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5VOL. 3 N. 3 - DECEMBER 2007
EUPHORBIA WORLD
Parasitic flowering plants on Euphorbia in
South Africa and Namibia
Maik Veste
University of Hohenheim
Institute of Botany and Botanical Garden
Garbenstrasse 30, 70599 Stuttgart, Germany
E-mail: maik.veste@t-online.de
Pjotr Lawant
B
y definition parasitic plants get nutrients,
carbohydrates, ions and water from their
host plants (Weber 1993) and they develop
special morphological features to survive on the living
tissue of other plants. e haustorium connects the
parasite with its host and allows the transportation
of water, inorganic and organic compounds into the
parasite. Holoparasites are unable to produce chlo-
rophyll for photosynthesis and are totally dependent
on their hosts. A second group are the semi-parasites;
their leaves produce chlorophyll and they are able to
photosynthesise.
From South Africa and Namibia 67 parasitic flower-
ing plants are known, 23 are stem parasites and 44 are
root parasites (Visser 1981). Mainly woody species are
potential hosts for the parasites, but also succulents like
Aloe dichotoma, Cotyledon and Lampranthus are listed as
hosts (Visser 1981, Veste 2005). e highest diversity
of parasites growing on succulents can be found on
Euphorbia species (table 1). Four root parasites and
four mistletoes belonging to four families can be found
on stem succulent Euphorbia species in South Africa
and Namibia.
Root parasites
e most remarkable root parasites of Africa with a
most “unplantlike” appearance belong to the Hydno-
raceae. In Southern Africa only two species from this
family occur: Hydnora africana and Hydnora triceps and
both species are restricted to parasites on Euphorbia spe-
Fig. 1: Hydnora africana exposing the entrance of the flower above
ground (Photo: Siegmar-W. Breckle).
Fig. 2: Hydnora triceps flower in Namaqualand. Most of the flower
is subterranean and only excavated for the picture.
6
EUPHORBIA WORLD
cies. Hydnora africana (Fig. 1) has a wide distribution
area from the Cape Peninsula northwards up the Nama
Karoo in Namibia and eastwards to Eastern Cape.
e rarest root parasite is Hydnora triceps (Fig. 2). e
botanist Johann F. Drège discovered this species near
Okiep in Namaqualand in the 1830. e distribution
is restricted to a small area of the Namaqualand and
southern Namibia. Only a few specimens were collected
and deposited in herbaria. For nearly 100 years this
Hydnora species sank into oblivion until 1988, when
Johann Visser, professor of botany at the University of
Stellenbosch, rediscovered H. triceps in the sandveld
of Namaqualand (Visser 1989). Until now only a few
records were made of this rare plant. Only two years
after the observation by Visser the author was able to
visit this population in the sandveld south of the road
between Springbok and Port Nolloth in the north-west-
ern part of Namaqualand. e area is characterized by
succulent dwarf shrubs, mainly mesembs and Euphorbia
species (Veste & Jürgens 2004). Meanwhile another
population was discovered by Maass & Musselman
(2004) near Rosh Pinah in southern Namibia. Hydnora
triceps is restricted only to Euphorbia dregeana, even
when other Euphorbia species grow nearby. Euphorbia
dregeana is distributed in Namaqualand and southern
Namibia. It is estimated that only 10 % of E. dregeana
in the Port Nolloth area and less than 0.5 % in southern
Namibia are parasitized by H. triceps (Maass & Mus-
selman 2004).
e hidden way of life of the parasites makes it
difficult to find this plant in the wild. Most parts of
the plant consist of a subterranean network of fleshy
so-called “pilot roots” connected to the root system of
the hosts. Fig. 3 shows such a subterreanean network
of Hydnora triceps connected to the roots of the nearby
Euphorbia dregeana. e Hydnora haustorium is directly
Parasite Family Host Reference
Root parasites:
Alectra vogelii Benth.Scrophulariaceae Euphorbia1) [1]
Buttonia superba Oberm. Scrophulariaceae Euphorbia1) [1]
Hydnora africana Thunb. Hydnoraceae Euphorbia1) [1]
Hydnora triceps Drege Hydnoraceae Euphorbia dregeana E. Meyer ex Boissier [2, 4]
Stem parasites:
Tapinanthus oleifolius (J.C.Wendl.) Danser Loranthaceae Euphorbia virosa Willd.1) [1, 5]
Viscum capense L. f. Viscaceae Euphorbia1) [1]
Viscum crassulae Eckl. & Zeyh. Viscaceae Euphorbia species aff. tetragona
Euphorbia grandidens Haw.
