COMPETITION FOR NODULATION OF PISUM-SATIVUM CV AFGHANISTAN REQUIRES LIVE RHIBOZIA AND A PLANT-COMPONENT

Nodulation of P. sativum cv. Afghanistan by Rhizobium leguminosarum strain Tom can be blocked by R. leguminosarum strain PF2 (isolated from P. sativum cv. Rondo) which does not form nodules on Afghanistan peas. PF2 was tested for its ability to produce bacteriocins and other compounds inhibitory to the growth of Tom. Neither strain was antagonistic toward the other. Similarly, there was no evidence for the production of inhibitors as the rhizobia grew in the plant rhizosphere. Apart from an already noted (Broughton et al. 1980) ability of PF2 to accumulate on the root surfaces 2-3 times faster than Tom, there was no reason to explain the complete blocking of nodulation. In other experiments using PF2 (and Tom) killed by exposure to massive doses of .gamma. irradiation, dead cells were still bound to the roots but they were incapable of blocking nodulation by living Tom. Finally, when both rhizobia were used to inoculate P. sativum cv. Rondo (they both form nodules on this plant), roughly 1/3 of the nodules contained Tom, 1/3 contained PF2, and 1/3 contained both strains (i.e., there was no evidence for competition). Competition in this system is dependent upon live bacteria and requires cooperation from the plant.