A comparative study of invasive Helianthus annuus populations in their natural habitats of Argentina and Spain

Abstract

Wild Helianthus annuus is native to North America but it is found in other parts of the world as well. The origin of exotic populations is uncertain, and probably they have evolved very differently in different countries. The goal of this work was to examine and compare invasive populations from Argentina and Spain, in an attempt to unravel their origin. Morphological and agro-ecological data were collected in their natural habitats during three exploration trips in 2007 and 2008. Study materials from both countries were represented by nine populations from central Argentina and seven from Spain, including six from Andalusia and one from Gerona. In Argentina, the wild H. annuus was found mainly on disturbed areas between roads and fences. In a few cases the populations invaded crop lands, however, they were located in the margins of the cultivated fields. Argentine populations reached more than 50,000 m 2 and a density of about 25 plants m-2 . In Spain, the populations were found in croplands. Only one small population was found in a non-tilled area, near an olive plantation. The largest population was about 1,500 m2 and comprised no more than 200 plants. Plants were screened for 24 morphological traits. Argentinean populations showed taller plants with higher number of heads of small size, while Spanish populations characterized by tougher stems, bigger heads with wider ligules and bracts. Plants were smaller and leaf size was larger in Gerona than in Andalusia. Discriminate analysis differentiated populations from Argentina and Spain by leaf shape, branching, plant height, head size and color. Moreover, there was a good differentiation among Argentinean populations, while populations from Andalusia were similar. Gene flow from wild and domestic sunflower to weedy populations is likely the source of genetic variation among them. In Argentina, wild H. annuus was introduced with agronomic purposes, and probably it escaped from cultivation and spread. Despite of intense gene flow within sunflower crop, populations seem to keep much more wild appearance than Spanish populations. According to previous studies, the origin of European populations probably was pollen contamination of commercial seed with wild seeds or crop-wild hybrids. Our morphological and agro-ecological data seem to confirm such hypothesis.