Roles of Ethylene and Indol-3yl-acetic Acid in Petiole Epinasty inHelianthus annuusand the Modifying Influence of Gibberellic Acid

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Abstract
The effect of 12 h exposure to ethylene upon epinastic curvature and elongation of a 5-cm segment in the attached petiole of Helianthus annuus has been investigated in either normal or GA2-treated plants. Curvature of segments occurred rapidly in the first. 6 h during exposure of normal plants to either 1.0 or 40.0 parts/106 ethylene, and continued slowly from 6 to 12 h. After the ethylene treatment, recovery from the induced curvature was completed in 12 h. In 0.2 parts/106 ethylene, recovery from the epinastic curvature began during the second half of the treatment 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 end of the treatment period, while in 1.0 parts/106 the onset of epinasty was delayed by about 6 h. In 0.2 parts/106 ethylene the epinastic response was slight. Ethylene accelerated elongation in the upper half of the petiole segment. This response was completed within 12 h in all concentrations and in both normal and GA3-treated plants. The mean elongation rate in the lower half was depressed from 4.6 to 1.0 mm 24h−1 in 40.0 parts/106 but immediately afterwards it rose to 14.2 mm 24 h−1. A similar response occurred in 1.0 parts/106. In contrast, the elongation of the lower half of the petiole segment was stimulated by 0.2 parts/106 ethylene. GA2-treated plants showed an initial depression of elongation in the lower half in 10.0 or 40.0 parts/106 ethylene, but in the second part of the treatment period the elongation rate recovered to that of the control segments. Both 0.2 and 1.0 parts/106 ethylene stimulated elongation growth in the lower half of segments in GA2-treated plants. Removal of the leaf lamina inhibited segment elongation, but did not affect the growth response of the upper half to 40.0 parts/106 ethylene. In contrast the lower half of the segment no longer showed the usual growth responses to 40.0 parts/106 ethylene, although these were partially retained when 10μg of IAA was applied to the cut end of the petiole.

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