Germination Responses to Temperature of Atrazine-Resistant and -Susceptible Biotypes of Two Pigweed (Amaranthus) Species

Abstract

Percent germination and germination rates of seeds of atrazine (6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine)-resistant and -susceptible biotypes of redroot pigweed (Amaranthus retroflexusL. # AMARE) and Powell amaranth (A. powelliiS. Wats. # AMAPO) from southern Ontario were compared in petri dishes using a 100-cell germination plate. The plate provided 100 separately controlled, alternating temperature combinations ranging from 0 to 45 C in increments of 5 C. The experiment was conducted under alternating light and dark conditions for 21 days and under constant dark conditions for 7 days. Initial seed viabilities exceeded 90% for all four biotypes. Under alternating light and darkness, the resistant biotype of Powell amaranth had significantly lower total germination and a slower rate of germination than the susceptible biotype, whereas the biotypes of redroot pigweed did not differ in germination response. Germination rate and total germination were positively correlated for all four biotypes. In constant darkness, the atrazineresistant biotypes of both species had significantly lower germination percentages compared to the susceptible biotypes. In general, percent germination of Powell amaranth was greater than that of redroot pigweed for day and night temperatures <25 C, whereas redroot pigweed had greater germination at higher temperatures. Greater seed dormancy of the resistant biotypes under some light and temperature combinations may have implications for the persistence of resistant populations in the field.