Colonization of rhizospheres and nodulation of two Vigna species by rhizobia inoculated onto seed: Influence of soil

Abstract

Changes in the populations of Rhizobium strains CB756str, CB985 and CB1024strspc in the rhizospheres of cowpea ( Vigna unguiculata) and black gram ( V. mungo) grown at three sites were evaluated. The population dynamics of the three rhizobia varied with soil type but the strain responses on the two legumes were generally similar. Most noticeable was the ability of CB756str to grow in the sandy soil (Beerwah) but not in the heavy clays (Narayen and Emerald). In contrast, the levels of CB1024strspc and CB985 generally increased in the clay soils. Nodulation (% due to the inoculum strain) did not always reflect events within the rhizosphere. Although not suited to Narayen, CB756str formed a similar proportion of the nodule population of black gram as CB1024strspc but this may have been due to higher seed inoculum levels of CB756str. At Emerald nodulation by all three strains of rhizobia was poor regardless of the success in colonization of the rhizosphere. Successful competition for nodule sites by native rhizobia may contribute to this discrepancy between Narayen and Emerald although lower seed inoculum levels at Emerald may also have been important. Nodule decay was consistently associated with a large increase in the number of rhizobia per root system. This is likely to be important in the survival of strains into the following season. Comparisons of nodulation by parent and mutant rhizobia suggested that resistance to antibotics may have slightly reduced nodule forming ability for CB1024strspc on black gram at Emerald.