Relative fitness of imazamox-resistant common sunflower and prairie sunflower

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.