Water relations of coastal and estuarine Rhizophora mangle: xylem pressurepotential and dynamics of embolism formation and repair

Physiological traits related to water transport were studied in Rhizophora mangle (red mangrove) growing in coastal and estuarine sites in Hawaii. The magnitude of xylem pressure potential (P-x), the vulnerability of xylem to cavitation, the frequency of embolized vessels in situ, and the capacity o f R. mangle to repair embolized vessels were evaluated with conventional an d recently developed techniques. The osmotic potential of the interstitial soil water (pi (sw)) surrounding the roots of R. mangle was c. -2.6 +/- 5.5 2 x 10(-3) and -0.4 +/- 6.13 x 10(-3) MPa in the coastal and estuarine site s, respectively. Midday covered (non-transpiring) leaf water potentials (Ps i (L)) determined with a pressure chamber were 0.6-0.8 MPa more positive th an those of exposed, freely-transpiring leaves, and osmotic potential of th e xylem sap (pi (x)) ranged from -0.1 to -0.3 MPa. Consequently, estimated midday values of P-x (calculated by subtracting pi (x) from covered Psi (L) ) were about 1 MPa more positive than Psi (L) determined on freely transpir ing leaves. The differences in Psi (L) between covered and transpiring leav es were linearly related to the transpiration rates. The slope of this rela tionship was steeper for the coastal site, suggesting that the hydraulic re sistance was larger in leaves of coastal R. mangle plants. This was confirm ed by both hydraulic conductivity measurements on stem segments and high-pr essure flowmeter studies made on excised leafy twigs. Based on two independ ent criteria, loss of hydraulic conductivity and proportions of gas- and li quid-filled vessels in cryo-scanning electron microscope (cryo-SEM) images, the xylem of R. mangle plants growing at the estuarine site was found to b e more vulnerable to cavitation than that of plants growing at the coastal site. However, the cryo-SEM analyses suggested that cavitation occurred mor e readily in intact plants than in excised branches that were air-dried in the laboratory. Cryo-SEM analyses also revealed that, in both sites, the pr oportion of gas-filled vessels was 20-30% greater at midday than at dawn or during the late afternoon. Refilling of cavitated vessels thus occurred du ring the late afternoon when considerable tension was present in neighborin g vessels. These results and results from pressure-volume relationships sug gest that R. mangle adjusts hydraulic properties of the water-transport sys tem, as well as the leaf osmotic potential, in concert with the environment al growing conditions.