Gas space and oxygen exchange in roots of Avicennia marina (Forssk.) Vierh. var. australasica (Walp.) Moldenke ex N. C. Duke,the Grey Mangrove

Aerenchyma pervades the cortex of the whole root system in Avicenniamarina var. australasica plants grown for 3 to 5 years in artificial-tidaltanks. The gas spaces appear to be schizogenous in all but the finest roots,where some apparently lysigenous gas spaces develop. Gas spaces in thepith are small. Pneumatophores examined at the time the tips are growingpossess subrisules on the tips as well as lenticels on mature regions. At othertimes subrisules are not seen. When the tide floods the pneumatophores,gas pressure and oxygen concentration go down – pressure by 1.7 kPa orless, but oxygen by as much as 3 mol m^-3, or more in some cases.On draining, pressure recovers immediately to atmospheric, but oxygenslowly rises to a plateau below the concentration in the atmosphere. Thechanges in oxygen concentration are consistent with oxygen supply bydiffusion. The contribution of oxygen made by the small influx of air onpressure recovery is only a minor fraction of the respiratory oxygenrequirement. Wet weather at low tide restricts gas exchange, affecting bothpressure and oxygen. The pressure and oxygen changes are repeated overmany tidal cycles. The results are compared with those for other speciesand situations in the literature.