Germination and Seed/Embryo Size in Holoparasitic Flowering Plants with “Dust Seeds” and an Undifferentiated Embryo

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“Dust seeds” with an undifferentiated embryo and unipolar germination are produced by holoparasitic species in 10 families of angiosperms. However, aside from a few Orobanchaceae species important in agriculture (crop weeds) and in Chinese traditional medicine we know little about seed germination in these achlorophyllous plants. Our primary purpose was to review the literature on seed germination in nine families of the mostly economically-nonimportant taxa of holoparasites and (briefly) of Cynomoriaceae, an economically important medicinal family for which several publications on its germination are in Chinese. A second purpose was to pull together information on sizes of dust seeds and their undifferentiated embryos and endosperm in both economically- and noneconomically-important holoparasitic plants. We found that not much progress has been made in understanding dormancy-break/germination in these holoparasites since 1969, when the author of a book on parasitic flowering plants pointed out how little we know about their germination. Lack of progress on germination of dust seeds of these holoparasites is not necessarily due to lack of research but at least in part to their nonresponsiveness to well-known treatments used to break dormancy in seeds of autotrophic and hemiparasitic flowering plants. For most species, seed length is <1.0 mm (in many <0.50 mm) and embryo length < 0.40 mm (in many <0.10 mm); endosperm is scant to copious. We suggest that seed conditioning, an essential step in germinating seeds of weedy Orobanchaceae holoparasites, be added to the protocol for germinating the dust seeds of noneconomically-important holoparasites with an undifferentiated embryo.