Growth, photosynthesis and water-use efficiency of two C-4 Sahelian grasses subjected to water deficits

Growth, photosynthesis and water use efficiency of two Sahel dominant C-4 g rasses: Schoenefeldia gracilis Kunth and Dactyloctenium aegyptium (L) P. Be auv were studied under well-watered and drought conditions. D. aegyptium ha d a faster rate of development than S. gracilis and responded to water-defi cits by decreasing leaf area in the earlier stages of development. However, under water-stress the overall reduction of leaf area suffered by S. graci lis was greater than in D. aegyptium. The decrease in net photosynthesis ob served under water-deficits resulted mainly from stomatal closure. Analysis of correlation between long-term water-use efficiency and several physiolo gical, 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 compensate d by an increase in the short-term water-use efficiency associated with sto matal closure. In S. gracilis, the long-term water-use efficiency under mod erate water-deficits increased but was not positively correlated with relat ive growth rate; it was associated with a high allocation to non-photosynth etic (support and vascular) tissues. The possible implications of this beha viour for the different 'strategies' for plant survival in the arid climate of the Sahel are discussed. (C) 2000 Academic Press.