Submergence impedes plant gas exchange with the environment. Survival depends upon internal aeration to provide O2 throughout the plant body, although short-term anoxia can be tolerated. During nights, plants rely on O2 entry from the floodwater and pO2 in roots declines so that some tissues become severely hypoxic or even anoxic. Underwater photosynthesis is the main daytime O2 source and also provides sugars. Capacity for photosynthesis under water, like in air, is determined by available CO2 and light; however, slow diffusion in water often limits CO2 supply. Underwater photosynthesis in some wetland species is enhanced by gas films on superhydrophobic leaf surfaces. Leaf gas films also increase night-time O2 uptake by submerged plants. Flooding events are forecast to increase and understanding of plant submergence tolerance should enable predictions of possible impacts on vegetation communities and also aid breeding of improved submergence tolerance in rice.