Water use efficiency of C₄ perennial grasses in a temperate climate

The C₄ perennial grasses Miscanthus x giganteus and Spartina cynosuroides are potential biomass crops. Evaporation and growth rates of 3-year-old rainfed and irrigated stands were measured over one growing season in southeast England. Leaf gas exchange provided an independent measure of instantaneous water use efficiency. Total water use was similar in both species, in each treatment. However, due to the higher productivity of M. x giganteus, the above ground biomass formed per unit of water loss ('water use efficiency') was higher in M. x giganteus than S. cynosuroides. In irrigated crops the values were 9.1 and 7.4 g kg^-1, respectively and in rainfed crops 9.5 and 8.2 g kg^-1, respectively. When normalized by the daily maximum vapour pressure deficit, the values for both crops were comparable with typical values for C₄ crops in a range of environments; 7.3-9.4 g kPa kg^-1. Soft water deficits reduced the crop coefficient (the ratio of water loss from the crop to Penman potential evaporation) in rainfed stands by ca. 50%. The crop 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.