INHERITANCE OF BIPYRIDYL HERBICIDE RESISTANCE IN ARCTOTHECA-CALENDULAAND HORDEUM-LEPORINUM

The mode of inheritance of resistance to bipyridyl herbicides in bipyr idyl-resistant biotypes of Arctotheca calendula and of Hordeum leporin um was investigated. F1 plants from reciprocal crosses between diquat- resistant and -susceptible plants of A. calendula showed an intermedia te response to diquat application that was nuclearly inherited. Treatm ent of F2 plants with 100 g ai ha-1 of diquat or 800 g ai ha-1 of para quat killed all homozygous-susceptible plants, caused severe injury to heterozygous plants but only slight or no injury to homozygous-resist ant plants. Back crosses of F1 to susceptible plants exhibited interme diate and susceptible phenotypes. The observed segregation ratios in F 2 and test-cross populations fitted predicted segregation ratios, 1:2: 1 (R:I:S) and 1:1 (I:S) respectively, showing that bipyridyl resistanc e is conferred by a single incompletely-dominant gene. Biotypes of par aquat-resistant and -susceptible H. leporinum were crossed reciprocall y. F1 plants from reciprocal crosses showed an intermediate response t o paraquat application. The F2 progeny showed segregation ratios that fitted the predicted segregation ratio of 1:2:1 (R:I:S) for inheritanc e of resistance being governed by a single partially-dominant gene.