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Phosphorus Nutrition of Rhizobium japonicum: Strain Differences in Phosphate Storage and Utilization¹

ABSTRACT

Phosphate accumulation from defined liquid medium was assessed in six strains of Rhizobium japonicum grown at high P (2 x 10^–3M orthophosphate in solution). The total cell P concentration ranged from 1.6 to 2.4% dry mass depending on strain, and correlated with subsequent growth after transfer to medium without added P. Stored cell P could support up to four or five generations. Steps to ensure high-P storage in inoculant cultures might have practical utility.

Growth ceased when total cell P dropped to 0.3%. Electron microscopy indicated a tendency of deficient cells to become elongated, distorted, and packed with lightly staining granules (perhaps poly ß hydroxybutyrate).

High-P cells of all the strains contained from 5 to 7 electrondense granules. Their total volume varied between 0.1 and 0.5% of the cell, depending on strain; and related linearly to percent of P in the cell. The granules occurred only when cell P was already high (> 1.5%) and disappeared within 1 day after transfer to medium without P. They may be polyphosphate storage granules. They provided P sufficient at most for 1 to 2 generations.

Cells grown at an intermediate external solution-P concentration buffered at 6 x 10^–6M, resembling that in fertile soil, had few granules and moderate stored P (1.3 to 1.7%). Invasion of P-depleted rhizospheres by rhizobia from soil probably cannot depend on previously stored P: it must also require efficient uptake from very dilute-P concentrations.

External gum was abundantly produced at low- and moderate-P supply, but not at the high P concentrations routinely supplied in laboratory media.

¹ Contribution from the Dep. of Land, Air, and Water Resources, Univ. of California, Davis CA 95616. Partly supported by grants from the National Science Foundation (NSF/RANN), U.S. Department of Agriculture and Agency for International Development.

² Postgraduate Research Scientist, Professor, and Graduate Research Assistant.

Received for publication August 22, 1980. Accepted for publication December 12, 1980.

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