HIGH-MOLECULAR-WEIGHT ORGANIC-COMPOUNDS IN THE XYLEM SAP OF MANGROVES- IMPLICATIONS FOR LONG-DISTANCE WATER TRANSPORT

The rise of sap in mangroves has puzzled plant physiologists for many decades. The current consensus is that negative pressures in the xylem exist which are sufficiently high to exceed the osmotic pressure of s eawater (2.5 MPa). This implies that the radial reflection coefficient s of the mangrove roots are equal to unity. However, direct pressure p robe measurements in xylem Vessels of the roots and stems of mangrove (Rhizophora mangle) grown in the laboratory or in the field yielded be low-atmospheric, positive (absolute) pressure values. Slightly negativ e pressure Values were recorded only occasionally. Xylem pressure did not change significantly when the plants were transferred from tal: wa ter to solutions containing up to 1700 mOs-mol kg(-1) NaCl. This indic ates that the radial reflection coefficient of the roots for salt, and therefore the effective osmotic pressure of the external solution, wa s essentially zero as already reported for other halophytes. The low v alues of xylem tension measured with the xylem pressure probe were con sistent with previously published data obtained using the vacuum/leafy twig technique. Values of xylem tension determined with these two met hods were nearly two orders of magnitude smaller than those estimated for mangrove using the pressure chamber technique (- 3 to -6MPa). Xyle m pressure probe measurements and staining experiments with alcian blu e and other dyes gave strong evidence that the xylem vessels contained viscous, mucilage- and/or protein-related compounds. Production of th ese compounds resulting from wound or other artifactual reactions was excluded. The very low sap now rates of about 20- 50cm h(-1) measured in these mangrove plants were consistent with the presence of high mol ecular weight polymeric substances in the xylem sap. The presence of v iscous substances in the xylem sap of mangroves has the following impl ications for traditional xylem pressure measurement techniques, develo pment of xylem tension, and long-distance water transport: (1) high ex ternal balancing pressures in the pressure chamber are needed to force xylem sap to the cut surface of the twig; (2) stable tensions much la rger than 0.1 MPa can be developed only occasionally because viscous s olutions provide nucleation sites for gas bubble formation; (3) the fr equent presence of small gas bubbles in viscous solutions allows water transport by interfacial, gravity-independent streaming at gas/water interfaces and (4) the increased density of viscous solutions creates (gravity-dependent) convectional flaws. Density-driven convectional fl ows and interfacial streaming, but also the very low radial reflection coefficient of the roots to NaCl are apparently the means by which R. mangle maintains water transport to its leaves despite the high salin ity of the environment.