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Murray-Darling Freshwater Research CentreMDFRC item.We immersed the root systems of three emergent aquatic monocots (Cyperus involucratus Rottb., Eleocharis sphacelata R. Br. (both Cyperaceae), and Juncus ingens N. A. Wakef. (Juncaceae)) in nutrient agar gels with pH indicators and pH microelectrodes to study H+ exchange by root systems. For all three species, the pH changed at the tips of the adventitious roots and around young laterals. Older, lignified surfaces had little effect on pH. The pH changes around the laterals were affected by the form of nitrogen in the nutrient gel When NO3− was supplied as the N source, there was a small pH increase, whereas NH4+ enhanced H+ release, causing the pH adjacent to the laterals to decrease below 4.5. In contrast, root tips acidified the media, irrespective of the nutrient composition. H+ exchange was quantified from titrimetric assays of H+ exchange in nutrient solutions. There was a net H+ efflux from the roots of all three species into complete nutrient solutions with NH4+ as the N source, ranging from 246 μmol H+ h−1 g−1 d. wt for C. involucratus, to 32 μmol H+ h−1 g−1 d. wt for J. ingens. The H+ efflux into similar nutrient solutions with NO3− as the N source ranged from −89 μmol H+ h−1 g−1 d. wt for C. involucratus to −0.8μmol H+ h−1 d. wt for J. ingens. By assuming that H+ exchange was mainly located in the laterals, rates of H+ excretion as high as 542 μmol H+ h−1 g−1 d. wt can be calculated for C. involucratus in NH4+ solutions. Rates of H+ excretion by C. involucratus were scarcely affected by solutions of most Other ions. The NH4+-induced H+ release was dramatically reduced by H+ pump inhibitors, attaining only 18% of control rates with 50 μM diethylstilbestrol and 38% of control rates with 5 μM dicyclohexylcarbodiimide.

Murray-Darling Freshwater Research CentreMDFRC item.The ability of diffusive gas transport and pressurized, convective flow to satisfy internal oxygen demands was examined for an aquatic sedge, Eleocharis sphacelata R. Br. Resistances to convection and diffusion through the plant were quantified from anatomical studies of the airspace dimensions, and these were used in mathematical models to calculate the fluxes required to satisfy oxygen demands measured in the tissue. The greatest resistance to diffusion in the underwater tissue was the submerged culm between the waterline and sediment surface (1560 Ms m−3 per m culm length). Resistances of the nodal intercalary meristem (52 Ms m−3) and rhizome internode (34 Ms m−3) were minor. In contrast, resistances to convection were low in the culms (38 MPa s m−3 per m culm length), and higher in the nodal meristems (93 MPa s m−3). The rhizome internodes had large cortical canals with a low convective resistance (0.75 MPa s m−3), and a parallel spongy pith with a very high resistance (518 MPa s m−3) that is probably short-circuited by convection. The resistance of the submerged culm means that diffusion is inadequate to satisfy oxygen demands in plants growing in >10 cm of water, and that convection is therefore essential in the natural habitat of this species (water to c. 2 m depth). However, a convective oxygen influx as low as 28 × 10−8 mol s−1 per culm (equivalent to a gas flow rate of 3 μl s−1 per culm) could satisfy the entire oxygen demand of the underwater tissue; this value is well below actual rates. At this flow rate, the spongy pith in the rhizome would also remain aerobic: it has a low resistance to diffusion (73 Ms m−3) and could receive sufficient oxygen by diffusion from the node. The data agree well with previous empirical measurements of convection in this species and show that diffusion and convection are both important processes for its aeration.