[1]
[3]
Viscum minimum Harv. Viscaceae Euphorbia polygona Haw.[1]
Table 1: Parasitic flowering plants on Euphorbia in South Africa and Namibia (after [1] Visser 1981, [2] Visser 1989, [3] Midgley et al.
1994, [4] own observation 1990/91). Note 1): Visser (1981) mentions only the genus and not the species.
Fig. 3: The subterranean network of the haustorial “roots” of
Hydnora triceps parasitic on Euphorbia dregeana. The scale in
the picture is 10 cm.
Fig. 4: Connection between Hydnora triceps and the root of
Euphorbia dregeana.
7VOL. 3 N. 3 - DECEMBER 2007
EUPHORBIA WORLD
food of the local Khoi in northern South Africa and used
for several Cape dishes (van Wyk & Gerike 2000). e
fruits have a sweet taste when baked on a fire.
Other root parasites growing on Euphorbia species
are Alectra vogelii and Buttonia superba belonging to the
Scrophulariaceae. Buttonia is a climber which rambles
through shrubs often some distance from the hosts and
occurs in the eastern provinces of South Africa.
Stem parasites
Xylem-tapping mistletoes are common in the arid
and semi-arid regions of Southern Africa. e richest
mistletoe flora can be found in the summer rainfall
area of the Nama Karoo, where thirteen species occur
(Dean et al. 1994). In the winter rainfall desert of the
Succulent Karoo eight species can be found. e mis-
tletoes are represented by the Viscaceae Viscum and the
Loranthaceae Moquiniella, Septulina and Tapinanthus
(Visser 1981). e host specificity varies between the
different species. Tapinanthus oleifolius is not restricted
to a specific host and it can be found on more than 30
hosts including Acacia, Aloe, Citrus, Ficus, Rhus, Tamarix
and even other mistletoes like Viscum and T. oleifolius
plants. T. o l e i f o l i u s is the only Loranthaceae in Southern
Africa growing on Euphorbia virosa (front cover). Eu-
connected to the xylem and the phloem of the host‘s
roots (Fig. 4). e structure of the pilot-roots bearing
the flowers and the haustoria remains unclear (Visser
1981, Weber 1993). However, its anatomical structure
shows characteristics for stems (Tennakoon et al 2005).
From buds along this vegetative body the haustorium
and the flowers are developed exogenously. It can take
more than one year for the development of a flower.
Only the entrance to the flowers is exposed above
ground to attract pollinators (Fig. 1). e flowers of
Hydnora are brown, warty or scaly on the outside and
orange on the inside (Fig. 5). ey have a putrid smell.
Within one year the flower grows to a height of 10 to 15
cm (3.95 to 5.9 inches). Flowering time is from June to
January. e fruits have a tough outer layer, which splits
and exposes a pulpy mass in which the seeds are embed-
ded. e fruits are eaten by small mammals and even
by birds. e delicious fruits are part of the traditional
Fig. 5: Cross-section of the flowers of (A) Hydnora africana, left,
(Photo: Siegmar-W. Breckle) and (B) of Hydnora triceps, above
(Photo: Maik Veste).
8
EUPHORBIA WORLD
phorbia virosa is up to 3 m (9.8 ft.) high and distributed
from the area south of Vioolsdrift on the Orange River
northwards to the Kaokoveld and southern Angola. A
large population of T. oleifolius growing on Euphorbia
virosa can be found in the Karas Mountains in southern
Namibia, where the mistletoe is also growing on Acacia
nebrownii and Ziziphus mucronatha. From Burkina
Faso Tapinanthus globiferus and T. ophiodes growing on
Euphorbia balsamifera are known (Boussim et al. 2004).
