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Murray-Darling Freshwater Research CentreMDFRC item.The supply of oxygen to respiring shoot tissue was investigated for three submerged macrophytes (Potamogeton crispus L., Egeria densa Planch, and Myriophyllum triphyllum Orchard). For all species, the response of oxygen uptake rates to the external O2 concentration was a rectangular hyperbola over the range 0–5.0 × 10−3m3 m−3. However, the response pattern for material with water-infiltrated lacunar airspaces was non-hyperbolic over this range. The change in response was interpreted as an increased substrate (O2) limitation, resulting from lower radial diffusion rates within the infiltrated material. Neither the uninfiltrated nor the infiltrated responses obeyed the linear and logarithmic formulae of the type observed for submerged macrophytes by earlier authors. These results suggest that the responses observed are affected by factors such as water velocity, internal restrictions to diffusion and the range of oxygen tensions investigated. Therefore, it is unlikely that one response formula can adequately account for the effects of oxygen concentration on submerged macrophyte oxygen uptake. The lacunar airspaces also represent a possible oxygen source for dark respiration. The consumption of oxygen from the airspaces was investigated by displacing the gas from the lacunae and measuring the subsequent increase in the rate of oxygen assimilation from the external liquid. Approximately 30% of the oxygen consumed by E. densa and P. crispus, and more than 40% of that consumed by M. triphyllum, was derived from the lacunar system. This O2 supply is a consequence of the higher oxygen concentration in the lacunae than in the external medium, due to the low solubility of oxygen in water. Storage of photosynthetically-produced oxygen in the lacunae could not be identified during a light/dark transient, due to rate changes caused by the effects of light on the respiratory metabolism. However, O2 partial pressure gradients artificially set up between the lacunae and water equilibrated within an hour, suggesting that excess oxygen would be lost to the water within this time.

Murray-Darling Freshwater Research CentreMDFRC item.Egeria densa is a large aquatic angiosperm with an extensive lacunar airspace system in the shoots and roots. Oxygen produced in the chloroplasts is partitioned to both the external liquid and the lacunar system. In the light O2 entering and pressurising the lacunae diffuses to the roots down a 5% per metre concentration gradient. At the roots, some of the transported O2 sustains aerobic respiration of the tissue, and the excess is released into the sediment, oxidizing the rhizosphere. This longitudinal O2 transport is a diurnal phenomenon; corresponding diurnal fluctuations of 0.4 µgO2. cm-2. min-1 were found for the rhizosphere O2 flux.Diffusion of CO2 from roots to shoots provides an additional carbon source for photosynthesis. This results in a root-derived CO2 concentration gradient along the shoot axis.In the dark dissolved O2 around the plants may be depleted. A lower demand for external O2 by shoots with gas-filled lacunae, than by shoots with water-infiltrated lacunae, demonstrated that radial O2 diffusion in the lacunae assists nocturnal aeration of Egeria.

Murray-Darling Freshwater Research CentreMDFRC item.A pressure transducer was used to measure internal gas pressures (pI) in the lacumar system of Egeria densa Planch. Under illumination, pI values up to 25 kPa above atmospheric developed in shoots in unstirred water. Upon circulation of the water, pI values decreased to 50–90% of the maximal values in unstirred water. Positive pressures, indicating partitioning of photosynthetic O2 into the lacunae, were recorded in both re-circulating and open-circuit flow. When the shoot was darkened, negative internal pressures (5–10 kPa below atmospheric) developed.The pressure gradient (Δp) required to drive O2 transport in the lacunar system was calculated from the Hagen-Poiseuille equation. This gradient was 9.15 × 10−4 kPa m−1. Manometric measurements of pI and pressure transfer rates showed that transient gradients of approximately 0.9 kPa m−1 developed following dark to light changes. However, pressure equilibration (mean rate = 0.02 m s−1) resulted in rapid attainment of constant pI under steady-state conditions. It is proposed that O2 transport in Egeria is sustained by simple diffusion, assisted by advective movements under internal pressure gradients that develop following changes in external conditions.