LONG-TERM XYLEM PRESSURE MEASUREMENTS IN THE LIANA TETRASTIGMA-VOINIERIANUM BY MEANS OF THE XYLEM PRESSURE PROBE

Diurnal changes of xylem pressure in the liana Tetrastigma voinierianu m have been measured under greenhouse conditions by means of the recen tly developed xylem pressure probe. During the early morning hours, te nsions in the vessels developed more or less rapidly with time, depend ing on light intensity. On sunny days, absolute negative pressures dow n to about -0.4 MPa (atmospheric = O.1 MPa) were recorded around noon in petiolar or stem xylem vessels, whereas on rainy or cloudy days the xylem pressure remained in the positive sub-atmospheric or slightly n egative pressure range. Towards the evening the tension in the vessels always decreased, i.e. the xylem pressure shifted to about atmospheri c, or even above-atmospheric, values during the night. Simultaneous xy lem pressure recordings at heights of 1 and 5 m frequently yielded eit her no gradient in tension at all, or far less than expected from the Cohesion Theory. Occasionally, tension gradients were even opposite to those predicted by this theory. Stem-to-leaves pressure gradients in accord with the Cohesion Theory were recorded only when tension had be en developed during sunny days in the upper branches of the liana, bec ause increases in tension were not immediately propagated to the xylem of the leaves at ground level, as would be expected from a strictly c oupled hydraulic system. Parallel recordings of the ''xylem tension'' using the pressure chamber yielded rather variable values ranging from 0.1 to 1 MPa; diurnal pressure changes could not be detected at all. The data are discussed on the basis of the equation for the chemical a ctivity of water. They strongly suggest that the xylem tension induced by transpiration is not the sole force for water ascent. Other forces , such as osmotic pressure or convectional and interfacial forces, whi ch to a remarkable extent have already been postulated for decades, se em to be equally important.