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

Abstract

AbstractPhosphate accumulation from defined liquid medium was assessed in six strains of Rhizobium japonicum grown at high P (2 × 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 β 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 × 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.