Abstract: | The effect of 12 h exposure to ethylene upon epinastic curvatureand elongation of a 5-cm segment in the attached petiole ofHelianthus annuus has been investigated in either normal orGA2-treated plants. Curvature of segments occurred rapidly inthe first. 6 h during exposure of normal plants to either 1.0or 40.0 parts/106 ethylene, and continued slowly from 6 to 12h. After the ethylene treatment, recovery from the induced curvaturewas completed in 12 h. In 0.2 parts/106 ethylene, recovery fromthe epinastic curvature began during the second half of thetreatment period. Pre-treatment of plants with 60 µg GA3,did not change the epinastic response to 40.0 parts/106 ethylene.In 10.0 parts/106 ethylene, recovery commenced towards the endof the treatment period, while in 1.0 parts/106 the onset ofepinasty was delayed by about 6 h. In 0.2 parts/106 ethylenethe epinastic response was slight. Ethylene accelerated elongation in the upper half of the petiolesegment. This response was completed within 12 h in all concentrationsand in both normal and GA3-treated plants. The mean elongationrate in the lower half was depressed from 4.6 to 1.0 mm 24h1in 40.0 parts/106 but immediately afterwards it rose to 14.2mm 24 h1. A similar response occurred in 1.0 parts/106.In contrast, the elongation of the lower half of the petiolesegment was stimulated by 0.2 parts/106 ethylene. GA2-treatedplants showed an initial depression of elongation in the lowerhalf in 10.0 or 40.0 parts/106 ethylene, but in the second partof the treatment period the elongation rate recovered to thatof the control segments. Both 0.2 and 1.0 parts/106 ethylenestimulated elongation growth in the lower half of segments inGA2-treated plants. Removal of the leaf lamina inhibited segment elongation, butdid not affect the growth response of the upper half to 40.0parts/106 ethylene. In contrast the lower half of the segmentno longer showed the usual growth responses to 40.0 parts/106ethylene, although these were partially retained when 10µgof IAA was applied to the cut end of the petiole. |