Witchweeds

The ability of viable witchweed seed to remain dormant in the soil for many years, only to germinate when favorable conditions prevail, makes eradication difficult (2). Usually, seed will not germinate unless pretreated in a warm, moist environment for several days before exposure to a chemical exuded from the roots of a host plant or some non-host plants (4J. An active chemical in the root... 

The compound (I) was given the trivial name strigol and it has been shown to be a very potent witchweed seed germination stimulant. 

The synthetic studies which have been described have resulted in significant improvements in the preparation of racemic strigol and have also provided access to several analogs which will subsequently be tested for seed germination activity in order to elucidate key structure-activity relationships. These results and further investigations will hopefully lead to effective synthetic compounds for the control of witchweed and related parasitic plants. 

Strigol is a natural product with unusually strong properties as a germination stimulant for parasitic weeds of the genera Striga (witchweed) and Orobanche (broomrape). 

The germination stimulant or stimulants from host plants have not yet been identified, but research on isolation and identification of these allelopathic compounds continues. Other nonhost plants, such as cotton, also release chemicals which stimulate the germination of witchweed seed and these crops can replace the cereal crops in witchweed-infected fields. If no acceptable host is present, the witchweed plant is unable to mature and produce seed. The importance of cereal crops as a staple food in underdeveloped countries makes growth of nonhost crops only partially acceptable, and there are numerous wild hosts that allow the witchweed to germinate, mature, and produce more seed (several thousand seeds can be produced by a single plant). Nevertheless, application of either natural or synthetic stimulants in the absence of a host plant is an effective way of reducing and eventually eliminating the witchweed problem. 

Most of the work on correlation of molecular structure with bioactivity in witchweed seed germination has been produced by two groups (7-10). Johnson, et al. (7-9) prepared and evaluated a large number of strigol analogs and many approached the activity of strigol. In many studies by others, the results of the bioassays are presented, but the compounds from Johnson are described only by GR-number. GR-7 and -24, probably the more promising of these compounds, have been used in extensive field studies, and their structures are known. 

Allelopathy found hopeful utilization in plant protection against parasite weeds. The main world parasitic species are the witchweeds (Striga spp.), broomrapes (Orobanche spp.), and eventually dodder (Cuscuta spp.). Witchweeds and broom-rapes attack many economically important crops especially throughout the semi-arid regions. Early detection of parasitic weed infestation and protection is difficult because of the growth habit of a root parasite and huge production of dust-like seeds viable up to 20 years (Kebreab and Murdoch 1999). 

Another useful reagent for alcohols is Pirkle s (-)-(/ )-l-(l-naphthalenyl)ethyl isocyanate (14)161. For example, it was used to separate the two diastereomeric carbamates obtained from rue-13. The carbamate shown by X-ray analysis to have structure 15 was cleaved162 to furnish (+ )-strigol (13), the witchweed germination factor, thus confirming the absolute configuration of 13163. 

R. Brown and coworkers observed that seeds of Striga, known as witchweed, a semi-parasitic plant of grasses and corn, germinated only in the presence of root exudates from the living host. They isolated the sirupy exudate, [VJ > + 20° (water), resembling a pentose, but did not specifically identify the active substance. A study of many carbohydrates, mostly pentoses, showed that only D-ihreo-pentulose, C 3) — 32° (water), had the required activity L-[Pg.416]

Worldwide, the competitive effect of weeds causes a 10% loss in agricultural production. Yield losses in rice and other grass crops in West Africa have been reported to range from 28-100% if weeds such as witchweed (Striga hermonthica) -a parasitic weed-are not controlled the greatest reductions occur on nutrient-poor soils. Left unchecked, weeds cause dramatic reductions in food production that eventually can... 

Fischer, N.H., Weidenhamer, J.D., Bradow, J.M. Dihydroparthenolide and other sesquiterpene lactones stimulate witchweed germination. Phytochemistry 1989 28 2315-2317. 

Compounds belonging to this structural type are equally recognized by seeds of Striga sp. and Orobanche sp. Comparative studies usually show higher levels of induction in witchweed (Striga hermonthica) than in... 

Dihydroparthenolide and other sesquiterpene lactones stimulate witchweed germination. Phytochemistry 28, 2315-2317... 

Menelaou, M. A. 1990. Stimulation of witchweed germination by sesquiterpene lactones a structure-activity study. Phytochemistry 29, 2479-2483... 

Hsiao, A. I., Warshem, A. D., and Moreland, D. E. 1988. Effect of chemicals often regarded as germination stimulants on seed conditioning and germination of witchweed (Striga asiatica). Ann. Bot. 62, 17-24... 

Egley, G. H. and Dale, J. E. 1970. Ethylene, 2-chloroethyl phosphonic acid, and witchweed germination. Weed Sci. 18, 586-589... 

Eplee, R.E. 1975. Ethylene a witchweed seed germination stimulant. Weed Sci. 23, 433-436... 

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