Variation within Strains of Clover Nodule Bacteria in Size of Nodule Produced and in “Effectivity” of Symbiosis

Abstract

The paper describes expts. on the influence of passage through the host legume and of storage in soil and on agar medium upon the symbiotic behavior of 2 strains of Rhizobium; the results referring to mean length and "effectivity" of nodules on red clover grown under bacteri-ologically controlled conditions. The original culture of an effective strain (Strain A) was found to be uniform in the behavior of isolations from replicate colonies although considerable variation was found between the responses of individual plants infected from the same colony. Neither plant passage with or without intervening plating or short-time culture on agar, nor selection from large or small nodules had any effect on the mean size or effectivity of the nodules produced either by strain A, or by an ineffective strain (Strain H.K.C.). After storing strain A in sterilized Woburn sandy soil, ineffective variants were found to constitute a considerable proportion of the bacterial population. These variants resembled the parent type in cultural and serol. characters. On passing these ineffective variants through the plant, 2 reversions to the effective parent type were found amongst the 13,400 nodules examined. These remained effective on further plant passage. On subsequent examination stock cultures on agar slopes, both of the effective parent type and of the ineffective variant, showed a very occasional tendency to produce new variants in effectivity. Uncorrelated variants in type of growth on agar also appeared under these conditions. These changes in effectivity are considered to be of the nature of "mutations".