Cultivar variation for imazamox resistance in wheat (Triticum aestivum L.): Insights into enzymatic assays for early selection.

Acetohydroxyacid synthase (AHAS, E.C. 2.2.1.6) is the target site of several herbicide classes including imidazolinones. Imidazolinone resistance in wheat is conferred by two major genes AhasL-D1 and AhasL-B1. The objective of this work was to evaluate the in vitro and in vivo AHAS activity and plant growth in response to imazamox of nine wheat cultivars. Dose-response curves for two-gene resistant cultivars were significantly different from the single-gene resistant and susceptible cultivars in the in vitro AHAS assay. Resistance levels at the in vivo AHAS and whole-plant assays for resistant cultivars were >10-fold higher than susceptible cultivars. Moreover, in vivo dose-response curves showed differences among cultivars with the same number of resistance genes. It was concluded that in the in vitro AHAS assay cultivar variability was due to differences in target-site sensitivity while the in vivo AHAS assay reflected the resistance at whole-plant level. Both in vitro and in vivo AHAS dose-response curves could be useful tools when exploring mechanisms involved in imidazolinone resistance in different wheat genetic backgrounds and for the selection of higher resistant genotypes.