Water Relations

Abstract

Water is the major constituent of plants, serving as cheap structural material and a biochemical reaction medium, as well as being exchanged for needed CO₂ in photosynthesis (Meidner and Sheriff, 1976; P. J. Kramer, 1983). Water possesses many physicochemical properties that are unusual (refs. above; Jones, 1983; Nobel, 1983). Its high polarity and strong hydrogen bonding give it high values of specific heat, dielectric constant (therefore a high solvent capability), viscosity, and surface tension (therefore a strong capillary rise and, perhaps surprisingly, a high tensile strength in narrow vessels [Pickard, 1981]). Water also expands on freezing, as very few liquids do, and thus winter’s ice is exposed for summer melting. Ninety percent or so of the plant cell’s water is in the large central vacuole, much of the remainder is in the cytoplasm, and some is in the cell wall. Water flows in and out of the cell passively, i.e., no active-transport carriers exist, nor could they be afforded metabolically. The flow occurs at anomalously high rates considering water’s poor solubility in cell-membrane lipids (Salisbury and Ross, 1985).