Resistance to imidazolinone (IMI) herbicides has been incorporated recently into domesticated sunflower through conventional breeding methods. However, there are concerns regarding gene flow of the IMI-resistance trait to wild species and possible accompanying ecological consequences. Hybrids of domesticated sunflower with both common sunflower and prairie sunflower were created, with and without the imazamox-resistance trait. The relative fitness of imazamox-resistant (IMI-R) hybrids was compared with their imazamox-susceptible (IMI-S) counterparts. Greenhouse experiments were conducted to study the growth of IMI-R and IMI-S common and prairie sunflower hybrids under noncompetitive conditions. The photosynthesis rate of IMI-S prairie sunflower was slightly higher than that of IMI-R plants. However, relative growth rate, net assimilation rate, leaf area, and total dry weight were similar in IMI-R and IMI-S common and prairie sunflower, whereas plant height of IMI-S hybrid was greater than that of IMI-R common sunflower hybrids. A replacement series study was conducted under field conditions in 2001 and 2002 to evaluate the relative competitiveness of IMI-R and IMI-S common and prairie sunflower. IMI-R and IMI-S hybrids of both sunflower species were equally competitive. The results suggest that, in the absence of IMI herbicides, genes controlling IMI-R do not reduce or increase the competitive ability of either common or prairie sunflower. Therefore, if the IMI-resistant trait is incorporated in these species, the frequency of IMI-resistance genes is unlikely to decrease, even in the absence of IMI selection pressure.

Nomenclature: Imazamox; common sunflower, Helianthus annuus L.; prairie sunflower, Helianthus petiolaris Nutt.