COMPARISON OF THE EFFECTS OF WHITE-LIGHT AND THE GROWTH RETARDANT PACLOBUTRAZOL ON THE ETHYLENE PRODUCTION IN BEAN HYPOCOTYLS

At a concentration of 17 mu mol.L-1, paclobutrazol (PP), a triazole pl ant growth retardant, effectively reduced the elongation and increased the thickness of hypocotyls in 6-day-old Phaseolus vulgaris L. cv. Ju liska seedlings, both in the light and in the dark. PP treatment did n ot increase the cell number in transverse sections of hypocotyls. The diameter of hypocotyls was uniform from the zone of intensive elongati on along the whole hypocotyl in etiolated plants, but those grown in t he light exhibited an additional lateral expansion at the base. Ethyle ne evolution was not reduced by PP in etiolated hypocotyls, and did no t differ significantly in the elongating apical and fully grown basal zones. PP reduced the ethylene release by the growing zones in green h ypocotyls, but not in the basal parts, which resulted in an increasing ethylene gradient towards the hypocotyl base. The level of 1-aminocyc lopropane-1-carboxylic acid (ACC), the immediate precursor of ethylene , was much higher in retardant-treated hypocotyls than in the controls , which was due in part to the reduced malonylation. The swelling of t he hypocotyl bases could be eliminated by inhibitors of ethylene biosy nthesis or action, or could be induced by 10 mu mol.L(-1)ACC in contro l plants in the light. None of these treatments had a significant effe ct on the lateral expansion of hypocotyls in etiolated seedlings. PP t reatment induced a similar effect to that of white light in etiolated seedlings, and amplified the effect of light in green plants with resp ect to the ACC distribution, and consequently, the ethylene production in the hypocotyls of 6-day-old bean seedlings. It can be concluded th at the lateral expansion of hypocotyl bases in PP-treated green plants is controlled by ethylene.