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Effect of Calcium on the Phosphorus Nutrition of Rhizobium meliloti¹

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.

¹ Contribution from Dep. of Land, Air and Water Resources, Univ. of California, Davis CA 95616. Supported in part by a grant from NSF RANN.

² Respectively, Research Assistant (now Microbiologist, Univ. of Hawaii NifTAL Project, Maui, HI 96779), and Professor of Soil Science, UC Davis.

Received for publication July 8, 1983. Accepted for publication October 31, 1984.

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