Viscum crassulae is frequently found on Euphorbia spec.
aff. tetragona and is also reported on E. grandidens.
e smallest mistletoe is Viscum minimum barely
growing taller than a few millimetres and is hard to
find. e visible part is composed of a small stem with
a single internode of 0.5 mm (0.02 in.), 2 or 3 leaves
and a single inflorescence (Fig. 6). Most of the plant
consists of branching haustorium and is inside the host
as an endoparasite. e distribution area ranges from the
Little Karoo towards the Eastern Cape. Visser (1981)
reported that Viscum minimum is parasitic almost exclu-
sively on Euphorbia polygona and E. horrida. However, in
an experiment it was possible to establish V. minimum
successfully on 28 succulent Euphorbia species from
the area south of the Sahara, Morocco and Madagascar
(Heide-Jørgensen 2004).
Birds are important for the seed dispersal of all mis-
tletoes. e berries are picked up by fruit-eating birds.
e berries are ripe when food is rather scarce and a large
variety of birds are attracted. ey defecate the seeds
of the Loranthaceae within minutes, while the Viscum
seeds are retained slightly longer. Most of the seeds are
distributed on the nearby plants up to 50 meters (164
ft.) from the fruiting plant (Visser 1981) and therefore,
mistletoes are distributed in clumps in the landscape.
Once the seed is removed from the fruit germination
can start and the hypocotyls grow towards the branch.
e penetration into the host’s tissue and the essential
contact with the xylem can take up to one year.
Ecophysiology of mistletoes
Even though Tapinanthus oleifolius is able to photo-
synthesise, the heterotrophic carbon gain from its host
Euphorbia has been found to be 55 % in young leaves
and more than 80 % in old succulent leaves (Richter
et al. 1995). e daily CO2 uptake of Tapinanthus was
96 mmol per m2 leaf surface per day in young leaves and
only 29 mmol per m2 per 24 h in old leaves. However,
daily transpiration of the mistletoe was 68 mol per m2
per 24 h in young leaves and 239 mol per m2 per 24 h
in old leaves (von Willert & Popp 1995). is shows
the importance of the influx of water, ions and organic
components into the leaves of the parasite. e water
supply of the xylem-tapping mistletoes by the host is an
important feature for this association. A water potential
gradient is essential for the water fluxes and the nutri-
ent fluxes from the hosts to the mistletoe. In general,
transpiration rate of the parasite is higher than its hosts,
e.g. Viscum crassulae had a four times higher transpira-
tion rate than its host Euphorbia grandidens (Midgley et
al. 1994).
Ty p ic a l f o r Ta p i n a n t h u s o l e i f o l i u s growing on Euphor-
bia virosa are its succulent leaves. Ions are transported by
the transpiration stream in the parasite resulting in a high
ion accumulation, especially of potassium and chloride
(Fig. 7). Succulence shows a clear interrelation with the
chloride content. Chloride induces the development of
the leaf succulence of Tapinanthus. A similar increase of
ion content and leaf succulence can be observed in mis-
tletoes growing on halophytic hosts. When Tapinanthus
oleifolius (Namibia) or Psilocephalus (syn. Loranthus)
acaciae (Arava-Valley, Israel) were parasitic on halophytic
Tamarix trees, water content and leaf volume increased
significantly in old leaves in comparison to individuals
growing on non-halophytic Acacia trees (Popp et al.
Fig. 6: Viscum minimum parasitic on Euphorbia horrida. Only the
small stem and 2-3 leaves and the inflorescence are visible (photo:
S.-W. Breckle).
9VOL. 3 N. 3 - DECEMBER 2007
EUPHORBIA WORLD
1995, Veste & Breckle 1995). e increase of succulence
dilutes the ions and other organic osmolytes (sugars,
organic acids, cyclitolts) in the cell and the osmotic
concentration is even lower than on the non-halophytic
host. As on other halophytes the increasing succulence
is a morphological adaptation for the mistletoe to the
increasing salt stress on halophytic hosts. u
Acknowledgements
Marianne Popp (Vienna) and Dieter J. von Willert
(Münster) for their discussions and information on the
parasites in Southern Africa. anks to Siegmar-W. Bre-
ckle (Bielefeld) for providing pictures from several para-
sites. e travel to South Africa and Nambia was funded
by the Deutsche Forschungsgemeinschaft (DFG) and
German Academic Exchange Service (DAAD) and
Volkswagen of South Africa.
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mmol = millimol)
