Water covers over 70 of the surface of Earth and is generally recognized as a renewable resource. Yet, however counterintuitive, the availability of fresh water limits plant growth over much of the land mass of the planet and posesmajor challenges for human society as a whole. For land plants, including crop species, fresh water is a basic requirement for life. Water is a common trigger for seed germination. Its uptake from the soil facilitates inorganic mineral nutrition, and its flux through vascular tissues of the plant circulates minerals and organic nutrients throughout the plant. Water (and solute) retention determines turgor, driving plant cell expansion and contributing to plant form and function, including stomatal movements. Finally, water loss by transpiration from the stomata of leaves is, at once, a byproduct of gas exchange and CO_2 uptake for photosynthesis and a driver for water flux and its circulation throughout the plant. In turn, plants exert major controls on the water and carbon cycles of the world. Roughly 32 × 10~(15) kg year~(-1) of water is drawn by land plants and transpired to the atmosphere, while terrestrial photosynthesis annually fixes about 120 × 10~(15) g of carbon (Schimel et al., 2001). Stomatal transpiration by plants is speculated to have made a significant contribution to recent changes in continental runoff and freshwater availability associated with the global rise in CO_2 (Gedney et al., 2006), although it must be weighed against the consequences of other human activities, especially land use (Piao et al., 2007).
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