II. GENUS OROBANCHE
Theophrastus Eresius (371–286 BC) mentions parasitic seed plants such as CuscutaandOrbanchein his
book on the history of plants (cited in Ref. 18). The name Orobanchederives, according to Dioscorides
(first century AD), from the Greek Orobos(pea) and angchein(to strangle).
The majority of Orobanchaceae occur in the Northern Hemisphere. There, the largest species diver-
sity is found in the warm and temperate zones.
The genus Orobanchecontains more than 100 species of total root parasites, all of which are achloro-
phyllous [25–27]. Most of the parasitic species and especially O. ramosaandO. aegyptiaca, included in
the section Trionychon Wallr., and O. cernua,O. cumana, and O. crenata, included in the section Os-
proleon Wallr., have a broad host range including plant families such as Solanaceae, Brassicaceae, and
Fabaceae. They are widespread in the Mediterranean countries, southeastern Europe, Middle East, and
North Africa [28]. They cause tremendous agricultural problems there, especially with tomato, tobacco,
and sunflower, on which they can grow rapidly, resulting in a serious reduction of crop yield
[19,26,29–36].
These plants have a complicated life cycle. Their seeds are dormant after ripening and in order to ger-
minate, as well as those of Striga[37–41] after contact with a germination stimulant, they require a
postripening phase under warm and dry storage conditions [42–45]. Also necessary is a conditioning
phase in a warm and humid surrounding [46–48]. It has been shown that not only the germination but also
the penetration and the development of the haustorium are subject to chemical signaling from the host
[49–52].
III. RAPD MOLECULAR MARKERS FOR SOLVING TAXONOMY
PROBLEMSA. O. ramosa L. O. aegyptiacaPers.—Two Species or a Species
Complex?O. ramosaL. and O. aegyptiacaPers. are two species of special interest because of the controversial data
on their taxonomy. It is a problem for several reasons [26]:
- There is an inherent morphological variability within populations of the plant, which is reflected
in several aspects of their biology: weediness, chromosome aberrations, and reproductive strate-
gies. - Their holoparasitism results in a reduced number of characters of taxonomy: they are nonpho-
tosynthetic, they have no leaves, and they produce only short abnormal roots. - The host plant may influence the morphology of the parasite.
These plants are especially difficult to distinguish in the fields because too few characters are used
in keys [26].
In spite of the economic importance and scientific interest of the Orobancheproblem, relatively lit-
tle is known about the genetic variabilityof the species within the genus. It is possible that genetic dif-
ferences may contribute to variation in host preferences, and knowledge of the genetic diversity of para-
site weed populations within and across species boundaries is important in evolving control strategies.
Genetic variation has been reported with regard to susceptibility of the host and the virulence of the par-
asite [53]. Some cytological studies reveal that the genus seems to contain several polyploid complexes,
which can produce new interspecific hybrids with unpredictable consequences concerning the range of
hosts [54]. Some information has also been gained from isoenzyme analysis [55]. However, isoenzyme
markers may be affected by environmental conditions and are expressed differentially at different stages
of development.
DNA polymorphism is a different physical property at the same locus [56–58]. If such a polymor-
phic DNA sequence is detected, one can distinguish the two genomic DNAs. DNA-based markers are not
dependent on environmental and developmental factors and have been applied successfully to discrimi-
nate between individual genotypes [59–74]. Random amplified polymorphic DNA (RAPD) is a technique
used to detect polymorphisms in a DNA sequence [75–78]. The method relies on amplification of poly-
790 MINKOV AND LJUBENOVA