Introduction: water movement through plants

Abstract

Loss of water by leaves (transpiration) is inevitable if carbon dioxide is to be taken up for photosynthesis. The resulting flow of water through the plant gives rise to a water stress in the tissues which is explicable with reference to a simple model. The importance of this stress to the growth and physiology of the plant is indicated. Consideration of the soil-plant-atmosphere continuum as a series of resistances reveals that the major resistance lies in the gaseous phase where the stomata are strategically placed to control the rate of flow through the system. The stomata therefore play a key role in controlling stress in the plant as do the resistances within the plant. These ‘plant resistances’, e.g. that of the root and leaf, are not simple in that the resistance to flow through them can be a function of the flow itself. The physiological reason for this is still obscure. The water stress in the soil round the roots is clearly of major importance as the source from which water is drawn; thus flow of water to the root surface and perhaps the soil-root interface may play an important role.