Gibberellic acid and nodulation of legumes

IN 1958 Thurber, Douglas and Galston¹ reported that potassium gibberellate applied to Phaseolus vulgaris plants had a marked reducing effect on the number of nodules formed and in some cases almost inhibited their formation. When this communication was published we were in the course of determining the effect of gibberellic acid on the nodulation of Trifolium repens and ultimately reported² that, at 25 p.p.m., gibberellic acid in agar culture had no effect on nodule numbers in this plant; and further that up to 1000 p.p.m. had no effect on the growth of Rhizobium trifolii. It was thus obvious that not all legumes reacted in the same way regarding nodulation. Prof. Galston explained these apparently conflicting results³ by stating that 'this difference in nodulation response is probably related to the already known fact that different genotypes of plants presumably of varying endogenous gibberellin-levels react quite differently to identical gibberellin applications'. Further experiments which we have recently carried out with small-seeded legumes and their corresponding Rhizobia furnish more evidence of this variability and of the fact that reduction in nodulation, where present, is not due to the effect of the gibberellic acid on the Rhizobia.