Evaluation of the species boundaries between Parthenium hysterophorus and P. glomeratum (Asteraceae-Ambrosiinae): morphological, anatomical, chromosomal and phytochemical evidence

Species boundaries between Parthenium hysterophorus L./P. glomeratum Rollins were examined. Our working hypothesis states that the delimitation of these taxa is based on the dissection of a continuous variation pattern from an elevational cline. We chose a 70 km gradient from 300-3500 m in the Province of Salta (Argentina) where both taxa are sympatric. The study was centred in phenetic analyses of morphological data. In addition, foliar and caulinar anatomy, chromosome numbers and karyotype, and sesquiterpene lactone composition were investigated. Transplant experiments were performed, individuais that occur naturally at high elevations were cultivated at lower ones to analyse the phenetic plasticity of some key characters. Our results indicate that based on morphological and anatomical data no group of specimens can be delimited. The main characters used to recognise both species are significantly correlated with the elevation at which they grow, suggesting a clinal variation. Cytological and phytochemical data show that there is variation among the populations and that no discontinuities can be detected to separate them. Accordingly, it is convenient to consider the studied population as belonging to P. hysterophorus, and P. glomeratum as a synonym of that species.

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