Oxygen dynamics during submergence in the halophytic stem succulent Halosarcia pergranulata

This study elucidated O-2 dynamics in shoots and roots of submerged Halosarcia pergranulata (Salicornioideae), a perennial halophytic stem succulent that grows on flood-prone mudflats of salt lakes. Oxygen within shoots and roots was measured using microelectrodes, for plants when waterlogged or completely submerged, with shoots in light or in darkness, in a controlled environment. Net photosynthesis (P-N) when underwater, at a range of dissolved CO2 concentrations, was measured by monitoring O-2 production rates by excised stems. The bulky nature and apparently low volume of gas-filled spaces of the succulent stems resulted in relatively high radial resistance to gas diffusion. At ambient CO2, quasi-steady state rates of P-N by excised succulent stems were estimated to be close to zero; nevertheless, in intact plants, underwater photosynthesis provided O-2 to tissues and led to radial O-2 loss (ROL) from the roots, at least during the first several hours (the time period measured) after submergence or when light periods followed darkness. The influence of light on tissue O-2 dynamics was confirmed in an experiment on a submerged plant in a salt lake in south-western Australia. In the late afternoon, partial pressure of O-2 (pO(2)) in the succulent stem was 23.2 kPa (i.e. similar to 10% above that in the air), while in the roots, it was 6.2-9.8 kPa. Upon sunset, the pO(2) in the succulent stems declined within 1 h to below detection, but then showed some fluctuations with the pO(2) increasing to at most 2.5 kPa during the night. At night, pO(2) in the roots remained higher than in the succulent stems, especially for a root with the basal portion in the floodwater. At sunrise, the pO(2) increased in the succulent stems within minutes. In the roots, changes in the pO(2) lagged behind those in the succulent stems. In summary, photosynthesis in stems of submerged plants increased the pO(2) in the shoots and roots so that tissues experience diurnal changes in the pO(2), but O-2 from the H2O column also entered submerged plants.