Response of chickpea and soybean rhizobia to salt: Osmotic and specific ion effects of salts

Abstract

Chickpea Rhizobium strain 2-ICAR-SYR-Ch-184 showed different responses (in terms of growth and survival) to a range of different salts. The chloride ions of Na, K and Mg were found to be more toxic than the corresponding sulphate ions. The osmotic effect was studied using polyethylene glycol (PEG), glycerol and sorbitol to raise the osmotic pressure to −1.0 and −2.0 MPa. PEG was toxic even at low concentrations. However, there was good growth at −1.0 and −2.0 MPa using glycerol and sorbitol. Although, the sensitivity of the strain to NaCl was increased at higher osmotic pressures, the harmful effect of salts on the growth of this strain could be attributed more to the specific ion effect rather than the osmotic effect. The influence of various organic osmotica on the growth of chickpea rhizobia and soybean rhizobia and bradyrhizobia was studied in salt-stressed media. Chickpea Rhizobium strain 2-ICAR-SYR-Ch-Ch184 showed an increase in final cell density when glutamate was added to 0.34 m NaCl. All other strains showed no improvement in growth with the addition of glutamate. Glycine betaine did not relieve the inhibition of growth of all strains except for a slight improvement in the growth of the salt-sensitive soybean Bradyrhizobium japonicum strain RCR 3407 at 0.08 m NaCl. However, all tested strains used glycine betaine as a sole carbon source regardless of the presence or absence of NaCl.