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.
Lange Zeit herrschte die Annahme SOBwasser-Rhizopoden w~ren, eigige seltene Formen ausgenomm.en, kosmopolitisch und die Frage nach der geographischen Verbrel. tung daher yon geringer Bedeutung.Da vlele der am beaten bekannten und nicht allzu seltenen Formen ubiquittir slnd und sich also in sehr verschledenen MiUeus (PH, CI-Gehalt, u.s.w., betreffend) Die damals schon bekannten F&lle, das Rhizopodenarten nur in beechr&nkten Gebiete bsobachtet werden (u.a. Nebela ceudata, N. ansata, Sphenoderia mecrolepia) ersch0tterten seine 0be~eugung: ,,Rhizopoden sind Kosmopollten" nicht. Er weist auf die relativ kleine Anzahl Ausnahmen, die bei weiteren Untersuchungen noch st~ndig abnimmt und auf die Tatsache, dab zwischen der Zahl der Arbeiten an eurol~ischem bisweilen auseereurp~i-schem Material noch ein grebes MissverhAItnis bestehe. Aus oekologischer Sicht seien an streng bestlmmtes Milieu gebundenen Formen (z.b. Nebela spec., Assulina spec., Placocysta spec. und Amphitrema flavum) dort in tropischen Gegende zu erwarten, wo die kllmatiechen Faktoren for das Zustandekommen eines Sphagnetum zusamentreffen. Ebenso sollten polare Seen dieselbe Rhizopodenfauna aufweisen kOnnen wle schweizerische Alpenseen.Wenn auch viele der Behauptungen AVERINZEW's (1906) noch immer anerkannt sind geblleben, es hat doch seine Auffassung, dab die oekologischen Faktoren for die Verbreitung der Rhlzopoden allein bestimmend =mien, nicht bew&hrt. Die Sachlage sollte sich welt kompllzlerter erwelsen als aus den damals bekannten Tatsachen zu folgen war.Verschledene Arten, welche schon im vorigen Jahrhundert insbesondere durch CERTES (1889) auf der S0dhemisph~lre gefunden wurden, sind zu Anfang dieses Jahrhun-* Addreer Burg.Weertl stmat 87-1. U14 HN Amhem, The Nethedsncl s. 152
HOOGENRAAD (1935) has given a survey of the rhizopad associations of several Sphagnetae of the Netherlands and a comparison with some populations from diverse localities in Europe, He based his analysis upon the three types distinguished by HARNISCH (1929) 3. Nebela carinata proved not to be restricted to one of HARNISCH's types but can be found in all three types; the absence in many associations is probably connected with its dependence of the humidity of the medium; it can be expected only in constantly wet Sphagnum.~ address: Bur. Weertstraat 87-1, 6814 HN Amhem, The Netherlands.For a comparison of the environmental conditions of our Dutch samples with samples of alpine we studied samples from Norway, Austria and Switzerland, collected from small bogs lying at the slopes of mountains and hills. Nebela tubulosa, Nebula marginata, Amphitrema flavum and Assulina scandinavica could be found in most of them. These species are rare in the Netherlands: Nebela tubulosa and Assulina scandinavica were found twice and Nebela marginata is known from three Iocalitlas.Looking at the difference between the lowland and the alpine Sphagnum deposits one gets the impression that the differences in the populations can be ascribed to the process of purification of the water. In the stagnant lowland pools the water is supplied by rain or the soil, interrupted by periods of dryness and concentration, while the alpine slopes have a constant supply from above and a renewal by flowing down. The Dutch localities, where I found the above mentioned alpine species, presented nearly the same circumstances: water flowing in from diluvial sands and flowing down to lower levels. These delicate differences in the water supply may be responsible for the presence or absence of some sensitive species in otherwise similar associations. A somewhat similar situation is presented by Nebela barbata and Nebela speciosa. Nebela barbata with a widespread geografical distribution (Eurasia, America, Indonesia) is a rather rare component among the characteristic sphagnicol associations and so it was surprising to observe Nebela speciosa (known from Europa and America) in an Amphitrema flavum association from te lake "Botshol".A more subtle analysis of these little differences between greatly similar associations may be obtained by the investigation of the presence of e.g. Desmidiaceae, which are also very sensitive to environmental conditions (pH, 02, chemistry, humidity).Recently, MEISTERFELD (1977) published a survey of his intensive investigations on the diverse environmental conditions, which are likely crucial for the presence and 50
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