Comparison of the effects of white light and the growth retardant paclobutrazol on the ethylene production in bean hypocotyls

At a concentration of 17 µmol·L^–1, paclobutrazol (PP), a triazole plant growth retardant, effectively reduced the elongation and increased the thickness of hypocotyls in 6-day-old Phaseolus vulgaris L. cv. Juliska seedlings, both in the light and in the dark. PP treatment did not increase the cell number in transverse sections of hypocotyls. The diameter of hypocotyls was uniform from the zone of intensive elongation along the whole hypocotyl in etiolated plants, but those grown in the light exhibited an additional lateral expansion at the base. Ethylene evolution was not reduced by PP in etiolated hypocotyls, and did not differ significantly in the elongating apical and fully grown basal zones. PP reduced the ethylene release by the growing zones in green hypocotyls, but not in the basal parts, which resulted in an increasing ethylene gradient towards the hypocotyl base. The level of 1-aminocyclopropane-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 the hypocotyl bases could be eliminated by inhibitors of ethylene biosynthesis or action, or could be induced by 10 µmol·L^–1ACC in control plants in the light. None of these treatments had a significant effect on the lateral expansion of hypocotyls in etiolated seedlings. PP treatment induced a similar effect to that of white light in etiolated seedlings, and amplified the effect of light in green plants with respect to the ACC distribution, and consequently, the ethylene production in the hypocotyls of 6-day-old bean seedlings. It can be concluded that the lateral expansion of hypocotyl bases in PP-treated green plants is controlled by ethylene.