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

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 GA3-treated plants. Curvature of segments occurred rapidly in the first 6 h during exposure of normal plants to either TO 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 pg 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 TO 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 TO mm 24 h_1 in 40-0 parts/106 but immediately afterwards it rose to 14-2 mm 24 h-1. A similar response occurred in TO parts/106. In contrast, the elongation of the lower half of the petiole segment was stimulated by 0-2 parts/106 ethylene. GA3-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 10 parts/106 ethylene stimulated elongation growth in the lower half of segments in GA3 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 10 pg of IAA was applied to the cut end of the petiole.