Recent CO2 levels promote increased production of the toxin parthenin in an invasive Parthenium hysterophorus biotype

Recent carbon dioxide (CO₂) concentrations promoted higher parthenin concentrations in an invasive Parthenium hysterophorus biotype. Mean concentrations of parthenin, an allelopathic and defensive sesquiterpene lactone, were 49% higher at recent (~400 ppm) than at mid-twentieth-century (~300 ppm) CO₂ concentrations, but did not vary in a non-invasive biotype, suggesting that recent increases in atmospheric CO₂ may have already altered the chemistry of this destructive weed, potentially contributing to its invasive success.