Competition Among Rhizobium leguminosarum Strains for Nodulation of Lentils ( Lens esculenta )

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Abstract
Thirty-one cultures of Rhizobium leguminosarum were screened for effectiveness (C2H2 reduction) on lentils (Lens esculenta). Fluorescent antibodies prepared against three of the most effective strains (Hawaii 5-0, Nitragin 92A3, and Nitragin 128A12) exhibited a high degree of strain specificity; the antibodies reacted strongly with their homologous rhizobia in culture and with bacteroids in nodules. They did not cross-react with one another, and only weakly with 5 of the 47 other R. leguminosarum cultures tested. In competition studies in the growth chamber, whenever strain Nitragin 92A3 was included in the inoculum mixture, it consistently (but not always significantly, P = 0.05) occupied the majority of nodules on all four cultivars used. However, some degree of strain X cultivar interaction was apparent: Hawaii 5-0 was of equal competitiveness (P = 0.05) with Nitragin 92A3 on three of the varieties (Commercial, Tekoa, and Benewah), but inferior (P = 0.01) on the Chilean variety; Nitragin 92A3 completely dominated (P = 0.01) Nitragin 128A12 on all cultivars; and Hawaii 5-0 was of equal competitiveness (P = 0.05) to Nitragin 128A12 on the Chilean variety and more competitive (P = 0.01) on the commercial variety and less so on the other two varieties. In field experiments, Hawaii 5-0 proved of equal competitiveness (P = 0.01) with Nitragin 92A3 in one soil (an Inceptisol) and superior (P ≤ 0.05) to it in another (an Oxisol). Incidence of double-strain occupancy of nodules varied from 0 to 36% in vermiculite, depending on the strains in the mixture and the host variety, and from 0 to 38% in the field, depending on the strains in the mixture and the soil type. The results suggest a close relationship between the competitiveness of a strain and its occurrence in doubly infected nodules.