The root and shoot tissues of flooding tolerant wetland plants are highly porous to enable internal gas phase diffusion of O2 during waterlogging or submergence. In the case of only partial submergence (snorkeling), the atmosphere can act as source of O2 . The aim of this study was to assess the effect of waterlogging, partial submergence and complete submergence in the dark as well as in light on O2 partial pressure (pO2 ) in roots of Rumex palustris (flooding tolerant) and R. acetosa (flooding intolerant). We used O2 microelectrodes to measure pO2 of adventitious roots during manipulations of the water level around the shoot. Root pO2 in both species declined significantly upon submergence but remained oxic also when shoots were completely submerged in the dark (0.8 and 4.6 kPa in R. acetosa and R. palustris, respectively). The snorkeling effect was substantial in R. palustris only. Submergence in light had less impact on root pO2 and the effect of snorkeling was also minor. Hence, the benefits of light (underwater photosynthesis) and air contact (snorkeling) upon growth and survival in submerged wetland plants can now be linked to enhanced internal aeration.