Growth Factor Requirements of the Root Nodule Bacteria.

It is known that yeast or plant extracts markedly stimulate the growth of the root nodule organism in media of purified ingredients. An essential growth factor or superior source of nitrogen has been invoked to explain the results, but neither theory has a broad experimental basis. The results of Allyn and Baldwin (1930) suggest that the extracts may affect favorably the oxidation-reduction potential of the medium. Allison, Hoover and Burk (1933) report that a specific co-enzyme for respiration of rhizobia can be extracted from commercial sucrose, azotobacter cultures and various plant or animal tissues. This extract markedly stimulates the growth of rhizobia in a medium of purified ingredients. Hoover and Allison (1935) state, "the growth of certain species, on the usual synthetic (sugar, inorganic salts, and nitrate) medium is negligible if pure ingredients are used, and that the addition of a small amount of growth factor is essential." Thorne and Walker (1936) also found that an extract of azotobacter (or other substances) was able to stimulate growth of several species of rhizobia in a medium of highly purified materials but did not regard it as essential for growth. By substituting ammonium chloride or asparagin for potassium nitrate, they grew four species in repeated transfer for more than two months with no appreciable decrease in rate of growth of the organisms. Addition of reducing substances to Allison and