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.