Influences of phosphate on the growth and nodulation characteristics of Rhizobium trifolii Public Deposited

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  • Nodulation success by two indigenous Rhizobium trifolii serogroups, 6 and 36, on subterranean clover (Trifolium subterraneum L.) was affected by the addition of phosphate to Abiqua soil (Ultic Haploxeroll). To understand the impact of phosphorus nutrition on the survival and nodulation characteristics of the two serogroups, studies were carried out to determine: (i) the growth characteristics of representative serogroup isolates, 6 and 36, in defined nutrient solution with phosphate concentrations typical of soil solution; and (ii) the influence of phosphate stress upon the nodulation characteristics of the two isolates at pH 5.5 and 6.5. An iron (III) oxide dialysis system was used to provide growth medium buffered at 0.00, 0.28, 0.51, 0.87, and 6.50 uM phosphate at pH 5.5. After depleting the internal P content of the organisms, the external critical phosphate requirements of 6 and 36 were found to be significantly different (0.56 and 0.93 uM respectively). Despite the relatively small difference between the critical phosphate requirements, 6 adapted to phosphate-depleted medium far better than 36 by maintaining viability in 0.00 uM P medium, whereas 36 lost viability within 48h in medium having < 0.56 uM P. At pH 5.5, P-deficient 36 was inferior to P-deficient 6 by taking 2 to 4d longer to initiate nodulation and forming less nodules in P-depleted plant growth medium. Nodulation by P-starved 36 was improved by either the presence of phosphate at pH 5.5, or by a higher pH of the plant growth medium depleted of phosphate. Rhizoplane enumeration showed that both 6 and 36 could proliferate even under stringently low inorganic phosphate conditions (5.4 to 9.2 generations in 7d) and achieved about 9 generations at 21d. P-deficient 36 proliferated significantly less than luxury P grown 36 regardless of the P status of the plant growth medium. The numbers of nodules formed by 6 and 36 were not related to the rhizoplane densities of the bacteria.
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