Hydraulic properties of living late metaxylem and interactions between transpiration and xylem pressure in maize

Abstract

A new approach to study dynamic interactions between transpiration andxylem pressure in intact plants is presented. Pressure probemeasurements were performed in living (immature) late metaxylem of maizeroots rather than in adjacent mature xylem. This eliminated technicallimitations related to the measurement of negative pressures. Waterrelations of single cells showed that turgor and volumetric elasticmodulus were significantly larger in living metaxylem than in corticalcells; hydraulic conductivity was similar in both types of root cells.Increasing transpiration induced an immediate decrease of xylempressure, and vice versa. Turgor in the living metaxylem could becontinuously recorded for more than 1 h. The relationship between xylempressure and transpiration yielded a root hydraulic resistance of 1.3 x10(9) MPa s m(-3). Control experiments indicated that the response ofliving xylem in the positive pressure range essentially paralleled thatof mature root xylem in the negative range. In mature xylem, pressuresas low as -0.55 MPa were recorded for short periods (several minutes).Several tests verified that the pressure probe was in contact withmature xylem during the measurements of tensions. The resultsdemonstrate convincingly that transpiration generates an effectivedriving force for water uptake in roots, a central feature of thecohesion theory.