Effect of Calcium on the Phosphorus Nutrition of Rhizobium meliloti

Abstract

Effects of calcium at 300 and 1500 µM on P nutrition were assessed in eight strains of Rhizobium meliloti in defined liquid medium. Evaluations included: P storage from “luxury” external concentration (1000 µM P); utilization of stored P after transfer to unreplenished low-P medium (0.06 µM); and growth at low concentrations of P buffered at 5, 0.5, and 0.06 µM 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.