Methods of measuring hydrokinetic pressure gradients in the xylem of plants in situ

Three techniques have been proposed for measuring hydrokinetic pressure gradients in the xylem of plants applying the Onsager equations of irreversible thermodynamics. In the first the same electrical gradient is applied in alternate directions in a tree and the resulting current difference ΔI is measured. ΔI is directly proportional to the hydrokinetic pressure gradient, and by this method has been found in Eucalyptus to be about 0.13 atm m −1 . Measurements of streaming potential gradients are also a linear function of the hydrokinetic pressure gradient and this has been calculated in maple to be between 0.6 to 1.2 atm m −1 . A direct measurement of transpiration flow by tracer methods has also been considered.