Complementary action of Gibberellic acid and auxins in Pea internode extension

Gibberellic acid (GA) induced extension of green pea-stem sections in light only if an auxin was also present. Of the auxins tested 3-indolylacetic acid, 2-methyl-4-chloro-phenoxyacetic acid, 2: 4-dichloro-phenoxyacetic acid and I-naphthylacetic acid were effective in increasing extension of sections and in elicit-ing a response to GA. Excised internodes from plants pre-treated with GA extended appreciably faster in vitro than those from untreated plants only if an auxin was supplied in the incubation medium. This and other evidence suggests that in the intact plant GA elicits a growth-response only in the presence of auxin. By comparing growth-rates of excised internodes in vitro and of intact internodes in vivo under comparable conditions, using untreated plants and plants pre-treated with GA, evidence has been obtained that in untreated plants growth-rate is somehow limited to a level below that made potentially possible by the endogenous auxin supply; treatment with GA appears to release the plant from this state of inhibition. Growth of intact pea internodes is considered to be regulated by a three-factor system, consisting of auxin, an inhibitory system, and a hormone with physiological properties similar to those of GA.