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An Evaluation of the Role of Ethylene in Herbicidal Injury Induced by Picloram or Clopyralid in Rapeseed and Sunflower Plants ¹

Department of Plant Science, University of Alberta, Edmonton, Alberta, Canada T6G 2P5

The role of ethylene in herbicidal injury induced by 4-amino-3,5,6-trichloropicolinic acid (picloram) or 3,6-dichloropicolinic acid (clopyralid) was investigated in sunflower (Helianthus annuus L.) and rapeseed (Brassica napus L. cv Altex). Picloram induces herbicide injury in both species, whereas clopyralid induces injury only in sunflower. Picloram applied to the third leaf of a rapeseed plant increased ethylene evolution several-fold. Clopyralid had no effect on ethylene production in rapeseed. In sunflower, both picloram and clopyralid elevated ethylene production. Ethylene biosynthesis induced by the herbicide treatment was not restricted to treated areas. When clopyralid was applied only to the lower stem and cotyledons of sunflower, the herbicide treatment resulted in an increase in the rate of ethylene production from the true leaves. Increased ethylene production preceded or coincided with the onset of morphological responses induced by a herbicide application to a susceptible species. The contrast in ethylene production by these two plant species cannot be accounted for by differences in absorption and translocation of clopyralid and picloram.

Treatment with aminoethoxyvinylglycine (AVG) before picloram or clopyralid application prevented an increase in ethylene production. Pretreatment with AVG also delayed the development of morphological changes induced by picloram or clopyralid. It appears that enhanced ethylene biosynthesis after application of picloram or clopyralid to the susceptible plant species was a factor involved in resulting morphological changes.

¹ Supported by Agricultural Research Council of Alberta through the Farming for the Future Program to J. C. H. and W. H. V. B., and by a Natural Sciences and Engineering Council of Canada Grant to M. S. S.