Gibberellic acid, a metabolic product of the fungus Gibberella fujikuori, has been intensively studied during the past few years for its remarkable effects on cell elongation, flowering, and dormancy in higher plants by Brian and Grove (1957), Brian (1958, 1959), Phinney et al. (1957), Bukovac and Wittwer (1958), Stodola (1958), and Stowe and Yamaki (1957, 1959). There is, however, little information concerning its influence on microorganisms. Brian et al. (1954) reported that a number of common bacteria and molds were unaffected by the acid, although the acid was rapidly decomposed in soil, presumably by microorganisms. Lu et al. (1958) found that gibberellic acid increased numbers of Azotobacter chroococcum in a sandy loam soil; subsequently Lu and Bollen (1958) observed that 50 ppm Gibrel,' a water-soluble potassium salt of gibberellic acid, hastened decomposition of native and added organic matter and stimulated nitrogen fixation by Azotobacter in this soil. Since heterotrophic microorganisms as well as higher plants, which are autotrophic, are known to be influenced by Gibrel, it became of interest to determine what effect this substance had on the activities of autotrophic bacteria in the soil. Experiments on nitrification and sulfur oxidation therefore appeared desirable. These autotrophic processes are not only carried on by a physiologically distinct class of bacteria but they also are of great importance in soil fertility. The rate and extent of these transformations are influenced by environmental factors, which are in part determined by soil type. Therefore, 11 different soils representing a wide range of conditions and geographical locations in the state of Oregon were selected for the investigatioii.
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Chandra et al. (1960) studied this question.