Witchweed seeds

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 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. 

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

Formation of the 3j8-acetoxyeti-5-enic esters has been used to obtain optically pure samples of (+)- and (—)-tran5-verbenoP and to resolve an alcohol intermediate in the synthesis of the witchweed seed germination stimulant (+)-strigol. A general synthesis of thiol esters from carboxylic acids, exemplified by the formation of the n-propylthio-, isopropylthio-, and t-butylthio-esters of cholic acid, comprises reaction with diethyl chlorophosphate-triethylamine, followed by the thallium(i) salt of the appropriate thiol. 

Witchweed seeds must be kept in a moist environment at 20 to 30°C for 1-3 weeks before the stimulants can initiate germination (4,15,39,40). This process is defined... 

Seeds. Witchweed seeds were collected from plants in experimental field of the USDA Methods Development Laboratory, Whiteville, N.C., USA and stored for three years at room temperature. Viability of these seeds was more than 95% according to a tetrazolium chloride viability test. 

Table I. Germination of Witchweed Seeds Conditioned in Water at 30°C for 7 and 14 Days Followed by 24-h Terminal Treatment with Brassino-lide, d/-Strigol, and Natural Stimulants at 30°C in the Dark ...

Figure 1. Germination of Witchweed Seeds Conditioned in Water and Brassinolide (10 7M) for 3-7 Days before Terminal Treatment with Natural Stimulants. Bars Represent Standard Errors of Means for 3 Replications over 3 Experiments.

Table V. Germination of Witchweed Seeds Conditioned in Various Plant Growth Regulators at 30°C for 7 Days with and without Terminal Treatments of Natural Stimulants (4-fold Diluted) from Sorghum at 30°C Conditioning media Germination (%)...

Table VII. Percent Germination of Witchweed Seeds Conditioned in the Combination of rf/-Strigol (ST) with Brassinolide (BR) at 30°C for 7-15 Days Followed by 24-h Terminal Treatments with d/-Strigol (10 7M) at 30° C...

The mechanism of action of brassinolide is still unknown therefore studies should be conducted further to understand the physiological effects of brassinolide on conditioning and germination of witchweed seeds. 

The authors thank Dr. R. E. Eplee, USDA, APHIS, Whiteville Methods Development Center, Whiteville, NC for the witchweed seeds used in this research and other helpful suggestions. 

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