Flooding tolerance in plants: from physiological and molecular perspectives

Submergence stress due to increased rainfalls is an agricultural problem in many areas in the world. Complete or partial submergence leads to an array of morphological, physiological and molecular changes in plants, which adversely affect plant growth and development and may lead to a drastic reduction in grain yield of many crops. The adverse effects of submergence stress can be mitigated by developing crop plants with improved submergence tolerance using various physiological and molecular approaches. For this purpose, a thorough understanding of physiological and molecular responses of plants, for improving crop to submergence, is imperative. Submergence stress affects plant growth at any developmental stage, though the impact of stress depends on the duration of submergence and species or genotype. At earlier stages, submergence stress mostly affects shoot growth, leaf area, plant–water contents and photosynthesis, while at later stages, it may adversely disturb the redox and molecular activity of plants. Furthermore, early senescence, production of reactive oxygen species (ROS) and imbalance in molecular mechanisms constitute the major plant responses to submergence stress. In order to cope with submergence stress, plants execute various mechanisms, including decrease in transpiration rate, increase in stomatal conductance and photosynthesis, scavenging of ROS, production of antioxidants and activation of Ca2+ fluxes and subsequent protein phosphorylation. Potential molecular strategies are needed to improve plant submergence stress tolerance, including traditional and modern molecular breeding protocols and transgenic approaches.