Studies of Water Quality on Rhizobia

The physiology of the response of R, meliloti to salt stress was investigated. It was found that the growth of the bacterium is not inhibited simply as a consequence of the increased osmolarity of the medium caused by the addition of salts but by the effect of a particular salt. The bacteria were shown to accumulate glutamic acid in response to inhibitory concentrations of sodium chloride suggesting that this amino acid may act as an osmoticum. However, the addition of glutamic acid to the growth medium did not protect the bacterium from inhibition by sodium chloride even though the amino acid was transported and metabolized.

The effects of salts commonly found in irrigation water on growth of Rhizobium meliloti, the bacterium that establishes a symbiotic relationship with alfalfa (Medicago sativa) were investigated to determine how this might affect alfalfa production. The growth rate of this bacteria was not affected by sodium, potassium, magnesium, chloride, sulfate, bicarbonate or phosphate, except in concentrations much greater than can be tolerated by alfalfa cultivars currently in use. Growth of this bacteria was not affected by alkaline pH. These results indicate that saline and alkaline conditions do not limit alfalfa yields because of the effects of these conditions on growth of the symbiotic bacteria.

The work was expanded to include the rhizobia that establish a symbiotic relationship with peanuts (Arachis hypogaea). 19 strains were tested for the effect of saline and alkaline conditions on the growth rate of these bacteria. With one exception, the growth of all strains was inhibited by modest levels of sodium chloride when the pH was slightly alkaline. These preliminary results indicate that saline and alkaline conditions could limit peanut yields because of the effect on these condition on growth of the symbiotic bacteria.