Antagonism by Clopyralid of Picloram-Induced Ethylene Biosynthesis in Rapeseed Plants

The effect o f clopyralid pretreatment (500 g/ha) on picloram-induced ethylene, ACC (1-aminocyclopropane-l-carboxylic acid), and MACC [l-(malonylamino)-cyclopropane1-carboxylic acid] was measured in rapeseed plants that were treated with 50 or 100 g/ha o f picloram. In contrast to plants that did not receive a clopyralid pretreatment, ethylene biosyn­ thesis was significantly reduced in plants pretreated with clopyralid prior to picloram. Piclo­ ram-induced levels o f ACC also were significantly reduced in plants receiving pretreatment with clopyralid. In contrast, there was no difference between the levels o f MACC in plants that were and were not pretreated with clopyralid. Therefore, the mechanism by which clopyralid pretreatment interferes with picloram-induced synthesis o f both ACC and ethylene may be manifested through the blocking o f de novo synthesis o f ACC synthase normally stimulated by picloram. The lack o f significant difference in MACC levels between plants that were and were not pretreated with clopyralid precludes the stimulation o f enhanced ACC conversion to MACC as an exclusive mechanism o f clopyralid’s antidoting activity. It is likely that the rate o f picloram-induced ACC synthesis by plants receiving pretreatment is within their capacity to convert ACC to MACC, thereby limiting the substrate available for conversion to ethylene. In contrast, it appears that the extent o f ACC synthesis by plants receiving no pretreatment su­ persedes their capacity for conversion to MACC. thereby resulting in greatly enhanced rates o f ethylene evolution and subsequent development o f injury symptoms.