Plant Water Relations

Abstract

Water is one of the most important constituents of life. Chemically, water is the hydride of oxygen. Oxygen, being more electronegative, exerts a strong attractive pull on its electrons. This unequal attraction results in small positive charge on two hydrogen molecules and a small negative charge on the oxygen molecule. The two lone pairs of electrons of the oxygen molecule result in bending of water molecule. The partial charges on oxygen and hydrogen molecules result in high electric dipole moment and polarity of water molecule. The distinct physical and chemical properties of water, namely, cohesion, surface tension, high specific heat, high heat of vaporization, lower density of ice, and solubility, are due to hydrogen bonding between water molecules (Fig. 1.1). Water forms solution with large number of compounds. It is thus usually referred as a universal solvent. The solvent action of water is of tremendous importance for the cells. All cells require water, dissolved ions, and sugars to survive. Cells undergo oxidation-reduction reactions, and water serves as the medium in which all these reactions are carried out. Plants, being immobile and autotrophic, have to depend on the supply of water and minerals from the soil and carbon dioxide and light from the atmosphere. Transport of water and minerals in the vascular strands is based on the differences in pressure and concentration gradients of both solutes and the solvent (water). The transport of minerals and water from the soil to xylem and from xylem to substomatal cavity is referred as short-distance transport. Once water enters the xylem elements, it is transported up to 100m or more by the transpirational pull created in the leaves. Therefore, there is need to have an essential long-distance transport by two different transport systems involving transport in opposite directions. This chapter shall focus on various mechanisms of short- and long-distance transport in plants.