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Abstract.A germination stimulant, alectrol, for the seeds of the angiospermous root parasites Alectra vogelii Benth. and Striga gesnerioides (Willd.) Vatke has been isolated from root exudates of its genuine host plant Vigna unguiculata Walp cv Saunders Upright. Its spectroscopic data lead to a chemical structure closely related to (+)-strigol. The compound is more active than strigol in stimulating seeds of the respective parasites, Alectra vogelii and Striga gesnerioides.Root parasite flowering plants of the genera Alectra, Striga (Scrophulariaceae), and Orobanche (Orobanchaceae) have developed an interesting chemical ecology. Apparently, they have found an unchallenged ecophysiological and biochemical niche. The parasitic seeds are able to recognize their correct host plants, because they germinate only in their presence. Basic to this phenomenon is the chemical communication between parasite and host: seed germination is initiated by chemical substances occurring in the root exudates of their respective host plants. There seems to be a great variety of these so-called germination stimulants because different host plants produce different compositions of different stimulants, as demonstrated by chromatographic investigations (Sunderland 1960, Visser et al. 1974, 1987. The first stimulating compound, strigol (Fig. 1), was isolated from cotton, Gossypium hirsutum, a false host,

A large-scale continuous system for recovery of root exudates from millet and cowpea was developed and tested extensively. Large quantities of germination stimulants were exuded into the medium flowing through the plant trays, which could be demonstrated by germination of seeds of three species of angiospermous parasitic plants,Alectra vogelii, Striga gesneriodes, and witchweed. Recovery of root exudates was affected by adsorption on the macroreticular polymer resin XAD-4 followed by desorption with methanol. Furthermore, indications were found that germination inhibitors are released into the medium. Subsequent testing confirmed the quantitative release of the germination stimulants as well as a large number of other compounds.