Acid-tolerance and symbiotic effectiveness of Rhizobium trifolii associated with a Trifolium subterraneum L.-based pasture growing in an acid soil

Abstract

The critical range of pH, below which growth in solid media of several strains of Rhizobium trifolii was inhibited, was between pH 4.55 and 4.85. This was consistent with growth of the strains over a range of pH in liquid culture. Concentrations of Al in excess of 150 μM were necessary to inhibit growth of R. trifolii in agar-culture at pH 5.10. Sensitivity of growth to Al was also influenced by the concentration of P in the medium. Acid-tolerance of R. trifolii isolates collected from a very acid soil was considered to be poor and was similar to that observed for R. trifolii strains TA1 and WU95. Only 96 of 481 isolates were capable of growth in solid media at pH 4.70, whereas all isolates made growth at pH 5.10. The proportion of isolates capable of growth at pH 4.70 was not related to the pH of the soil from which they were isolated, but was influenced by the method of isolation. A higher proportion of more acid-tolerant isolates were obtained from clover nodules formed in dilute soil suspensions than from single nodules removed from plants in the field. The symbiotic eflectiveness of 127 isolates of R. trifolii from the acid soil, relative to the effectiveness of R. trifolii strain TA1 in association with Trifolium subterraneum, ranged between 27 and 112% with a mean of 74% over all isolates. Significant negative correlations were found between the symbiotic eflectiveness and the growth of individual isolates in solid media at low pH.