Rumex palustris, a flooding-tolerant plant, elongates its petioles in response to complete submergence. This response can be partly mimicked by enhanced ethylene levels and low O2 concentrations. High levels of CO2 do not markedly affect petiole elongation in R. palustris. Experiments with ethylene synthesis and action inhibitors demonstrate that treatment with low O2 concentrations enhances petiole extension by shifting sensitivity to ethylene without changing the rate of ethylene production. The expression level of the R. palustris gene coding for the putative ethylene receptor (RP-ERS1) is up-regulated by 3% O2 and increases after 20 min of exposure to a low concentration of O2, thus preceding the first significant increase in elongation observable after 40 to 50 min. In the flooding-sensitive species Rumex acetosa, submergence results in a different response pattern: petiole growth of the submerged plants is the same as for control plants. Exposure of R. acetosa to enhanced ethylene levels strongly inhibits petiole growth. This inhibitory effect of ethylene on R. acetosa can be reduced by both low levels of O2 and/or high concentrations of CO2.