Mechanisms limiting the geographical range of the parasitic weed Orobanche crenata

Factors and mechanisms limiting the geographical range of the root parasitic plant Orobanche crenata (crenate broomrape) were investigated and the risk of a spread beyond its current distribution area was assessed. Information on distribution and ecophysiology of O. crenata was gathered from literature and laboratory studies. A simulation model of seedbank dynamics served to test hypotheses and identify risk areas. The geographical range of O. crenata is strongly affected by mechanisms acting on the seedbank. Relevant processes include seed conditioning, germination, loss of viability, and dormancy induction and release. These are driven by the dynamics of soil temperature and moisture, and the presence or absence of hosts. Populations of O. crenata can only establish where a distinct precipitation seasonality prevails, with a warm and dry period being followed by warm and wet conditions, and where the presence of conditioned seeds coincides with that of a host. Climatically suitable regions include all Mediterranean climate areas and part of the monsoon, savanna and winter-dry climate regions of Central America, Africa, Australia and South Asia. In humid areas, both temperate and tropical, preponderance of dormancy-inducing wet conditions and lack of a warm-dry period facilitating dormancy release prevent the completion of the O. crenata life cycle and thus establishment of the species.