Plant Growth Substances Produced by Micro-organisms of Soil and Rhizosphere

S ummary : Micro-organisms isolated from rhizospheres and rhizoplanes of wheat plants, and from root-free soil, produced growth regulating substances with the properties of gibberellins and indolyl-3-acetic acid (IAA). Substances inhibiting extensions of pea plant internodes and lettuce hypocotyls were also produced, especially by bacteria from the root region of seedlings 6 days old. Bacteria producing growth promoting substances were most abundant on roots of older plants. Seedlings grown aseptically with added gibberellic acid (GA₃) and IAA, or grown with a soil inoculum, developed similarly and differed in their morphology from those grown aseptically without additives.