Oxygen and pressure changes measured in situ during flooding in roots of the Grey Mangrove Avicennia marina (Forssk.) Vierh.

Abstract

Manometers and oxygen electrodes were attached to the root system of mangroves grown in pots flooded to soil level. Measurements of internal pressure and oxygen concentration were made at the tip and middle of cable roots and on a pneumatophore, in artificial tidal conditions in a constant temperature growth-room. At the end of a period of ‘low tide’ there was a substantial oxygen gradient within the root system, from 6.41 ± 0.26 mol m −3 at the middle of the cable root to 4.81 ± 0.37 mol m −3 at the cable root tip (means and standard errors, n = 8). All oxygen concentrations fell during ‘high tide’ when the whole plant except the shoot was submerged, and the internal gradients gradually diminished, until by the end of a 6-h flooding the concentration throughout was in the range 4.59 ± 0.37 mol m −3 to 2.82 ± 0.41 mol m −3. Concentrations and gradients recovered over a few hours after unflooding. The oxygen concentrations measured in the roots fell among those previously published. Pressure measurements within the root system showed pressures similar to atmospheric or slightly positive (with respect to atmospheric) throughout the root system at ‘low tide’, a rapid decline in pressure reaching −1.0 to −1.7 kPa after flooding, with a slow partial recovery to −0.8 to −0.4 kPa when flooding lasted much longer than normal high tide, and finally a rapid return to atmospheric pressure on unflooding. We suggest that the above-atmospheric pressure at ‘low tide’ is consistent with the possibility of thermo-osmotic pressurisation, and further tracer experiments are needed to test whether this would result in significant gas exchange by mass flow. The low pressure during ‘high tide’ is likely to result from removal of respiratory carbon dioxide from the gas space: it would be likely to result in an influx of air through the exposed shoot, and a brief rapid influx through the pneumatophores on unflooding.