Effect of Calcium on the Phosphorus Nutrition of Rhizobium meliloti

Effects of calcium at 300 and 1500 µ M on P nutrition were assessed in eight strains of Rhizobium meliloti in defined liquid medium. Evaluations included: P storage from “luxury” external concentration (1000 µ M P); utilization of stored P after transfer to unreplenished low‐P medium (0.06 µ M ); and growth at low concentrations of P buffered at 5, 0.5, and 0.06 µ M with an iron oxide dialysis system. The strains stored P in luxury medium, but unlike other rhizobia, they needed high Ca to utilize the stored P. They either grew or died following transfer to low‐P medium, depending on the Ca concentration and the Ca concentration at which they had grown previously. Ability to grow in media buffered at low P concentrations also contrasted with that of other rhizobia, in two respects: no strain of R. meliloti grew at 0.06 µM P, regardless of Ca concentration; and some strains needed high Ca to grow at 0.5 and 5 µM P. Two isolates from Medicago rugosa and Melilotus indica were less Ca‐demanding than six isolates from Medicago sativa . Previous reports that R. meliloti has low calcium requirements may be correct only for the luxury P levels that are conventional in defined media. Our evidence for high Ca requirement at realistic P concentrations agrees with data from soil experiments.