Water Relations

Abstract

Water is the most abundant constituent of living things. The living tissues of plants usually contain more than 70 per cent by weight of water, and maintenance of a satisfactory water content is essential for the plants to remain alive. Even quite small variations in plant-tissue water-content can markedly influence processes of growth and metabolism. Living things on this planet first evolved in water, and for hundreds of millions of years all plants lived totally immersed in water. One of the major hindrances to colonisation of the land was the great difficulty in controlling the water content of plant tissues exposed to the desiccating influence of the air; but eventually plants capable of such control did evolve. The most successful land plants of today can sustain continued water loss from aerial parts by maintaining an efficient uptake of water from the soil. Their tissue water balance is thus a dynamic one, a continual balancing of loss and gain. This explains why an emphasis on dynamics, an interest in movement of water, typifies the modern approach to plant water relations.