Comparative growth and atrazine response of resistant and susceptible populations of Amaranthus from southern Ontario.
Growth characteristics and response to atrazine were studied in atrazine resistant (R) and susceptible (S) populations of Amaranthus retroflexus and A. powellii from several localities in southern Ontario. 6 wk after sowing, plants of A. powellii were taller and had more leaves and heavier stems, while plants of A. retroflexus had greater leaf weights. R plants of A. powellii were taller and had heavier stems than S plants. R and S plants of A. retroflexus did not differ significantly in growth characteristics. Differences in response to atrazine between R and S populations were essentially the same for A. powellii and A. retroflexus. S plants showed extensive leaf chlorosis and a marked decrease in leaf and stem wt. after treatment with atrazine. R plants, especially of A. powellii, showed increased leaf wt. and leaf number, suggesting a stimulatory effect of the herbicide. R populations were generally less variable with respect to growth characteristics than S populations. The within-population variability of R and S populations tended to increase with increasing levels of atrazine. At maturity, in the absence of atrazine, A. retroflexus and A. powellii differed in ht. and biomass production but not in the number of days to flowering or in reproductive effort. R and S plants did not differ significantly in the amount of seed/plant in non-competitive conditions. Plants from more southerly populations in southern Ontario were taller and produced more biomass at maturity than plants from more northerly populations.