Water relations of coastal and estuarine Rhizophora mangle: xylem pressure potential 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 of R. mangle to repair embolized vessels were evaluated with conventional and recently developed techniques. The osmotic potential of the interstitial soil water (πsw) surrounding the roots of R. mangle was c. -2.6±5.52×10-3 and -0.4±6.13×10-3 MPa in the coastal and estuarine sites, respectively. Midday covered (non-transpiring) leaf water potentials (ΨL) determined with a pressure chamber were 0.6-0.8 MPa more positive than those of exposed, freely-transpiring leaves, and osmotic potential of the xylem sap (πx) ranged from -0.1 to -0.3 MPa. Consequently, estimated midday values of P x (calculated by subtracting πx from covered ΨL) were about 1 MPa more positive than ΨL determined on freely transpiring leaves. The differences in ΨL between covered and transpiring leaves were linearly related to the transpiration rates. The slope of this relationship was steeper for the coastal site, suggesting that the hydraulic resistance was larger in leaves of coastal R. mangle plants. This was confirmed by both hydraulic conductivity measurements on stem segments and high-pressure flowmeter studies made on excised leafy twigs. Based on two independent criteria, loss of hydraulic conductivity and proportions of gas- and liquid-filled vessels in cryo-scanning electron microscope (cryo-SEM) images, the xylem of R. mangle plants growing at the estuarine site was found to be more vulnerable to cavitation than that of plants growing at the coastal site. However, the cryo-SEM analyses suggested that cavitation occurred more 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 proportion of gas-filled vessels was 20-30% greater at midday than at dawn or during the late afternoon. Refilling of cavitated vessels thus occurred during the late afternoon when considerable tension was present in neighboring vessels. These results and results from pressure-volume relationships suggest that R. mangle adjusts hydraulic properties of the water-transport system, as well as the leaf osmotic potential, in concert with the environmental growing conditions.

On the use of a bubble formation model to calculate diving tables.

Previous decompression tables for humans were based upon unsupported assumptions because the underlying processes by which dissolved gas is liberated from blood and tissue were poorly understood. Some of those assumptions are now known to be wrong, and the recent formulation of a detailed mathematical model describing bubble nucleation has made it possible to calculate diving tables from established physical principles. To evaluate this approach, a comprehensive set of air diving tables has been developed and compared with those of the U.S. and British Navies. Conventional decompressions, altitude bends, no-stop thresholds, and saturation dives are all successfully described by one setting of four global nucleation parameters, which replace the U.S. Navy's matrices of M-values. Present air diving tables show great irregularity, even within sets created by the same authors. In contrast, this new approach is remarkably self-consistent, permitting accurate interpolation and extrapolation.

Application of bubble formation model to decompression sickness in fingerling salmon.

Recently a new cavitation model has been proposed in which bubble formation in aqueous media in initiated by spherical gas nuclei stabilized by surface-active membranes of varying gas permeability. In previous application of the varying permeability model, good agreement has been obtained with experimental limits in pressure reduction for gelatin, rats, and humans following steady-state exposures. We new extend this investigation to fingerling salmon and demonstrate that a satisfactory description of the decompression data of D' Aoust et al. (Undersea Biomed Res 1980; 7:199-209) is provided by the model with parameter values that are similar to those found for other physical and biological systems. This adds further evidence for the generally of the model as well as for the importance of bubble nucleation as the primary and controlling event in decompression sickness.

On the use of oxygen to facilitate decompression.

Oxygen is widely used at elevated partial pressures to facilitate decompression, yet the optimum dosage and the magnitude of the beneficial effects are poorly known. This is because oxygen enhancements, expressed as increases in the allowed pressure reductions, are small and easily masked by individual variation. Furthermore, oxygen can also produce detrimental results, and the range from a therapeutic to a toxic dose is narrow. Berhage and McCracken recently reported two massive investigations involving 1185 rats and 60 experimental conditions. These authors suggest that the conventional concept of an "equivalent air depth" (EAD) is untenable and that oxygen must be considered in calculating the totat tissue gas tension. We find instead that the observations of Berghage and McCracken are compatible with a model in which the tensions of oxygen and carbon dioxide dissolved in tissue are taken into account, and that this model, in turn, agrees with EAD predictions of oxygen enhancements for subtoxic oxygen pressures.

Application of a bubble formation model to decompression sickness in rats and humans.

Although decompression sickness results from bubble formation in blood or tissue, pressure schedules currently in use are essentially empirical and contain little input from cavitation theory. The recent convergence of three lines of investigation suggests that a synthesis of practice and theory may now be possible. The data consist of pressure reduction limits for gelatin, rats, and humans following steady-state exposures. From the gelatin studies, a model has been developed in which bubble formation is initiated by spherical gas nuclei stabilized by surface-active skins of varying gas permeability. We demonstrate that the model is also in good agreement with data on rats and humans over a wide range of pressures and that the model parameters assume sensible values in each case. This suggests that cavitation theory can provide a rationale for current diving practice and can serve to secure, consolidate, and extend this practice.

Stabilization of gas cavitation nuclei by surface-active compounds.

Gas bubbles are the primary agent in producing the pathogenic effects of decompression sickness. Numerous experiments indicate that bubbles originate in water, and probably also in man, as pre-existing gas nuclei. This is surprising considering that gas phases larger than 1 micron should rise to the surface of a standing liquid, whereas smaller ones should dissolve rapidly due to surface tension. Several stabilizing mechanisms have been suggested, and each has been refuted on experimental grounds. In this article, we propose a new model that arises out of a systematic study of the earlier theories. We review these theories and conclude that gas cavitation nuclei must be held intact by surface-active skins that are initially permeable. The first quantitative analysis of bubble formation data from supersaturated gelatin is summarized and leads to the further conclusion that skins can become impermeable if the ambient pressure is increased rapidly by a sufficient amount. Our model owes much to Sirotyuk, who "demonstrated experimentally that stabilization of gas bubbles acting as cavitation nuclei in water is always attributable to the presence of surface-active substances in the water".

Bubble formation within decompressed hen's eggs.

Decompression sickness follows a reduction in ambient pressure and is a result of bubble formation in blood or tissues. The origin of such bubbles is the subject of considerable controversy, and a number of mechanisms have been proposed to account for them. In testing these mechanisms, freshly-laid hen's eggs provide a particularly intriguing model--namely, an intact biological system in which bubbles form readily and many of the proposed processes are excluded.