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 [3H]-and [2H2]GA2o in the hypocotyls 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,, GA19, GA2o, and GA44. In hypocotyls of seedlings exposed to ethylene, the concentration of GA,, known to be directly active in regulating shoot elongation in a number of species, was reduced. Ethylene treatment reduced the metabolism of [3H]GA2o and less [2H2]GA1 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 GA20 metabolism was direct or indirect. In seedlings treated with exogenous GA, or GA3, 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, this is not sufficient per se to account for the inhibition of elongation caused by ethylene.Seedlings of most, if not all, plant species produce ethylene as well as GAs.2 Gibberellins promote elongation of stem cells directly through the action of GA, in several species (e.g. 13, 14), while ethylene inhibits elongation of stems but promotes radial growth. It has been shown that GA treatment will reverse the effect of ethylene on elongation (e.g. 21) and that ethylene inhibits GA-induced elongation (e.g. 3).As part of ongoing studies on the role of GAs and other hormones in control of growth and development in Helianthus (4, 5, 17), we have investigated GA metabolism and its interaction with ethylene in sunflower seedlings. We have used ethylene as a tool to modify elongation in our investigation of GA metabolism.We describe the effect of GA and ethylene treatment on elongation of hypocotyls of sunflower seedlings, and report In experiment 2, to test the effect of exogenous GA on elongation, seeds were sown in Terragreen (a baked clay medium) in 10 cm pots and grown in Perspex chambers for fumigation with ethylene from the time of planting (15). The chambers were illuminated with fluorescent Gro-Lux lights (320 ,umol m-2 s-') during a 16 h day. The temperature was 26°C during the day, and 14°C at night. The seedlings were watered automatically with half-strength Hoaglands solution for 45 s h-1. The CO2 concentration in the chambers was maintained at 350 ,uL L'. After 7 d, seedlings were treated with GA3. Three days later, seedlings were harvested and hypocotyl length was measured. This experiment was repeated, and later repeated again with GA3 and GA, applied to plants grown in conditions slightly modified from those described above (i.e. ma...