Effect of salt stress on nodulation and nitrogen fixation in legumes.

It is now well established that almost all phases of root nodule development in legumes are adversely affected by saline conditions in the rooting medium. There is also a general agreement that the rhizobia are more tolerant to salt stress than the host plant, but they show considerable strain variability in growth and survival under saline conditions. Inhibitory effect of salinity on nodulation has been attributed to decrease in rhizobial colonisation and shrinkage and lack of root hair formation. Salt stress also induces premature senescence of already formed nodules. Both N2-fixation activity and nodule respiration are inhibited sharply on exposure of plants to saline conditions. The decrease in N2-fixation has been ascribed to direct effect on nitrogenase activity or an indirect effect through decrease in leghemoglobin content, respiratory rate, malate concentrations in nodules and photosynthate availability. Salinity increases oxygen diffusion resistance in the nodules and alters their ultrastructure. Decrease in N2-fixation in nodules under salinity is also accompanied by parallel decrease in the activity of H2O2-scavenging enzymes like catalase, ascorbate peroxidase and the level of antioxidants like ascorbic acid. Nodules appear to undergo osmoregulation under saline conditions by accumulating physiologically compatible solutes like proline, sugars (pinnitol) and lactic acid. The intensity of the adverse effects of salinity on nodule functioning depends on plant species, rhizobial strain, duration of exposure to saline conditions, nature, concentration and mode of salt application.