Wetlands Ecology and Management 9: 211–218, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. 211 Gas space and oxygen exchange in roots of Avicennia marina (Forssk.) Vierh. var. australasica (Walp.) Moldenke ex N. C. Duke, the Grey Mangrove William G. Allaway, Mark Curran, Lauren M. Hollington, Malcolm C. Ricketts & Nich- olas J. Skelton School of Biological Sciences A12, The University of Sydney, NSW 2006, Australia, Fax: +61 2 9351 4771; E-mail: allaway@bio.usyd.edu.au Key words: aerenchyma, diffusion, lysigenous, pressure, schizogenous, subrisule Abstract Aerenchyma pervades the cortex of the whole root system in Avicennia marina var. australasica plants grown for 3 to 5 years in artificial-tidal tanks. The gas spaces appear to be schizogenous in all but the finest roots, where some apparently lysigenous gas spaces develop. Gas spaces in the pith are small. Pneumatophores examined at the time the tips are growing possess subrisules on the tips as well as lenticels on mature regions. At other times subrisules are not seen. When the tide floods the pneumatophores, gas pressure and oxygen concentration go down – pressure by 1.7 kPa or less, but oxygen by as much as 3 mol m -3 , or more in some cases. On draining, pressure recovers immediately to atmospheric, but oxygen slowly rises to a plateau below the concentration in the atmosphere. The changes in oxygen concentration are consistent with oxygen supply by diffusion. The contribution of oxygen made by the small influx of air on pressure recovery is only a minor fraction of the respiratory oxygen requirement. Wet weather at low tide restricts gas exchange, affecting both pressure and oxygen. The pressure and oxygen changes are repeated over many tidal cycles. The results are compared with those for other species and situations in the literature. Introduction Avicennia marina has an adventitious root system (Baylis, 1950): roots emerge from the hypocotyl, and after briefly growing downwards they become hori- zontal. These develop into the cable roots which can spread out for many metres from the stem. Vertical, upward-growing pneumatophores arise from the cable roots and emerge from the mud. They are exposed to the air at low tide but covered by water when the tide is high. Cable roots and bases of pneuma- tophores produce fine roots, with several orders of branching, beneath the mud surface. Pneumatophores are aerenchymatous with lenticels (Haberlandt, 1884; Baker, 1915), and abundant gas spaces occur through- out the root system (Baylis, 1950). Root architecture is broadly similar – although with differences of de- tail – in other species of Avicennia that have been examined (see Chapman, 1976). The oxygen supply function of this aerenchymatous root system was sug- gested in the 19th century and demonstrated in the 20th (see Haberlandt, 1884; Chapman, 1976). In a landmark field study Scholander et al. (1955) demon- strated the decline of oxygen concentration inside the root during high tide and its recovery during low tide, together with a gradual reduction in gas pressure in- side pneumatophores while covered by the water and a very quick recovery on emergence. The low pressure could induce bulk flow of air into the pneumatophore (Scholander et al., 1955). Oxygen concentrations in- side roots were elegantly demonstrated in situ by Andersen and Kristensen (1988). We have grown A. marina from propagules in artificial-tidal tanks. These plants produced root sys- tems as described above: in Sydney, pneumatophores appeared after about one year (Curran, 1985; Curran et