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

Abstract

proposed more than 100 years ago by Dixon and Joly (1894). According to this theory, long-distance water Simultaneous measurements were made with the transport in xylem is driven by tension (negative pressure) xylem pressure probe on exposed, transpiring leaves gradients generated by the evaporation of water from and with the Scholander pressure chamber on both leaves and transmitted through continuous water columns transpiring and covered, non-transpiring leaves of from the leaf apoplast to the roots. Evidence supporting sugarcane and maize plants. Xylem tensions inferred the Cohesion-Tension theory consists largely of thousands from pressure chamber balancing pressures on non- of indirect estimates of xylem pressure obtained in excised transpiring leaves were similar to those measured leaves and branches with the Scholander pressure chamdirectly with the xylem pressure probe in transpiring ber (Scholander et al., 1965). The recent development of leaves. However, tensions inferred with the pressure the xylem pressure probe (xylem probe) by Balling and chamber on transpiring leaves that were placed in Zimmermann (1990) has allowed the direct measurement plastic bags just prior to excision were up to 0.6 MPa of xylem pressure in single vessels of intact plants. greater than those measured concurrently with the Measurements with the xylem probe have confirmed xylem pressure probe. These findings suggest that the existence of tensions in the xylem of both transpiring relatively large differences in water potential between and non-transpiring plants, but the values obtained are the xylem and bulk leaf tissue can exist during periods often substantially smaller than those inferred from indirof rapid transpiration, and they confirm that the ect techniques, such as the pressure chamber and psychrobalance pressure of an excised, previously transpiring meter (Balling and Zimmermann, 1990; Zimmermann leaf is only a measure of the bulk average equilibrium et al., 1991, 1994a, b). This has led some researchers to leaf water potential and not of the true xylem pressure question the validity of estimates of xylem tension that existed prior to excision. obtained with the pressure chamber and to postulate that the currently accepted hundred-year-old Cohesion