Mechanisms for coping with submergence and waterlogging in rice

Plants require water for growth but excess water that occurs during submergence or waterlogging is harmful or even lethal. A submerged plant is defined as "a plant standing in water with at least part of the terminal above the water or completely covered with water" (Figure 1; Catling 1992). Submergence subjects plants to the stresses of low light, limited gas diffusion, effusion of soil nutrients, mechanical damage, and increased susceptibility to pests and diseases (Greenway and Setter 1996; Ram et al. 1999). Basically, flooding (i.e., submergence) can be classified into "flash flooding" and "deepwater flooding" in accordance with the duration of flooding and the water depth (Bailey-Serres et al. 2010; Catling 1992; Jackson and Ram 2003). Flash flooding, which generally lasts less than a few weeks, is caused by heavy rain but the depth is not very deep. On the other hand, deepwater flooding, which lasts for several months, occurs during the rainy season, and the water depth reaches several meters (Catling 1992; Hattori et al. 2011).Waterlogging is defined as a condition of the soil in which excess water limits gas diffusion (Figure 1; Setter and Waters 2003). Oxygen diffusivity in water is approximately 10,000 times slower than in air, and the flux of O2 into soils is approximately 320,000 times less when the soil pores are filled with water than when they are filled with gas (Armstrong and Drew 2002, Colmer and Flowers 2008). The principal cause of damage to plants grown in waterlogged soil is inadequate supply of oxygen to the submerged tissues as a result of slow diffusion of gases in water and rapid consumption of O2 by soil microorganisms. Oxygen deficiency in waterlogged soil occurs within a few hours under some conditions. In addition to the O2 deficiency, production of toxic substances such as Fe2+, Mn2+, and H2S by reduction of redox potential causes severe damage to plants under waterlogged conditions (Drew and Lynch 1980; Setter et al. 2009). Thus, grow