Effectiveness ofLotusRoot Nodules

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Abstract
An extract from the roots of Lotus pedunculatus plants was found to contain a compound toxic towards fast-growing Lotus rhizobia. This compound was identified as a flavolan, which has a prodeiphinidin: procyanidin ratio of 75:25. A fast-growing strain of Rhizobium (NZP2213) which forms ineffective root nodules on L. pedunculatus was four times more sensitive to this flavolan (ED50 = 25 ¨g ml−1) than another strain (NZP2037, ED50 = 100 ¨g ml−1) which forms effective root nodules on this species. The roots of another Lotus species, L. tenuis, on which both strains of Rhizobium form effective root nodules, also contained a flavolan (≦ 95% procyanidin) but both strains were relatively insensitive to this flavolan (EDED50 = 350 to 500 ¨g ml−1) L. pedunculatus plants bearing ineffective root nodules contained two to three times more flavolan in their roots (5–7 mg g−1 fr.wt.) than uninoculated control plants. Experiments with seven other Lotus species and with hybrid plants developed between L. pedunculatus and L. tenuis showed a relationship between the prodeiphinidin: procyanidin ratio of the flavolan in their roots and the effectiveness of root nodules formed on these plants by NZP2213. Quantitative binding studies of the flavolan from L. pedunculatus to NZP2037 and NZP2213 indicated that, while the affinity constants for binding were similar for both strains, the surface of strain NZP2037 contained four times more binding sites than NZP2213, possibly correlating with this strain's ability to tolerate higher concentrations of this flavolan. It is suggested that the differential sensitivity of these two strains of Rhizobium to flavolans is related to their ability to form effective root nodules on Lotus species.