Growth, photosynthesis and water-use efficiency of two C4Sahelian grasses subjected to water deficits

Growth, photosynthesis and water use efficiency of two Sahel dominant C4grasses:Schoenefeldia gracilis Kunth and Dactyloctenium aegyptium (L) P. Beauv were studied under well-watered and drought conditions. D. aegyptium had a faster rate of development than S. gracilis and responded to water-deficits by decreasing leaf area in the earlier stages of development. However, under water-stress the overall reduction of leaf area suffered by S. gracilis was greater than in D. aegyptium. The decrease in net photosynthesis observed under water-deficits resulted mainly from stomatal closure. Analysis of correlation between long-term water-use efficiency and several physiological, structural and growth characteristics shows that long-term water-use efficiency in D. aegyptium was positively correlated with relative growth rates. The reduction in growth resulting from water-deficits was compensated by an increase in the short-term water-use efficiency associated with stomatal closure. In S. gracilis, the long-term water-use efficiency under moderate water-deficits increased but was not positively correlated with relative growth rate; it was associated with a high allocation to non-photosynthetic (support and vascular) tissues. The possible implications of this behaviour for the different ‘strategies’ for plant survival in the arid climate of the Sahel are discussed.