Diurnal changes in xylem pressure and mesophyll cell turgor pressure of the liana Tetrastigma voinierianum: the role of cell turgor in long-distance water transport

Long-term xylem pressure measurements were performed on the liana Tetrastig ma voinierianum (grown in a tropical greenhouse) between heights of Im and 9.5 m during the summer and autumn seasons with the xylem pressure probe. S imultaneously, the light intensity, the temperature, and the relative humid ity were recorded at the measuring points. Parallel to the xylem pressure m easurements, the diurnal changes in the cell turgor and the osmotic pressur e of leaf cells at heights of 1 m and 5 m (partly also at a height of 9.5 m ) were recorded. The results showed that tensions (and height-varying tensi on gradients) developed during the day time in the vessels mainly due to an increase in the local light intensity (at a maximum 0.4 MPa). The decrease of the local xylem pressure from positive, subatmospheric or slightly abov e-atmospheric values (established during the night) to negative values afte r daybreak was associated with an almost 1 : 1 decrease in the cell turgor pressure of the mesophyll cells (on average from about 0.4 to 0.5 MPa down to 0.08 MPa). Similarly, in the afternoon the increase of the xylem pressur e towards more positive values correlated with an increase in the cell turg or pressure (ratio of about 1 : 1). The cell osmotic pressure remained near ly constant during the day and was about 0.75-0.85 MPa between 1 m and 9.5 m (within the limits of accuracy). These findings indicate that the turgor pressure primarily determines the corresponding pressure in the vessels (an d vice versa) due to the right hydraulic connection and thus due to the wat er equilibrium between both compartments. An increase in the transpiration rate (due to an increase in light intensity) results in very rapid establis hment of a new equilibrium state by an equivalent decrease in the xylem and cell turgor pressure. From the xylem, cell turgor, and cell osmotic pressu re data the osmotic pressure (or more accurately the water activity) of the xylem sap was calculated to be about 0.35-0.45 MPa; this value was apparen tly not subject to diurnal changes. Considering that the xylem pressure is determined by the turgor pressure (and vice versa), the xylem pressure of t he liana could not drop to - in agreement with the experimental results - l ess than -0.4 MPa, because this pressure corresponds to zero turgor pressur e.