Water use efficiency of C-4 perennial grasses in a temperate climate

The C-4 perennial grasses Miscanthus x giganteus and Spartina cynosuroides are potential biomass crops. Evaporation and growth rates of 3-year-old rai nfed and irrigated stands were measured over one growing season in southeas t England. Leaf gas exchange provided an independent measure of instantaneo us water use efficiency. Total water use was similar in both species, in ea ch treatment. However, due to the higher productivity of M. x giganteus, th e above ground biomass formed per unit of water loss ('water use efficiency ') was higher in M. x giganteus than S. cynosuroides. Tn irrigated crops th e values were 9.1 and 7.4 g kg(-1), respectively and in rainfed crops 9.5 a nd 8.2 g kg(-1), respectively. When normalized by the daily maximum vapour pressure deficit, the values for both crops were comparable with typical va lues for C-4 crops in a range of environments; 7.3-9.4 g kPa kg(-1). Soil w ater deficits reduced the crop coefficient (the ratio of water loss from th e crop to Penman potential evaporation) in rainfed stands by ca. 50%. The c rop coefficient of irrigated crops exceeded 1.5 in mid-season. Difficulties in making comparisons between values of water use efficiency derived from different measurements of leaf gas exchange and crop growth are discussed. (C) 1999 Elsevier Science B.V. Ail rights reserved.