Investigations of how phenoxy‐resistant Carduus nutans biotypes survive herbicide spraying

Abstract

Biotypes of nodding thistle (Carduus nutans) have developed within New Zealand that require over six times the normal rate of MCPA, MCPB or 2,4‐D to kill them. Radiolabelled 2,4‐D was applied to both resistant and susceptible nodding thistle plants to investigate the mechanism of resistance. There was no difference in penetration of the 2,4‐D into the foliage of the resistant and susceptible plants. However some minor differences in translocation were detected, with the 2,4‐D apparently less mobile within the resistant plants. These differences were variable and did not fully explain the resistance. Significantly more radioactivity was released from the roots of susceptible plants. There were also small but significant differences in the quantities of radioactivity that could be removed from within the treated plants by ethanol, suggesting more binding within susceptible plants. Significant differences were detected in rates of metabolism of 2,4‐D within the root systems. More radioactivity was present as water‐soluble compounds in the resistant plants than the susceptible ones. Of the ether‐soluble components extracted, about 14 times more 2,4‐D was estimated to be present in susceptible plants than in resistant plants, with chromatography suggesting that compounds in the resistant plants were metabolites of 2,4‐D.