Respiration is a complex series of reactions which occur in all living cells to release energy from food. Oxygen is generally required for respiration as this releases more energy per glucose molecule. This type of respiration is referred to as aerobic respiration.
Plants in waterlogged soil do not have adequate oxygen for aerobic respiration and yet they have to release energy from food in order to survive. So, they resort to the second form of respiration referred to as anaerobic respiration.
Anaerobic respiration is a form of respiration that does not utilize oxygen. This form of respiration (anaerobic) involves incomplete oxidation of carbohydrate into some carbonic compounds such as ethyl alcohol and carbon dioxide. The amount of energy released is much less when compared to aerobic respiration but this still makes it possible for them to obtain the energy they require to survive.