No auxinic herbicide–resistance cost in wild radish (Raphanus raphanistrum)

There is great difficulty in understanding the initial effects of auxinic herbicides on plants. Therefore, research on understanding the mechanistic basis of auxinic herbicide resistance in plants is very difficult. This is especially so in a genetically diverse cross-pollinated species like wild radish. We should expect diversity in resistance mechanisms and AHRI researcher Dr Danica Goggin in researching the mechanistic basis of auxinic herbicide resistance has revealed this complexity in wild radish (Goggin et al, Annals of Botany 2018, 122, 627-640), with her paper “2,4-D and dicamba resistance mechanisms in wild radish: subtle, complex and population-specific”.

In this study, Danica Goggin used 11 very well characterised resistant wild radish populations and two susceptible populations grown in the presence and absence of competition from wheat so as to examine for any auxinic herbicide resistance fitness cost. She found no evidence of a fitness cost. The two susceptible and 11 resistant populations were equally suppressed by wheat competition, with growth and seed production equally reduced by competition. Thus, across these 11 resistant populations that had all been earlier studied at the mechanistic level (Goggin et al 2018), there was no evidence of a fitness cost associated with auxinic herbicide resistance.

This research was supported by an Australian Research Council grant, with Nufarm as a linkage partner.

Abstract

Wild radish (Raphanus raphanistrum L.) is a problematic and economically damaging dicotyledonous weed infesting crops in many regions of the world. Resistance to the auxinic herbicides 2,4-D and dicamba is widespread in Western Australian R. raphanistrum populations, with the resistance mechanism appearing to involve alterations in the physiological response to synthetic auxins and in plant defense. This study aimed to determine whether these alterations cause inhibition in plant growth or reproduction that could potentially be exploited to manage 2,4-D–resistant populations in cropping areas. Therefore, the morphology and seed production of resistant and susceptible populations were compared in an outdoor pot study, with plants grown in the presence and absence of competition by wheat (Triticum aestivum L.). The susceptible and resistant R. raphanistrum populations were equally suppressed by wheat competition, with plant growth and seed production being decreased by approximately 50%. Although resistant populations produced less vegetative biomass than susceptible populations, there was no negative association between resistance and seed production. Therefore, it is unlikely that any nonherbicidal management practices will be more efficacious on 2,4-D–resistant than 2,4-D–susceptible R. raphanistrum populations.