Waterlogging is the major obstacle for sustainable agriculture. Plants subjected to waterlogging suffer from substantial yield losses. Under natural environmental conditions, plants often get exposed to transient or permanent waterlogging. Flooding induces a number of alterations in important soil physiochemical properties like soil pH, redox potential and oxygen level. Thus, the plants growing on the waterlogged soil face the stressful environment in terms of hypoxia (deficiency of O2) or anoxia (absence of O2). These oxygen deficient conditions substantially hamper plant growth, development and survival. Plants under O2-restrictive environment exhibit metabolic switch from aerobic respiration to anaerobic fermentation. It is evident from the available literature that most of the genes expressed under flooding stress are potentially involved in the synthesis of enzymes known to play active role in the establishment of this fermentative pathway. Plants undergo this metabolic change in order to get continuous supply of Adenosine triphosphate (ATP). Under waterlogged conditions, plants exhibit several responses including hampered stomata conductance, net CO2-assimilation rate and root hydraulic conductivity. Furthermore, plants grown under waterlogged conditions often face the oxidative damage induced by the generation of reactive oxygen species. These reactive oxygen species in turn affects the integrity of membranes and induce damage to the efficiency of photosystem II, thereby, causing considerable decrease in net photosynthetic rates. Moreover, these perturbations in physiological mechanisms may affect the carbohydrate reserves and translocations. In fact, waterlogging tolerant and sensitive plant species could be discriminated on the basis of their efficient carbohydrate utilization. Waterlogging is also known to induce adverse effects on several physiological and biochemical processes of plants by creating deficiency of essential nutrients like nitrogen, magnesium, potassium, calcium. Apart from these waterlogging-induced alterations in physiological mechanisms, plants growing under flooded conditions also exhibit certain morphological changes entailing the formation of adventitious roots, initiation of hypertrophied lenticels and/or establishment of aerenchyma. Therefore, the aim of this review is to highlight the major morphological, physiological and biochemical adaptations of plants to tolerate the flooding stress.
Key words: Hypoxia, anoxia, fermentation, Adenosine triphosphate (ATP), reactive oxygen species (ROS), antioxidants.
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