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The role of calcium in mediating resistance to several auxinic herbicides (i.e., 2,4-dichlorophenoxyacetic acid, [4-chloro-2-methylphenoxy] acetic acid, (±)-2-(4-chloro-2-methylphenoxy) propanoic acid [mecoprop], 3,6-dichloro-2-methoxy-benzoic acid [dicamba], or 4-amino-3, 5,6-trichloro-2-pyridinecarboxylic acid [picloram]) was investigated by modulating calcium dynamics of a susceptible (S) and resistant (R) biotype of wild mustard. The inhibitory effects of the auxinic herbicides on root length of the S seedlings were significantly reduced upon pretreatment with calcium in the presence of the calcium ionophore A23187. Conversely, the addition of verapamil, a calcium channel blocker, to the R seedlings increased their sensitivity to the auxinic herbicides. Valinomycin, a potassium channel ionophore, did not ameliorate the effect of the auxinic herbicides on both biotypes of wild mustard, thus indicating that the observed effects were specific for calcium. These results demonstrate that calcium plays a crucial role in the resistance of wild mustard to auxinic herbicides at the level of intact seedlings, thereby supporting our previous results using intact protoplasts.

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The broadleaf auxinic herbicide clopyralid was applied to three varieties of corn (Pioneer 36B08, Pioneer 3730, and Pioneer 3559) to determine whether it was phytotoxic to this crop. The effects of clopyralid on the growth and development of corn were compared with those induced by the auxinic herbicides dicamba, 2,4-D, picloram, and fluroxypyr. When compared with the other auxinic herbicides, clopyralid, applied as a foliar spray at the three- and six-leaf stages of development, caused the least damage to all three varieties of corn. Among the auxinic herbicides tested, fluroxypyr and dicamba caused severe damage to the three varieties, whereas picloram and 2,4-D had significant detrimental effects on the growth and development of varieties 36B08 and 3730. Similar results were also obtained when corn seeds were germinated in petri dishes containing increasing concentrations of the auxinic herbicides. In addition to correlating these growth and development effects with auxinic herbicide–induced physiological changes in corn, we compared the effects of clopyralid and dicamba on proton efflux from isolated protoplasts of the three varieties. Results of these biophysical studies are consistent with those from our growth and developmental studies and confirm that clopyralid is least effective in eliciting a response in corn when compared with dicamba. We conclude that clopyralid does not cause the deleterious effects seen with other auxinic herbicides when sprayed under optimal environmental conditions, i.e., high humidity and temperature.