Effects of Environmental, Site and Phenological Factors on Hymenoxon Content of Bitterweed (hymenoxys Odorata)1,:

Bitterweed plants were collected biweekly from four ranches located on the Edwards Plateau of Texa~ The area covered was approximately 220 km from north to south and 80 km wide. The study started in December when bitterweed plants were in the seedling stage and continued to the following June, when seed heads were present and most plants were desiccated. Environmental conditions and phenological stage of bitterweed populations were recorded and hymenoxon was determined. Hymenoxon, a sesquiterpene lactone with an exocyclic ~x-methylene--y-lactone moiety, is the principal toxic constituent in bitterweed. Hymenoxon concentrations were highest in seedlings and decreased as the plants matured. Levels ranged from 1.1 to 4.5% (dry matter basis) in bitterweed collected in December and from 0 to .8% in June collections. When sampled at approximately the same phenological stage, there were significant differences in hymenoxon content of bitterweed growing at different locations, which appeared to be unrelated to environmental conditions. Whether these differences are due to genetic differences in plant populations or to phenotypic differences is unknown. (