RHIZOSPHERE BACTERIAL POPULATION RESPONSES TO ROOT COLONIZATION BY A VESICULAR-ARBUSCULAR MYCORRH1ZAL FUNGUS *

Authors


  • *

    Adapted from a dissertation by the senior author submitted to the academic faculty of Colorado State University in partial fulfillment of the requirements for the Ph.D. degree.

  • Present address of senior author: USDA-ARS, Western Regional Research Center, 300 Buchanan Street, Albany, California 94710, USA.

Summary

A mixture of bacteria and a vesicular-arbuscular mycorrhizal fungus isolated from field-collected sods of blue grama (Bouteloua gracilis (H.B.K.) Griffiths) were tested for their interaction in the rhizosphere of pot-grown blue grama plants. Populations of the inoculated bacterial species and actinomycete populations, as influenced by the presence or absence of Glomus mosseae (Nicol. and Gerd.) Gerdemann and Trappe, were enumerated by dilution plate counts from rhizosphere and non-rhizosphere soil samples. Total bacterial counts and the population of one bacterial species in the non-rhizosphere soil of pots containing plants were significantly greater than in soil of pots without plants. The population of two bacterial species and actinomycetes were not significantly different in the non-rhizosphere soil of both mycorrhizal and non-mycorrhizal plant treatments when compared to the soil of pots without plants. In the rhizosphere of mycorrhizal plants, the total bacterial population and colony counts of one of the four bacterial isolates, when expressed as colony-forming units (CFU) per gram of root dry weight, were significantly reduced compared with controls. The numbers of CFU per gram of rhizosphere soil of one bacterial species were significantly increased by the presence of the mycorrhizal fungus. Although no significant negative correlation was observed between populations of bacterial species in the rhizosphere soils, significant positive correlations between specific bacterial populations depended on whether or not the roots were mycorrhizal.

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