Effect of high salts concentrations on the growth of rhizobia and responses to added osmotica

Twenty-eight reference strains, 79 rhizobia isolated from Trigonella foenum graecum (fenugreek), 26 strains isolated from root nodules of Ceratonia si liqua (carob tree), 30 strains isolated from nodules of Adenocarpus decorti cans and five isolated from Cytisus arboreus root nodules, were screened fo r their tolerance to increased concentrations of NaCl. Nine isolates of fen ugreek were able to grow on medium containing 10% NaCl and one strain still grew at 14%, The effect of a range of salts at 2.5 and 5% (w/v) on the gro wth of rhizobia was assessed and it was shown that this effect depended on the ion form and the strains used. In general, NaCl appeared to be a good i ndicator of the tolerance of the strains to salts. The concentrations of th e salts used were high and even at 5%, growth was not completely arrested i n the less tolerant strains. Some substances, such as glutamate, proline, g lycine betaine and CaCl2, were tested as osmotica, The effect of the two am ino acids and betaine was confirmed with all the strains used whereas the a lleviating effect of CaCl2 was not observed in all strains. This salt had d ifferent effects on two isolates of fenugreek. These results revealed a gre at diversity in salt tolerance, correlated with different responses to othe r stress conditions, which may be due to diversity in microbial ecology.