The Control of Water and Carbon Dioxide Flux in Water-stressed Sugar Beet

Abstract

Changes in leaf and canopy water potential of sugar beet growing in soil of decreasing water content depended on soil water potential and were independent of water flux from the plant when this was varied by changing the water vapour content of the air. The calculated hydraulic conductance of the plant increased as flux increased and decreased as leaf water potential decreased and as the plant aged. The conductances to water vapour of individual leaves and of the canopy decreased as leaf water potential decreased and increased with increasing humidity of the air. The latter effect was independent of changes in leaf water potential. The conductances were not affected by the rate of evaporation or leaf temperature. The rate of photosynthesis was directly related to leaf conductance except in severely stressed, mature leaves in which leaf water potential had a more direct effect on photosynthesis. Stomatal conductances, transpiration, and photosynthesis were greater in young leaves than mature leaves on the same plant and at the same leaf water potential. These results are discussed in relation to current agricultural irrigation practices used for sugar beet.