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

Abstract

Summary. Long-term xylem pressure measurements were performed on the liana Tetrastigma voinierianum (grown in a tropical greenhouse) between heights of 1 m and 9.5 m during the summer and autumn seasons with the xylem pressure probe. Simultaneously, the light intensity, the temperature, and the relative humidity were recorded at the measuring points. Parallel to the xylem pressure measurements, the diurnal changes in the cell turgor and the osmotic pressure 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 tension 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 above-atmospheric values (established during the night) to negative values after daybreak was associated with an almost ] : 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 pressure towards more positive values correlated with an increase in the cell turgor pressure (ratio of about 1 : 1). The cell osmotic pressure remained nearly 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 (and vice versa) due to the tight hydraulic connection and thus due to the water equilibrium between both compartments. An increase in the transpiration rate (due to an increase in light intensity) results in very rapid establishment of a new equilibrium state by an equivalent decrease in the xylem and cell turgor pressure. From the xylem, cell turgor, and cell osmotic pressure 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 apparently not subject to diurnal changes. Con-*Correspondence and reprints: Institut fiir Pflanzenphysiologie, Universit~t Salzburg, Hellbrunnerstral3e 34, A-5020 Salzburg, Austria. E-mail: friedrich.bentrup @ sbg.ac.at sidering that the xylem pressure is determined by the turgor pressure (and vice versa), the xylem pressure of the liana could not drop toin agreement with the experimental results -less than -0.4 MPa, because this pressure corresponds to zero turgot pressure.

Changes in osmotic and turgor pressure in response to sugar accumulation in barley source leaves

“…In both cell types, the change is largely attributable to an increase in the osmotic pressure of the protoplast. This increase in PBS turgor pressure is very different from the observation of Thurmer et al (1999) who demonstrated that an increase in transpiration led to a decrease in the mesophyll cell turgor as well as xylem pressure. Considering the pathway of water flow from rhizosphere to the point of evaporation in terms of an electrical circuit model as described by Molz and Ferrier (1982), this indicates that, relative to the hydraulic resistance of the leaf (from PBS or mesophyll to the evaporation point), the hydraulic resistance to water flow from rhizosphere to leaf was considerably greater over the 9.5 m of the liana compared with the 0.2-0.3 m of barley.…”

“…In sugar beet plants, an inverse relationship between stomatal conductance and turgor pressure was demonstrated by Palta et al (1987). Thurmer et al (1999) found a similar diurnal change in turgor pressure in mesophyll cells of Tetrastigma, with the amplitude dependent on irradiance. However, there was a non-linear relation between transpiration and turgor pressure in mesophyll and epidermal cells of Tradescantia (Nonami and Schulze 1989).…”

Diurnal changes in xylem pressure of the hydrated resurrection plant Myrothamnus flabellifolia: evidence for lipid bodies in conducting xylem vessels

“…in the liana T. voinierianum, Benkert et al, 1995 ;Zimmermann et al, 1995a,b ;Thu$ rmer et al, 1999 ;and in sugarcane and crop plants, F. Thu$ rmer et al, unpublished data ;Schneider et al, 1997a,b ;Wegner & Zimmermann, 1998).…”

“…Because of the quite tight hydraulic link water equilibrium should exist between the vessels and the tissue cells at any time (Balling & Zimmermann, 1990 ;Zimmermann et al, 1995a,b ;Thu$ rmer et al, 1999) :…”

Diurnal changes in xylem pressure and mesophyll cell turgor pressure of the lianaTetrastigma voinierianum ? the role of cell turgor in long-distance water transport: a reply

“…Since, in the preceding paragraph, we have refuted the doubts concerning the validity of the xylem pressure recordings, we see no reason to question the osmotic values calculated by means of eq. 3 (Thtirmer et al 1999). We are aware, of course, that osmometry of reliably taken xylem sap samples is needed.…”