OXYGEN AND PRESSURE CHANGES MEASURED IN-SITU DURING FLOODING IN ROOTSOF THE GREY MANGROVE AVICENNIA-MARINA (FORSSK) VIERH

Manometers and oxygen electrodes were attached to the root system of m angroves 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 tid e' there was a substantial oxygen gradient within the root system, fro m 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 pl ant except the shoot was submerged, and the internal gradients gradual ly diminished, until by the end of a 6-h flooding the concentration th roughout 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 sys tem 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, wi th a slow partial recovery to -0.8 to -0.4 kPa when flooding lasted mu ch longer than normal high tide, and finally a rapid return to atmosph eric pressure on unflooding. We suggest that the above-atmospheric pre ssure at 'low tide' is consistent with the possibility of thermo-osmot ic pressurisation, and further tracer experiments are needed to test w hether this would result in significant gas exchange by mass flow. The low pressure during 'high tide' is likely to result from removal of r espiratory carbon dioxide from the gas space: it would be likely to re sult in an influx of air through the exposed shoot, and a brief rapid influx through the pneumatophores on unflooding.