This chapter first explains why plants have a greater demand for water than animals. Following a look at the physico-chemical properties of water, the water potential concept is introduced, which is used to analyse the movement of water into and through plants. Most plant species have to maintain a hydrated state; that is, they are homoiohydric. After temperature, precipitation is the most dominant environmental factor determining the distribution of vegetation at the global scale. Plants respond to fluctuations in water supply with a range of mechanisms, which are discussed at the molecular level in this chapter. These include adjustment of osmotic potentials, regulation of the stomatal aperture, and modulation of resistance to water flow by aquaporins—water channels residing in cellular membranes. Also, a plant actively modulates its growth, depending on water availability. The water status is sensed by a plant in an unknown fashion and is translated into adequate responses. These are predominantly mediated by the phytohormone abscisic acid. The corresponding signal transduction events are explained in this chapter. The final section discusses two photosynthesis variants that are characterized by higher water use efficiency and therefore represent adaptations to water scarcity. Both C4 photosynthesis and crassulacean acid metabolism photosynthesis have evolved independently many times and are of major ecological importance. Evolutionary trajectories can be postulated that illustrate how complex traits can arise in distinct steps.
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