Manometric Measurement of Turgor Pressures in Laticiferous Phloem Tissues2

Abstract

A manometric technique for the determination of turgor pressures in laticiferous phloem tissues has given reproducible results in Hevea brasiliensis and a few other tree species. In Hevea, early morning pressures are in the range 7.9–15.0 atmospheres, falling during the day and recovering at night. These diurnal pressure changes are positively correlated with atmospheric relative humidity and negatively correlated with changes in temperature, evaporation, leaf water deficit, and stomatal opening. They are not displayed by trees devoid of leaves. Thus the loss in turgor is probably the result of withdrawal of water from the phloem tissues under transpirational stress. Pressures at the base of the trunk normally exceed those at the top, the gradient usually approximating to 1 atmosphere/10 metres at night and rising up to six times this figure during the day. This increase probably reflects the development of tension gradients in the xylem during transpiration. A general turgor gradient from base to crown does not preclude mass flow in sieve-tubes in the opposite direction provided that rates of loading and unloading are such that a sufficient osmotic gradient is maintained in them in the required direction. No marked long-term effects of regular tapping on turgor pressure have been noted in Hevea trees and there is no evidence for seasonal changes in turgor under our conditions.