INVESTIGATIONS OF MECHANISMS OF RESISTANCE TO BIPYRIDYL HERBICIDES INARCTOTHECA-CALENDULA (L) LEVYNS

The mechanism of resistance to diquat and paraquat was investigated in a bipyridyl-herbicide-resistant biotype of Arctotheca calendula (L.) Levyns. No differences were observed in the interactions of these herb icides with Photosystem I, the active site, in thylakoids isolated fro m resistant and susceptible biotypes. Likewise, absorption of herbicid e through the cuticle and gross translocation were identical in plants of the two biotypes. Foliar application of either 25 g ha(-1) diquat or 200 g ha(-1) paraquat rapidly inhibited CO2-dependent O-2 evolution of leaf segments of the susceptible biotype. O-2 evolution of leaf se gments of the resistant biotype was less affected by these treatments. Fluorescence imaging was used to observe visually, as fluorescence qu enching, the penetration of herbicide to the active site. These experi ments demonstrated that diquat appears at the active site more slowly in the resistant biotype compared to the susceptible biotype. HCO3-dep endent O-2 evolution of thin leaf slices was less inhibited by diquat in the resistant biotype than in the susceptible biotype, The mechanis m of resistance to the bipyridyl herbicides in this biotype of A. cale ndula is not a result of changes at the active site, decreased herbici de absorption or decreased translocation, but appears to be due to red uced herbicide penetration to the active site.