Physiological and genetic aspects of herbicide resistance in giant buttercup (Ranunculus acris L. subsp. acris)

Abstract

Giant buttercup, (Ranunculus acris L. subsp. acris), has become a major weed of dairy pastures traditionally controlled by application of the chloro-phenoxy herbicide MCPA. A population of giant buttercup was confirmed as being resistant to the field rate, (1.125kg a.i.. ha⁻¹), of MCPA. Reliable controls of resistant plants are only being achieved at four times this rate of MCPA.

Leaf morphology did not differ between susceptible and resistant plants and did not appear to be a factor in conferring resistance. Mean petiole length was shorter in resistant plants, with a greater variation and length found in susceptible plants. It is not clear how this may confer resistance.

Uptake of MCPA was not different between susceptible and resistant plants, averaging 18.1% and 20.8% respectively. Partitioning of MCPA was different between susceptible and resistant plants, with resistant biotypes translocating less ¹⁴MCPA to stolons; 15.2% and 5.0% respectively. The difference tended to diminish with time. The level of decarboxylation of detached leaves of MCPA was also significantly greater in resistant plants. These two factors appeared to be the major causes of resistance to MCPA in giant buttercup.

No translocations, deletions or additions to chromosomes of resistant plants were detected, nor were changes in ploidy levels apparent. No gross chromosome structural changes could be linked to the occurrence of resistance to MCPA.

The resistance of giant buttercup to MCPA seems to be in the ability of resistant plants to regrow from auxiliary buds found on the stolon. Reduction of active herbicide at these sites is the probable reason for their survival.