Sap salinity effects on xylem conductivity in two mangrove species

Xylem sap salinity and conductivity were examined in two mangrove ecosystem tree species. For Avicennia germinans, extracted xylem sap osmotic potentials ranged from -0.24 to -1.36 MPa versus -0.14 to -0.56 MPa for Conocarpus erectus. Xylem sap of Conocarpus did not vary in osmotic potential between sites nor between predawn and midday. In Avicennia, values were more negative at midday than predawn, and also more negative at hypersaline than hyposaline sites. After removing embolisms, specific conductivity (K-s) was measured as a function of salinity of the artificial xylem sap perfusion. For both species the lowest K-s values, about 70% of the maximum K-s, were obtained when stems were perfused with deionized water (0 mM; 0.0 MPa) or with a 557-mM saline solution (-2.4 MPa). Higher K-s values were obtained in the range from -0.3 to -1.2 MPa, with a peak at -0.82 +/- 0.08 MPa for Avicennia and -0.75 +/- 0.08 MPa for Conocarpus. The variations in K-s values with minima both at very low and very high salt concentrations were consistent with published results for swelling and shrinking of synthetic hydrogels, suggesting native hydrogels in pit membranes of vessels could help regulate conductivity.