A comparison of methods for studying pressure and solute potentials in xylem and also in phloem laticifers ofHevea brasiliensis

Abstract

In this study, several ways have been examined to measure latex pressure, including some novel methods, with the aim of finding better methods than are presently available. A novel method, similar to the pressure probe as used for single cells, has been applied to laticifers, exerting a back-pressure to prevent exudation. The type of capillary bubble manometer developed by Bourdeau and Schopmeyer for resin pressures (1958) and then used for latex pressures by Buttery and Boatman (1964, 1966) has been improved and the construction and standardization of bubble manometer gauges for routine field use has been greatly simplified. It is concluded that the back-pressure method in its present state of development provides neither sufficient rapidity nor precision on account of the slow response time. This method does not justify the two operators needed. The requirement for relatively large quantities of escaping latex limits its applications and, in certain instances, pressurization might be incomplete. Nevertheless, it has provided some additional confidence in the accuracy of the other methods because readings can often be repeated many times on a single puncture, which is impossible with a bubble manometer. Also the gauge used is linear and therefore more accurate at high pressures than a bubble manometer. The newly designed Buttery and Boatman gauge requires fewer parts and utilizes standard plastic pippetor tips. These greatly speed and simplify construction of the gauges, which are needed in large numbers as they are essentially disposable. These gauges have proved to be remarkably reliable, even with minimal latex volumes, and have always indicated the highest latex pressures of all methods used at a particular site, suggesting that leakage rates are minimal at the junction between gauge and laticifers. The plastic tip holders facilitate the manual insertion of these delicate gauges into the tree bark. Xylem sap is contaminated by phloem sap if collected from petioles using a pressure bomb. However, this problem can be overcome in stems by bark ringing. Alternatively, the vacuum extraction method can be used which does not seem to suffer from this contamination problem. Several ways have been tested to maximize the reliability of latex serum samples for analysis in a vapour osmometer. Pressure and solute components of water potential can be integrated to give total water potentials. Centrifugation provides the best method to remove rubber particles in Hevea latex : valid osmotic potentials of serum can be made providing appropriate precautions are observed.