For a number of generations, we followed the frequency, performance
and fecundity of an initial mixture (50[ratio ]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 0.5 and, after 2 yr, R
plants had disappeared in all replicates in two
treatments and averaged <0.126 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 0.5 in all treatments and replicates during 1995 to average 0.772
on south-facing sites and 0.675 on north-facing
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.