MicrobioliB 



microbial types present to environmental factors. It has the short- 

 coming that it is only a partial characterization of the root micro- 

 flora. More complete characterization, and translation of the results 

 to binomial terminology, are needed. Perhaps it should be pointed out 

 that physiological methods of studying the rhizosphere microflora are 

 currently in a state of fltix. ■ Apparently the use of seven cultural 

 media as originally proposed proved to be unnecessarily cumbersome. 

 Recently, workers have been atempting to develop nutritional grouping 

 schemes based on fewer substrates. Katznelson at al. (16), for example, 

 have reported studies using the following three media: (a) basal medium 

 consisting of inorganic salts and glucose; (b) this medium plus amino 

 acids; and (c) the basic medium plus yeast and soil extracts. Gyllen- 

 berg (12) has made a different simplification; he used (a) basal medium, 

 (b) basal medium plus amino acids, and (c) basal medium plus both amino 

 acids and B vitamins. Very recently, Katznelson et al. (17) have 

 employed manometric techniques for characterizing the rhizosphere micro- 

 flora. 



Rhizosphere microfloras differ between plants. Not only is this true 

 between species, but it is also true for differing varieties within 

 species, as well as for plants differing in age and in conditions of 

 growth. Indeed even for individual plants there are differences depend- 

 ing upon part of the root system examined. 



The rhizosphere differences between species as well as between the 

 higher taxonomic categories of plant life are extremely kaleidoscopic. 

 Our knowledge of the rhizosphere microbiology is as yet too inadequate 

 to permit any satisfactory discussion of the microfloras of different 

 plant species. In order to emphasize that such differences do exist, 

 let us for the moment speak somewhat superficially of the legumes. 



It is well known that many leguminous species, even though by no means 

 the majority, form root symbioses with the legume-nodule bacteria. 

 Among the nodulating legumes, there is considerable specificity between 

 the two partners in the synbiosis. Thus the soybean rhizobia do not 

 nodulate alfalfa and clover even though they are capable of proliferat- 

 ing on the roots of these legvimes. Koreniako (18) has noted that 

 rhizobia grow abundantly in the rhizospheres of some non-leguminous 

 plants, for example, wheat and cotton, but not in those of others, for 

 example, corn and flax. In brief, rhizobia can grow on roots of legumes, 

 but they may or may not form nodules thereon. They cannot nodulate non- 

 legxmes, but they reportedly can grow on the roots of some non-legtiminous 

 plants. 



Different varieties of the same plant species have been shown to have 

 differing rhizosphere microfloras. Lochhead, Timonin, and West (19) 

 noted that the bacteria associated with wilt-resistant and wilt-sus- 

 ceptible varieties of tobacco and flax differed qualitatively and 

 quantitatively, even when both varieties were grown entirely free of 

 disease. Timonin (20) extended their work and showed striking differ- 

 ences in the fungal flora in the rhizospheres of wilt-resistant 

 susceptible plants. 



