This is a very interesting question and a great deal of work has been carried out by plant scientists to figure out the mechanisms that allow rice, and other aquatic plants, to survive under water. Although rice is a grass, it is classed as semi-aquatic. This means that, unlike many other grasses, it is adapted to grow in very wet to waterlogged conditions. Rice possesses several traits that allow it survive periods submerged in deep water after heavy rains. When submerged in water, rice plants produce special air channels called aerenchyma which connect the the roots to the shoots and allow the transport of oxygen to submerged tissues. Another trait which aids survival is that when leaves are submerged, a very thin layer of air is trapped on the surface of the leaves which allows the plants to exchange oxygen and carbon dioxide with the surrounding water. These mechanisms essentially mean that when rice plants are flooded, they are able to breathe. Other plant species that are not adapted to aquatic environments would suffocate and die in these conditions.