OROBANCHACEAE (broomrape family)

Plants parasitic on the roots of other plants, sometimes lacking chlorophyll, annual or perennial herbs, sometimes woody and/or tuberous at the base, the roots sometimes brittle, and appearing dense and coralloid, occasionally reduced or absent. Stems various, sometimes thickened or succulent, sometimes white or yellowish brown, sometimes strongly purplish- to blackish-tinged or blackening upon drying. Leaves alternate or opposite and sometimes also basal, sometimes appearing densely spiraled, then reduced to linear to ovate scalelike structures. Stipules absent. Leaf blades various. Inflorescences terminal and/or axillary, spikes, racemes, or of solitary flowers, at least the lower nodes subtended by bracts, inconspicuous bractlets sometimes also present below each flower. Flowers perfect, hypogynous, zygomorphic. Calyces 2–5-lobed, the lobes sometimes minute and toothlike, persistent at fruiting. Corollas often bilabiate, variously colored, 4- or 5-lobed, the tube usually well-developed, sometimes persistent at fruiting. Stamens 4, the filaments attached in the corolla tube, sometimes slightly unequal, the anthers usually not exserted (exserted elsewhere), attached near their midpoints, the anther sacs sometimes appearing spreading or asymmetrical (one of the sacs then reduced and nonfunctional), white or yellow. Staminodes absent. Pistil 1 per flower, of 2 fused carpels. Ovary usually 1-locular, with numerous ovules, the placentation parietal. Style 1, often persistent at fruiting, the stigma 1, variously shaped, entire or 2-lobed. Fruits capsules, dehiscent longitudinally from the tip. Seeds numerous, minute. About 96 genera, about 2,100 species, nearly worldwide.

With the few possible exceptions noted below, members of the Orobanchaceae (as currently delimited) are all root parasites. In some cases, the haustorial attachments to the host root are so fine that they are routinely lost when plants are excavated. For practical reasons, the species often are considered to belong to two broad physiological classes. The first, hemiparasitic, are plants that contain chlorophyll and thus merely supplement the sugars produced through photosynthesis with those that they receive from their hosts. The second, holoparasitc, are plants that contain no chlorophyll and thus have lost their ability to photosynthesize, siphoning all of their nutrition from the host. holoparasitic. Several studies have shown that for selected species of hemiparasites in the family it is possible to grow plants to reproductive maturity in pot culture in the absence of a host, especially when the plants are provided with extra fertilizer (Lackney, 1981; Mann and Musselman, 1981; Crank, 1990). However, in all cases, such plants underperform those grown with host plants and there is no evidence to suggest that such plants are successful in completing their life cycles in the wild.

Traditionally, the Orobanchaceae were treated in a much more restricted sense, comprising only about 17 genera and 230 species of holoparasites (Cronquist, 1981, 1991). However, a number of taxonomists had long questioned the separation of Orobanchaceae from the traditional, broadly circumscribed Scrophulariaceae, especially from the hemiparasitic genera in that family (Boeshore, 1920; Kuijt, 1969; Thieret, 1971; E. Fischer, 2004). These parasitic genera had been included in one or two tribes in the traditional classification of Scrophulariaceae. Characters such as axile vs. parietal placentation, the number of seeds per fruit, and other minor features were thought to vary too greatly to be of use in distinguishing families and some genera were ambiguously placed in either family. Even the transition from hemiparasitism to holoparasitism did not serve to separate the families, as the mainly African genus Harveya Hook. was noted to contain both chlorophyllous and achlorophyllous members. Molecular studies have resulted in the break-up of the former Scrophulariaceae (see the treatment of that family for more discussion) and have agreed on three things: 1) the parasitic lifestyle evolved only once within the overall group; 2) loss of chlorophyll evolved several times within the lineage of parasitic genera; 3) Orobanchaceae (in the broad sense to include all of the parasitic taxa) are more closely related to families such as Phrymaceae, Paulowniaceae, and even Acanthaceae and Bignoniaceae, than to the core group of genera remaining in Scrophulariaceae in the restricted modern sense (dePamphilis et al., 1997; Wolfe and dePamphilis, 1998; Nickrent et al., 1998; N. D. Young et al., 1999; Wolfe et al., 2005; Bennett and Mathews, 2006; Tank et al., 2006; Albach et al., 2009). These studies further suggested that the small paleotropical genus Lindenbergia Lehm. is the closest extant nonparasitic genus to the rest of the Orobanchaceae. Most recently, two other small autotrophic Asian genera have been implicated as related to the lineage that includes the parasitic genera: Rehmannia Libosch. ex Fisch. & C.A. Mey. and Triaenophora (Hook. f.) Soler. (A. R. Jensen et al., 2008; Albach et al., 2009; Xia et al., 2009).

On a global scale there remain many interesting problems of generic circumscription that require further study. The genus Orobanche itself has been shown to consist of five well-separated lineages, each of which might better be classified as a separate genus (J.-m.Park et al., 2008).

Some members of the Orobanchaceae, especially some Old World taxa of Orobanche and some species of Striga Lour. (witchweed) are important parasitic weeds of crop plants and are considered noxious weeds by the U.S. Department of Agriculture. However the North American species of Orobanche have not been implicated as crop pests. The negative effects of native hemiparasitic genera such as Agalinis, Aureolaria, and Dasistoma, upon the establishment and survival of tree seedlings in plantations and other plantings were documented by Musselman and Mann (1978), but their relative importance in Missouri forests and pine plantations is not known.