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Fred, Edwin Broun; Baldwin, Ira Lawrence; McCoy, Elizabeth / Root nodule bacteria and leguminous plants
(1932)
Chapter 10: Relationship between leguminous plants and bacteria, pp. 160-191
Page 169
ROOT NODULE BACTERIA that Leguminosae may benefit from the mere presence of rhizobia, regardless of whether nodules are formed. Other workers, however, have failed to confirm this observation. Cross-inoculation groups. The specificity of the relationship existing be- tween the leguminous plant and its rhizobia is worthy of note. Members of the genus Rhizobium are apparently able to infect only members of the family Leguminosae, and conversely, practically all species of the Leguminosae are infect- ed by some species of Rhizobium.2 Not all species of Leguminosae are infected by the same type of Rhiobiunim, however. Recognition of this specificity has lead to a classification of leguminous plants into groups, within which the rhizobia are interchangeable. In many cases a single plant group includes several different species or even genera of the Leguminosae; in other cases a single plant species is the only known host to a particular Rhizobium. A detailed list of the plant-bacteria or so-called cross-inoculation groups has been given in Chapter 8. There is as yet no satisfactory explanation for the specificity which exists. A correlation between the acidity of the cell sap and the limiting acidity for growth of the rhizobia concerned has been suggested. Baldwin, Fred, and Hastings, 1927, have established a certain correlation between the protein constitution of the seeds and the cross-inoculation grouping. Neither of these explanations establishes an exact correlation, and it seems probable that some other factor or factors in the physiological complex of the host determines infection or non-infection. The work of Richmond, 1926c, is interesting in this connection. By grafting the tops of navy bean plants, Phaseolus vulgaris, on the roots of the lima bean, Ph. limensis, and vice versa, he demonstrated that the conditions in the root are the factors determining infection. On grafted plants with lima bean roots, only rhizobia of the lima bean are able to induce nodule formation. And conversely, on plants with navy bean roots, the rhizobia of the navy bean alone may enter. Seeds produced on such grafted plants were apparently altered in such a way that the plants of the second generation could be infected by either the rhizobia of the navy bean or of the lima bean. The data supporting this latter statement are meager. Recently Hansen and Tanner, 1931, have repeated Richmond's graft- ing experiment and have arrived at negative conclusions as far as nodulation is concerned. They have also done serological work on the seed proteins of normal and grafted plants and have found no indication of changed nature of the seed protein complex. Strain variation. Even before the specificity of the plant-bacteria relationship was fully recognized, it was noted that nodules on the roots of leguminous plants did not always result in fixation of atmospheric nitrogen and consequently in- creased plant growth. Frank, 1892b, described two types of nodules on the pea, distinguished as "amylodextrin and albuminoid nodules." The former were characterized by their reddish-brown color with iodine, a reaction due chiefly to absorption of iodine by numerous granulae of amylodextrin in the bacteroids and probably also by accumulated starchy reserve in the plant cells. Beijerinck, 1888, divided nodules into two broad groups--one in which the plant gains the 2The few knowvn exceptions are listed in chapter 3. 169
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