Short communication. Comparative measurements of xylem pressure in transpiring and non-transpiring leaves by means of the pressure chamber and the xylem pressure probe

Simultaneous measurements were made with the xylem pressure probe onexposed, transpiring leaves and with the Scholander pressure chamber onboth transpiring and covered, non-transpiring leaves of sugarcane and maizeplants. Xylem tensions inferred from pressure chamber balancing pressureson non-transpiring leaves were similar to those measured directly with thexylem pressure probe in transpiring leaves. However, tensions inferred withthe pressure chamber on transpiring leaves that were placed in plasticsbags just prior to excision were up to 0.6 MPa greater than those measuredconcurrently with the xylem pressure probe. These findings suggest thatrelatively large differences in water potential between the xylem and bulkleaf tissue can exist during periods of rapid transpiration, and theyconfirm that the balance pressure of an excised, previously transpiringleaf is only a measure of the bulk average equilibrium leaf water potentialand not of the true xylem pressure that existed prior to excision.Keywords: Cohesion-Tension theory, xylem pressure probe, pressure chamber,xylem tension.