Susceptibility to imazamox in Italian weedy rice populations and Clearfield® rice varieties

The introduction of imidazolinone-tolerant rice varieties has made selective Oryza sativa (weedy rice) control possible. We hypothesise that Italian weedy rice populations have variable degrees of susceptibility to imazamox prior to imidazolinone-tolerant variety introduction. To this end, 149 Italian weedy rice populations collected from fields never before cultivated with imidazolinone-tolerant varieties were tested in a glasshouse-based, whole-plant response screening study. Imazamox was applied to all populations post-emergence at a rate of 70 g a.i. ha 1, resulting in 70-90% shoot biomass reduction in the majority of cases. The results prompted a second study of the seedling dose-response of four weedy rice populations from the initial study group. Three imidazolinone-tolerant and one conventional rice variety were also included. The seedling roots were cut six days after germination and exposed to different concentrations of imazamox. The root re-growth associated with each concentration-exposure was then measured. Imazamox concentrations to inhibit weedy rice root growth by 50% varied by about two orders of magnitude, or between 0.0018 mM and 0.12 mM. Even with this result, imidazolinone-tolerant varieties were at least 31.8 times less susceptible than weedy rice populations, suggesting that Italian weedy rice populations were not tolerant to imazamox before introduction of these varieties.