Studies on the Mechanism of Resistance to Simazine in Common Groundsel

Abstract

Studies were conducted to determine the effect on photosynthesis, the absorption, and the metabolism of simazine [2-chloro-4,6-bis(ethylamino)-s-triazine] in two biotypes of common groundsel (Senecio vulgarisL.). Simazine inhibited photosynthesis in the susceptible (S) plants, but resistant (R) plants were unaffected. When the herbicide was removed from the S biotype after 24 hr, photosynthesis resumed. Both biotypes absorbed the herbicide equally well and were unable to metabolize simazine during 96 hr. The greatest concentration of14C activity (80 to 90%) was located in the chloroform-soluble fraction of the foliage of each biotype and was identified as simazine. Small amounts of radioactivity (10 to 15%) occurred in the water-soluble fraction, but there was no differential increase in water-soluble simazine metabolites. A different mechanism of resistance than previously reported for triazine-resistant plants may be operative in one groundsel biotype.