Growth of Rhizobium Strains at Low Concentrations of Phosphate1
- K. G. Cassman,
- D. N. Munns and
- D. P. Beck2
A technique was developed for assessing effects of P on growth, in defined liquid media at orthophosphate concentrations comparable to those in soil solutions. Phosphate was supplied in sorbed form on iron oxide (goethite) separated from the liquid medium by a dialysis membrane. Depending on the level of sorbed P, concentrations in solution were established in the range 0.05 to 30 µM. Analytical monitoring indicated that concentrations above 0.1 µM were buffered against depletion by cell growth until populations exceeded 106 cells per ml.
The system was used to compare P responses of seven strains of soybean and cowpea rhizobia grown from P-depleted inocula at initial densities of 104 cells per ml. Two P-efficient strains grew as rapidly at the lowest concentration (0.05 µM) as in a control medium at 2,000 µM. They would seem unlikely to be P-limited in the field. Other strains were less efficient. One strain could barely grow at P concentration below 1 µM; this strain could have difficulty colonizing P-deficient soils or normal rhizospheres.
Large differences in tolerance to low P among strains may be agronomically important. The technique could be simplified for routine screening of strains; and its operation at submicromolar concentrations could be improved for research on nutrition and requirements of P-efficient rhizobia and other organisms.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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