In a study on the mechanism of stimulated petiole elongation in submerged plants, oxygen concentrations in petioles of the flood-tolerant plant Rumex palustris were measured with micro-electrodes. Short-term submergence lowered petiole partial oxygen pressure to c. 19 kPa whereas prolonged submergence under continuous illumination depressed oxygen levels to c. 8–12 kPa after 24 h. Oxygen levels in petioles depended on the presence of the lamina, even in submerged conditions, and on available light. In darkness, petiole oxygen levels in submerged plants dropped quickly to values as low as 0.5–4 kPa. It is hypothesized that prolonged submergence in the light is accompanied by a decrease in carbon dioxide in the petiole. Submergence-enhanced petiolar elongation rate was compared with emergent plants. Peak daily elongation rates occurred at the end of the dark period in emergent plants, but in the middle of the light period in submerged plants. We suggest that this shift in daily elongation pattern is induced by dependence of growth on photosynthetically derived oxygen in submerged plants. Implications of reduced oxygen for ethylene production are raised. Levels of 1-aminocyclopropane-1-carboxylic acid synthase and 1-aminocyclopropane-1-carboxylic acid oxidase and ethylene sensitivity are cited as potential factors in hypoxia-induced ethylene release.