imposed drought or periodic drought. Both species responded similarly to stressors of shade and drought. Few significant interactions between species ·and light regime or species and water regime were found, Species responded similarly if grown together or in a monoculture. Therefore, this study did not indicate competitive reactions or ecophysiological reasons why P. maximum should dominate shady habitats, and T. triandra should dominate open grasslands.