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

A new approach to study dynamic interactions between transpiration andxylem pressure in intact plants is presented. Pressure probe measurementswere preformed in living (immature) late metaxylem of maize roots ratherthan in adjacent mature xylem. This eliminated technical limitationsrelated to the measurement of negative pressures. Water relations of singlecells showed that turgor and volumetric elastic modulus were significantlylarger in living metaxylem than in cortical cells; hydraulic conductivitywas similar in both types of root cells. Increasing transpiration inducedan immediate decrease of xylem pressure, and vice versa. Turgor in theliving metaxylem could be continuously recorded for more than 1 h. Therelationship between xylem pressure and transpiration yielded a roothydraulic resistance of 1.3 x 10⁹ MPa sm^-3. Control experiments indicated that the responseof living xylem in the positive pressure range essentially paralleled thatof mature root xylem in the negative range. In mature xylem, pressures aslow as -0.55 MPa were recorded for short periods (several minutes). Severaltests verified that the pressure probe was in contact with mature xylemduring the measurements of tensions. The results demonstrate convincinglythat transpiration generates an effective driving force for water uptake inroots, a central feature of the cohesion theory.Keywords: Hydraulic conductivity, negative pressure, rootdevelopment, turgor, water transport, Zea mays.