Competition For Nudule Occupancy On Phaseolus Vulgaris By Rhizobium Etli Strains From Two Ecoregions Of China

The legume-rhizobium symbiosis and the distribution of rhizobia were determined by host selection and environmental factors, especially soil pH. In this study, using the single strain inoculation and co-inoculation of rhizobia with legume plants and BOX-PCR fingerprinting technique, the competitiveness of Rhizobium etli strains nodulating with common bean(Phaseolus vulgaris L.) from the northern(ecoregion I) and the southern(ecoregion II) regions of China was estimated by the number of nodules and nodule occupancy of plants, the results are as follows.Based on the pH tolerance and BOX-PCR fingerprinting patterns of 36 R. etli strains, which isolated from six sampling sites, five strains were selected as representatives including S9, M10, L101, H4 and NC1 and were employed in the plant experiments. Among them, strains L101 and S9 were from ecoregion I with soil pH 6.52 and 5.46, respectively; strains M10 and NC1 were from ecoregion II with soil pH 6.97 and 7.22, and H4 was from ecoregion II with soil pH 8.16.Competitiveness between R. etli strains L101 and strains M10 and NC1 was carried out in the plant experiments, and the vermiculite medium with original pH 5.5, 7.0 and 8.0 were used. The results showed that the L101 was a better competitor under the condition of pH 7.0 and 8.0 than M10 and NC1, while the strains M10 and NC1 were more competitive under pH 5.5 and occupied more proportion of total number of nodules, as high as 58.92% and 50%, respectively. The result suggested that the competition of R. etli strains isolated from different soils with similar pH values in the different ecological regions showed distinct responses to environment pH, similar results were found for R. etli strains isolated from different soils of same ecological region.Through the comparison of the growth of strain S9 and H4 in the TY media with initial pH 5.5, 7.0 and 8.0 separately. Results showed the two strains reached stationary phase at 48 h and the OD600 were 0.81 and 0.76 in the pH 5.5 medium, respectively, which was better than that in the pH 7.0 medium. However, in the pH 8.0 medium, two strains grew slowly, OD600 of S9 and H4 was 0.65 and 0.32 in 48 h respectively. The pH of media increased after growing.In the greenhouse, common beans without inoculation, separately inoculated and coinoculated with strain S9 and H4 were cultivated in neutral vermiculite for 35 days, for studying the effect of competitiveness of rhizobia on the plant growth of legume host. The result showed the nodule number and the dry weight of plants co-inoculated displayed a significant increase compared with the other treatments(P<0.05), indicating co-inoculation with two effective rhizobia can result in the increase of common bean plant growth.Co-inoculation of strain S9 and H4 was carried out in different conditions. In vermiculite pouches, with increasing of pH from 5.5 to 7.0 and 8.0, strain S9 showed a decrease in the nodule occupancy. The Ganzhou soil from ecoregion II(pH =5.49) and Daqing soil from ecoregion I(pH = 8.16) non-sterilized and sterilized were permitted discrimination competition for nodulation between strain S9 and H4. In sterilized soils, strain S9 occupied 87.69% nodules of the Ganzhou soil which was far greater than H4. When co-inoculation in the non-sterilized soils, strain S9 occupied more nodules in both the Ganzhou soil and Daqing soil with 68.40% and 66.67%, and strain H4 was the second competitor with nearly 30%. All these results indicating that without indigenous strains, the competitiveness for nodulation of R. etli strains isolated from acid and alkaline soils of different regions was effected by p H, and the effect was associated with the pH value of isolated soil of strains. The strain isolated from acid soil was more competitive in acid vermiculite medium and sterilized acid soil, while the strain isolated from alkaline soil was a better competitor in alkaline vermiculite medium. In non-sterilized soil, the competitiveness of strain S9 was not related with the adaptability of acid condition which may directly and indirectly affected by soil microorganism.