Salinity and its effects on the functional biology of legumes
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Salinity has plagued soil fertility and drastically affected growth and survival of glycophytes in irrigated regions of the world since the beginning of recorded history. It is particularly common in arid and semi-arid areas where evapotranspiration exceeds annual precipitation, and where irrigation is therefore necessary to meet crop water needs. Salt buildup in the soils and groundwater has threatened its productivity and sustainability. Plant responses to salt stress include an array of changes at the molecular, biochemical and physiological levels. Salt stress involves a water deficit induced by the salt concentration in the rhizosphere, resulting in disruption of homeostasis and ion distribution in the cell and denaturation of structural and functional proteins. As a consequence, salinity stress often activates cell signaling pathways including those that lead to synthesis of osmotically active metabolites, specific proteins, and certain free radical scavenging enzymes that control ion and water flux and support scavenging of oxygen radicals or chaperones. ROS detoxification forms an important defense against salt stress. Legumes are a key component of sustainable agriculture and can offer many economic and environmental benefits if grown more widely in crop rotations because of their ability to fix nitrogen in the root nodules in a symbiotic interaction with soil rhizobia. Due to their capacity to grow on nitrogen-poor soils, they can be efficiently used for improving saline soil fertility and help to reintroduce agriculture to these lands. However, in legumes, salt stress imposes a significant limitation of productivity related to the adverse effects on the growth of the host plant, the root-nodule bacteria, symbiotic development and finally the nitrogen fixation capacity. This paper reviews responses of legumes to salinity stress with emphasis on physiological and biochemical mechanisms of salt tolerance.