Ethylene-Mediated Regulation of Gibberellin Content and Growth in Helianthus annuus L.

Elongation of hypocotyls of sunflower can be promoted by gibberellins (GAs) and inhibited by ethylene. The role of these hormones in regulating elongation was investigated by measuring changes in both endogenous GAs and in the metabolism of exogenous [(3)H]- and [(2)H(2)]GA(20) in the hypocotyis of sunflower (Helianthus annuus L. cv Delgren 131) seedlings exposed to ethylene. The major biologically active GAs identified by gas chromatography-mass spectrometry were GA(1), GA(19), GA(20), and GA(44). In hypocotyls of seedlings exposed to ethylene, the concentration of GA(1), known to be directly active in regulating shoot elongation in a number of species, was reduced. Ethylene treatment reduced the metabolism of [(3)H]GA(20) and less [(2)H(2)]GA(1) was found in the hypocotyls of those seedlings exposed to the higher ethylene concentrations. However, it is not known if the effect of ethylene on GA(20) metabolism was direct or indirect. In seedlings treated with exogenous GA(1) or GA(3), the hypocotyls elongated faster than those of controls, but the GA treatment only partially overcame the inhibitory effect of ethylene on elongation. We conclude that GA content is a factor which may limit elongation in hypocotyls of sunflower, and that while exposure to ethylene results in reduced concentration of GA(1) this is not sufficient per se to account for the inhibition of elongation caused by ethylene.