The effect of growth pH on growth and survival of Sinorhizobium medicae WSM 419 in acid soils

The present research project was undertaken to study whether S. medicae WSM 419 can induce an acid tolerance response during growth at mild acid stress. Secondly, the effect of growth pH on the survival of S. medicae WSM 419 in acid soils and acid peat was assessed together with the effect of acid peat inocula on the survival of WSM 419 in acid soils. The acid tolerance response was inducible in S. medicae WSM 419. Cells of WSM 419 grown in Tryptone Yeast broth at an acid pH (pH 5.8) were more tolerant to an acid shock in culture at pH 4.0, pH 4.5, pH 5.0, pH 5.5 and pH 5.7 than cells grown in the same medium at neutral pH (pH 7.0). Strain WSM 419 grown at pH 5.8 and exposed to an acid pH at 5.5 and pH 5.7 grew, whereas cells grown at pH 7.0 died. WSM 419 grown at pH 5.8 died slowly when exposed to acid pH at pH 4.0, pH 4.5 and pH 5.0, while those grown at pH 7.0 rapidly died. The rate of cell death was influenced by the pH in which the cells had been grown. Decimal reduction time (D value) of WSM 419 grown at pH 5.8 (Mean Generation Time 5.7 h) and exposed to pH 4.0 was 94 min, whereas for cells grown at pH 7.0 (MGT 2.56 h) the D value was 52 min. When WSM 419, grown at pH 7.0 and pH 5.8, was inoculated into soils at pH 4.50 (Badgingarra), pH 5.30 (Lancelin) and pH 5.52 (Bodallin) the largest populations developed in the soil at pH 5.52. The pH of the growth medium of WSM 419 affected the population recoverable from acid soils. Inoculum cultures grown at pH 5.8 developed larger populations when added to soils at pH 4.50, pH 5.30 and pH 5.52 after 28 days than inoculum cultures grown at pH 7.0. The pH of the growth medium of inoculum cultures of WSM 419 affected the level of population that developed in acid peats. Inoculum cultures grown in acidic TY broth at pH 5.8 developed larger populations after 28 days in a range of acid peats at pH 4.5, pH 5.0, pH 5.5 and pH 6.0 than cultures grown at pH 7.0.The pH of the peat did not affect the increase in rhizobial population. These results indicated that it may be possible to manipulate the growth of inoculum cultures of rhizobia to enhance their growth in acid peat. In a glasshouse experiment, inoculum cultures grown at pH 5.8 and pH 7.0 were used to inoculate acid peats (pH 4.5, pH 5.0, pH 5.5 and pH 6.0) and the peats were then inoculated into acid soils (pH 4.50 and pH 5.30 and pH 5.52). The peat inoculum at pH 4.5 performed better in Badgingarra acid soil at pH 4.50. Peat prepared using cultures grown at pH 5.8 performed better in Lancelin sand than peat prepared using cultures grown at pH 7.0. However, this effect was not observed using these inocula in Bodallin and Badgingarra soils. In conclusion, acid tolerance response was inducible in S. medicae WSM 419. The cells of WSM 419 grown at pH 5.8 were more tolerant of an acid shock than cells previously grown at pH 7.0. Indeed, the growth pH affected the rate of cell death and the boundaries of the four-zone response model. The tolerance of WSM 419 to an acid shock in the laboratory was not reflected with the symbiotic performance of WSM 419 in acid soils. Thus, even though growth pH affected the level of population that develop in acid soils, it seems that this does not get reflected in the capacity of WSM 419 to nodulate M. polymorpha. The pH of the growth medium of inoculum cultures of WSM 419 affected the level of population that develop in acid peats, and the pH of the peat did not affect the level of increase in the population of WSM 419 during incubation. All the acid peat inocula contained greater population of rhizobia than that proposed as an acceptable standard for Australian manufacturers. Thus, it may be possible to manipulate the growth pH of inoculum cultures of WSM 419 to enhance their growth in acid peat. The pH of the growth medium of inoculum cultures together with the pH of the peats may alter their performance in acid soils