Water is by far the resource that plants absorb in largest amounts from their environment. However, only a tiny fraction of the water taken up is used for the production of new assimilates. e largest part is lost to the atmosphere as stomata open to allow diusion of carbon dioxide into leaves. For maize alone, this expensive exploit represents annually ca. 3 × 1014 L of water worldwide, enough to ll 250 million Olympic swimming pools (London 2012 Olympic Games had two of them) (FAO 2012). Although transpiration prevents excessive heating of sunlit leaves and ensures long-distance transport of nutrients from the bulk soil to the upper plant organs, the contrasting transpiration eciencies of C3 and C4 species is an indication that actual crop water use oen exceeds theoretical requirements. However, the fact that breeding for reduced crop water use has oen led to negative results (Blum 2005) stresses that a more detailed consideration of the dynamics of water uxes in the soil-plantatmosphere system is needed (Gregory et al. 2005).