The Physiology of Flowering Plants - KHAM PHA MOI

Chapter 3Water relations3.1 IntroductionLiquid water is absolutely necessary for life as we know it. Firstlyit is the solvent and reaction medium of all living cells, whichcontain some 75–90% water by weight; secondly it is a reactant inmany metabolic processes; and thirdly, as the hydration waterof macromolecules, it forms part of the structure of protoplasm,existing as ‘liquid ice’ in a labile but ordered structure. Thephysicochemical properties of water (H 2 O) are unique; heavywater (D 2 O or DHO), containing deuterium, the heavy isotope ofhydrogen, differs sufficiently to be toxic. In multicellular organisms,water provides the transport medium. Additionally, forplants, water is one of the raw materials for photosynthesis andproduces the turgor pressure of water-filled vacuoles which givesmechanical rigidity to thin-walled tissues, while some movementsof plant organs occur as a result of turgor pressure changes. Plantcell expansion is driven by turgor pressure and hence growth ratesdepend on hydration levels.On ‘dry’ land, the highly hydrated body of a terrestrial plant inmany situations tends to lose water to the environment, especially tothe atmosphere, in accordance with gradients of free energy of water.There are few habitats where plants do not suffer some water shortageat least intermittently. The necessity for maintaining an adequateinternal water content has been a major factor in the evolution ofland plants with respect to structure and numerous aspects of physiology.It is not an exaggeration to say that the colonization of land byplants has depended upon the evolution of systems for the absorptionand conservation of water. Another difficulty for land plants isthe transport of water from the underground supply tapped by theroots to the aerial shoots. For the tallest flowering plants this maymean moving water some 100 m against gravity.This chapter deals with the forces and factors involved in wateruptake and loss in flowering plants, the mechanisms of water movementwithin the plants, and the controls exerted on water exchangeand water transport by the plant and the environment respectively.