Safa, Sabir Barsoum (1984)
Biology of Striga hermonthica.
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Striga hermonthica (Scrophulariaceae: Rhinanthoideae) is aparasitic weed on sorghum and millet in Africa. It has not been possible, so far, to breed varieties of these crops immune to striga. Variation in many floral and vegetative features and in seed coat ornamentation indicates a diversity of genotypes within populations of hermonthica. The colour and structure of hermonthica flowers and their production of nectar are adaptations for cross-pollination by "long-tongued" insects like butterflies and moths; the floral biology of the species promotes outbreeding. Evidence for a gametophytic system of self-incompatibility was obtained for plants from Sudan using fluorescence microscopy to study the behaviour of pollen on stigmas. Plants grown from seed samples of hermonthica populations from Ethiopia, Sudan, Nigeria,Niger, Upper Volta, Mali, Ghana and Ghambia were tested and were also found to be self-incompatible. Crosses made between plants from different countries and from different host species were found to be compatible and set seed. Because there are no physiological barriers to gene exchange, variability is maintained throughout the range of the species by obligate outbreeding. There is much variation in the response of a host variety when tested, not only against different samples of hermonthica seed from different localities, but between repetitions using the same seed sample. This variability occurs in controlled conditions and in sick plots, as well as in the field. It indicates that bulk samples of striga seed are also variable for genes determining the host-parasite relationship. This brings into question the effectiveness of the standard techniques used for screening crops for resistance to the parasite. Evidence for the breakdown of resistance to striga was obtained from field observations in Sudan.
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Institution: University of London, Royal Holloway College (United Kingdom).