Environmental effects on the fitness of triazine‐resistant and triazine‐susceptible Brassica rapa and Chenopodium album in the absence of herbicide

Abstract

For a number of generations, we followed the frequency, performance and fecundity of an initial mixture (50 : 50) of triazine resistant (R) and susceptible (S) but otherwise near-isogenic lines of Brassica rapa (for 3 yr) and of Chenopodium album (for 2 yr) in neighbouring experimental garden plots, which differed in aspect (north-facing versus south-facing), shading, and transparent cover from precipitation. Each of eight treatments was replicated four times per species. Seed from each plot was kept separate and sown back in its plot of origin to provide the next generation. For both Brassica and Chenopodium, R frequency changed between generations. In Brassica R frequency declined consistently from an initial 0n5 and, after 2 yr, R plants had disappeared in all replicates in two treatments and averaged 0n126 in the remaining treatments. However, R frequencies in the smaller second generation samples in each year were less consistent. In Chenopodium, R frequency increased consistently from an initial 0n5 in all treatments and replicates during 1995 to average 0n772 on south-facing sites and 0n675 on northfacing sites. However, in 1996 Chenopodium R frequency did not change significantly from that in 1995 for any treatment. Brassica S plants were more fecund that R plants, except in shaded conditions, and south-facing plots produced significantly more S seeds than north-facing plots. Both Brassica plants and plots were more productive in 1996 than in 1995. By contrast, Chenopodium was much less productive in 1996, a season with more climatic extremes between plots than 1995. In 1995, R plants were more fecund than S plants in south-facing plots and in covered conditions, but this difference was not detected in 1996. South-facing Chenopodium plots were more productive than north-facing plots in 1995, but the reverse was the case in 1996. Cover resulted in an increase in seed production in Chenopodium on south-facing plots in 1995, but a decline in 1996. We conclude that selection might act against triazine-resistant plants in B. rapa under the climatic conditions in the UK, but in C. album, which is more susceptible to extreme conditions of light and water stress, triazine-resistant phenotypes might predominate in low stress cool moist conditions. We suggest that a relationship might exist between the overall climatic tolerance of a species, and climate thresholds beyond which herbicide-resistant phenotypes might spread. Thus, in the absence of herbicide, it is not inevitable that the frequency of R phenotypes will decline in field conditions.

The Biology of Canadian Weeds. 137. Brassica napus L. and B. rapa L.

“…In natural habitats, triazine-resistant weedy B. rapa germinated slower (Mapplebeck et al 1982), the maximum depth of seedling recruitment was less (Mapplebeck et al 1982), and the frequency of triazine-resistant plants rapidly decreased in a weedy population in natural habitats (Plowman et al 1999). Carbon exchange rates were lower (up to 28%) in triazine-resistant domesticated B. napus and B. rapa genotypes compared with triazine-susceptible genotypes, despite similar chlorophyll a and b contents (Hobbs 1987).…”

“…In weedy B. rapa in Canada and elsewhere, acquired resistance to triazines is clearly associated with a fitness penalty (Mapplebeck et al 1982;Plowman et al 1999). In natural habitats, triazine-resistant weedy B. rapa germinated slower (Mapplebeck et al 1982), the maximum depth of seedling recruitment was less (Mapplebeck et al 1982), and the frequency of triazine-resistant plants rapidly decreased in a weedy population in natural habitats (Plowman et al 1999).…”

Pleiotropic effects of herbicide-resistance genes on crop yield: a review

“…Greenhouse and field studies using nuclear isogenic material from two crop species looked at yields of: (1) first‐generation hybrids between the triazine‐resistant and susceptible lines; (2) the F 2 offspring of this cross that share, on average, the same genome; or (3) advanced back‐cross generations . Yields of resistant plants in pure stands of oilseed rape cultivars in Canada were reduced by 22 to 36% and yields for pure stands of foxtail millet in France were reduced by 22% .…”

A herbicide-resistant ACCase 1781 Setaria mutant shows higher fitness than wild type

“…We used both types of material in our experiments. Finally, estimation of fitness differences over a whole life cycle or even several generations has also been recommended as a definitive measure to quantify gene effects (Plowman et al, 1999, Roux et al, 2006. To do this, we carried out a 3-year field experiment to detect gene frequency changes in progeny populations of the interspecific hybrids in stressful and control field conditions.…”