GROWTH FACTOR RELATIONSHIPS OF SOIL MICROORGANISMS AS AFFECTED BY PROXIMITY TO THE PLANT ROOT

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Abstract
A comparison was made of the growth factor requirements and growth factor synthesizing capacity of bacteria from control soil, rhizosphere, and rhizoplane of wheat. Organisms for which growth factors were essential were proportionately much less abundant at or near the root than in more distant soil; this is ascribed chiefly to lower percentages of forms requiring thiamine, biotin, and vitamin B12. At the rhizoplane proportions of bacteria requiring growth factors were as low as, or lower than, in the rhizosphere. Bacteria capable of synthesizing growth-promoting substances were proportionately much more numerous in the root zone than in control soil. Somewhat greater capacity was shown by rhizoplane than by rhizosphere isolates. Fungi characteristic of the rhizoplane showed the highest capacity for vitamin production.The occurrence of growth-promoting substances at or near the root is attributable chiefly to microbial synthesis rather than to root excretion. This is the reverse of the situation believed to exist respecting amino acids. The preferential stimulation of growth factor synthesizing organisms is regarded as an indirect effect of plant growth—a reflection of the direct stimulation of amino acid requiring forms, a group with pronounced capacity for vitamin synthesis.