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Parasitic flowering plants are defined by the production of specialized nutrition-deriving structures, the haustoria, that form a functional link to their hosts. Species of Striga (witchweeds) are obligate hemiparasites, and connection to a host plant is fundamental for them to survive. Seeds of Striga cannot germinate until a ‘chemical’ such as strigol and sorgolactone exuded by the host root indicates the vicinity of a host. Host-recognition factors that can activate development programs in Striga spp. are termed xenognosins (Lynn et al., 1981). Atsatt (1977) proposed that parasitic plants probably use host defence chemicals as cues to stimulate the germination and growth of the haustorium, and which have originally evolved in the host to deter harmful organisms. Akiyama et al. (2005) suggest that plants release chemicals (sesquiterpene lactones) from their roots as signals fostering their symbiosis with arbuscular mycorrhizal fungi, and that these signals are used by Striga to detect host roots. Several strigolactones found in root exudates of various plant species (Yasuda et al., 2003) stimulated germination in seeds of Striga species under laboratory conditions.
Striga is an r-strategist; that is, it allocates lots of energy to produce large numbers of minute seeds to reduce the risk associated with host finding. Producing many minute seeds increases the chance that at least a few seeds will get close enough to the roots of a suitable host plant. Numbers of seeds per plant average 58000 in S. asiatica, and numbers over 200000 almost certainly occur in well-grown S. hermonthica.
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