The Relationship between Transpiration, Root Water Uptake, and Leaf Water Potential

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Abstract
The effect of changing the transpiration rate on leaf water potential and water balance has been examined to show if permeability of the plant (predominantly the roots) is constant or varies with the transpiration rate. Measurements of leaf effective thickness, water potential, transpiration, and uptake of water by roots were made on sunflower, barley, and maize plants grown in solution culture and subjected to a range of atmospheric conditions and root treatments: cooling, low osmotic potential, and removal of part of the root system. Leaf water potential changed little under a wide range of atmospheric conditions and rates of water flux in the three species, so that the root permeability to water increases as the rate of transpiration, and therefore flow across the root surface, increases. Equality between uptake and loss of water and thereby maintenance of constant leaf water potential is assisted by stomatal changes, which appear to be in response to conditions at or in the root rather than a direct response to changes in bulk leaf water potential.