Plant Water Relationships

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Life is intimately associated with water, and particularly with water in its liquid phase. Water is the form in which the H atom, an essential element of all organic molecules, is absorbed and then assimilated in the course of photosynthesis (page 147). About 500 g of water is absorbed by roots to produce 1g organic material and thus may be considered as a plant nutrient, in the same way as CO2 or NO 3 − are also plant nutrients. The quantity of water required for the photosynthetic process, however, is small and amounts to only about 0.01% of the total quantity of water used by the plant. Most functions in which plant water is involved, are of a physical nature. Water is a solvent for many substances such as inorganic salts, sugars and organic anions. It is also the medium in which all biochemical reactions take place. Water molecules are adsorbed at the surfaces of particles forming hydration shells, which influence physical and chemical reactions. Water in liquid form allows the diffusion and mass flow of solutes, and for this reason is essential for the translocation and distribution of nutrients and metabolites throughout the entire plant. Water is also important in the vacuoles of plant cells as it generally exerts an intracellular pressure on the protoplasm and cell wall (so called turgor pressure) thus maintaining the rigidity of leaves, roots and other plant organs. These few examples indicate the overall importance of water in plant physiology.