Productivity and Intraspecific Competitive Ability of a Velvetleaf (Abutilon theophrasti) Biotype Resistant to Atrazine

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Abstract
Studies were conducted to determine the relative competitive ability and productivity of an atrazine-resistant biotype (WRB1) and an atrazine-susceptible accession (WSA1) of velvetleaf from Wisconsin. Noncompetitive productivity studies conducted in the field and greenhouse during 1992 and 1993 showed that instantaneous relative growth rate, net assimilation rate, and leaf area ratio of the WRB1 biotype and WSA1 accession were similar. There were no consistent differences between the WRB1 biotype and WSA1 accession in plant height, shoot dry biomass, and leaf area over time. The WRB1 biotype and WSA1 accession did not differ in noncompetitive seed yield. A replacement series experiment conducted in the field in 1992 and 1993 showed that the WRB1 biotype and WSA1 accession did not differ in shoot dry biomass or seed yield at densities of 36, 64, and 100 plants m−2. Results suggest that resistance to atrazine has not reduced the noncompetitive productivity or the intraspecific competitive ability of the WRB1 biotype relative to the WSA1 accession. Thus, even in the absence of atrazine selection intensity, the frequency of atrazine resistance gene(s) is unlikely to decrease in the velvetleaf population from which the WRB1 biotype was selected.