The effects of conditioning temperature and length of the conditioning period on germination of seeds of the parasitic weed Orobanche crenata were investigated in vitro. Seed samples from Syria and Egypt, collected in various years, were compared. Optimum temperature for conditioning was 15–20 °C for Syrian seeds and 20 or 15–20 °C for Egyptian seeds. At 20 °C a conditioning period of 18 days was required for maximum germination. When conditioning at 20 °C was prolonged beyond 7 weeks, germination decreased. This effect was more pronounced for Egyptian seeds than for Syrian seeds. Germination of Syrian seeds was strongly reduced after conditioning for more than 4 weeks at 15 or 10 °C. A stepwise decrease in temperature during conditioning did not enhance this effect. The results of a tetrazolium test indicated that the seeds most probably did not die but developed secondary dormancy. Key words: Orobanche, germination, conditioning, temperature, dormancy.

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The interbreeding potentials of nine populations of spider mites were examined. The populations were gathered from horticultural crops in different glasshouses in a limited area of Aalsmeer. A great variety of genetic incompatibilities between these populations was found. In most crossings a partial sterility of the hybrids, as measured by egg hatchability, occurred. Some crosses showed reciprocal differences. It was suggested that genetic divergence of these adjacent populations arises especially from the haplo‐diploid nature of spider mites. Zusammenfassung GENETISCHE AFFINITÄT ZWISCHEN ANEINANDERGRENZENDEN SPINNMILBEN‐POPULATIONEN Die Kreuzbarkeit von neun Spinnmilben‐Populatiorien wurde untersucht. Die Populationen wurden von Gartenpflanzen verschiedener Gewächshäuser in einem bestimmten Gebiet von Aalsmeer gesammelt. Es zeigte sich eine große Variabilität in der genetischen Verträglichkeit zwischen diesen Populationen. Bei den meisten Kreuzungen trat, gemessen an der Schlüpffähigkeit der Eier, eine partielle Sterilität der Hybriden auf. Einige Kreuzungen wiesen reziproke Unterschiede auf. Es wird vermutet, daß die genetischc Divergenz dieser aneinandergrenzenden Populationen speziell auf der haplo‐diploiden Natur der Spinnmilben beruht.

No abstract

The aquatic weed hydrilla (Hydrilla verticillata Royle) was discovered growing in the Potomac River, south of Alexandria, VA, in Kenilworth Aquatic Gardens, Washington, D.C., and in the Chesapeake and Ohio (C&O) Canal near Seneca, MD. Cultures in Florida of the Kenilworth clone produced male flowers. This is the first report of the occurrence of the male in the U.S. Two distinct isoenzyme patterns have been identified for plants from various locations in the U.S., corresponding to a monoecious strain and a dioecious female. The occurrence of the wild colonies of the monoecious Hydrilla greatly increase the potential for physiological diversity through sexual reproduction, which may have serious consequences for the management of this weed.

No abstract

Germination and longevity of purple witchweed seeds stored in nylon gauze bags in the soil in situ were tested in northern Bénin over a 2-yr period, covering the rainy seasons in 1994 and 1995. The seeds were collected at Ina Station in November 1993 from corn and sorghum fields. It appeared that germination percentages of the seeds, which were stimulated by GR24 to germinate, as well as their viability according to a tetrazolium test, decreased steadily in wet soil. During the 1994 rainy season, germination percentage of seeds, which reached maximum values of 30 to 74%, decreased to values of 11 to 17%. During the 1995 rainy season, the number of germinating seeds decreased further, and at the end of this season the germination percentage approached zero. Seed viability also decreased in line with the decrease in germination. In addition to the study on longevity under field conditions, seeds also were exposed to various water regimens in pots. In the pot experiment, purple witchweed seed viability and germination declined in moist soil treatments. The dying-off process observed contradicts the common opinion on longevity ofStrigaseeds in their natural environment. “Wet dormancy” was not observed in the course of the rainy season.

The genus Striga contains some of the most damaging parasitic plant species of the world, and it causes large problems in agricultural systems of Africa and Asia. Effective, cheap, and easy applicable methods for Striga control do not yet exist. One method with great potential may be the use of resistant crops. However, resistant crops are not widely available, hence, these genetic traits will have to be obtained from another source. Here we report the results of a screening in which it is shown that several native African savannah grass species possess a high level of resistance to Striga hermonthica (Del.) Benth., Striga aspera (L.) Kuntze, and interspecific hybrids of these two Striga species. This trait could be of potential value for the development of resistant crops. Resistance to Striga was not expressed at the levels of germination and attachment. Root exudates of all 14 grass species tested induced Striga seeds to germinate. Furthermore, attachment to the host roots was always observed. Subsequent development of the Striga seedlings, however, was absent or heavily impaired in resistant grasses. Preliminary observations suggested that the parasite could not breach the endodermal barrier of the roots. The host range of interspecific hybrids of S. aspera and S. hermonthica appeared to be a combination of the host ranges of their parents. They could not attack grass species that were resistant to both parents.Key words: Striga aspera, Striga hermonthica, interspecific hybrids, germination, attachment, tropical grasses, resistance.