Broom and threadleaf snakeweed reduce rangeland productivity throughout the western United States. These closely related species are only distinguishable at flowering and are highly variable in morphology and phenology. The objective of this study was to evaluate the genetic variation among snakeweed populations from Arizona, New Mexico, Texas, and Wyoming using isozyme analysis. Plants from each population were transplanted into pots and maintained in a greenhouse for 6 mo prior to analysis. Protein was extracted from young leaf tissue and separated using starch gel electrophoresis. Genetic variation was assessed based on expressed isozyme patterns of 10 enzymes. Of 10 enzymes tested, 16 loci were identified. Total genetic diversity (HT) was large for the eight polymorphic loci in broom snakeweed and three polymorphic loci in threadleaf snakeweed. Genetic variation among populations (DST) of both species contributed the major portion of diversity, but diversity among individuals within each population (HS) was also present. Phylogenetic analysis using Nei's genetic identity and distance determined broom snakeweed populations from New Mexico and Texas were more alike than those from Wyoming and Texas, or those from Wyoming and New Mexico. Compared to genetic differences among New Mexico broom snakeweed populations defined in a previous study, genetic diversity was greater among populations from different states. In addition, genetic variation was smaller in threadleaf snakeweed populations relative to broom snakeweed populations. Results concurred with previous research designating broom snakeweed as the progenitor of threadleaf snakeweed. Local populations of each species have probably developed due to long-term environmental selection.

Broom and threadleaf snakeweed are major rangeland weeds in the western United States, and picloram is the major herbicide used for their management. Previous work has shown that these species are most susceptible to picloram applied in autumn or when precipitation is high and that differences in herbicide absorption and tissue sensitivity as measured by picloram-induced ethylene production do not fully explain variation in seasonal response. Therefore, the role of picloram metabolism in seasonal susceptibility to picloram was examined. Because snakeweed is characterized as highly genetically variable, picloram metabolism was evaluated monthly for 3 yr among populations from two species as well. Picloram metabolism was examined monthly for 3 yr among two populations of threadleaf and nine populations of broom snakeweed grown in a common garden. Metabolism ranged from 30 to 70% of picloram applied, and picloram was converted to two metabolites more polar than picloram regardless of species or population. Although metabolism was greatest in the year with the most precipitation, rate of metabolism was unrelated to precipitation received in the 7-d period before treatment. Application timing as defined by a given month or specific phenological stage was not related to the level of metabolism. We conclude that variation in picloram metabolism is not involved in differential susceptibility across season or population.