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Host plants of Osyris lanceolata (African Sandalwood) and their influence on its early growth performance in Tanzania

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Abstract

Identification of the host plants of the hemi-parasitic African sandalwood (Osyris lanceolata) and the influence of some on its early growth performance was investigated at Image, Nundu, Sao Hill and Iringa in the southern highlands of Tanzania. The aim was to identify host plants that support the growth of O. lanceolata, and to evaluate the potential of some in promoting its early growth under artificial establishment. The results revealed that O. lanceolata parasitises a wide range of hosts although some were preferred. The preferred hosts were Rhus natalensis, Dodonaea viscosa, Tecomaria capensis, Catha edulis, Apodytes dimidiata, Brachystegia spiciformis, Maytenus acuminatus and Aphloia theiformis. Of the preferred hosts, Brachytegia spiciformis, Rhus natalensis and Casuarina equisetifolia promoted most effectively the early growth of O. lanceolata in terms of height, diameter and overall root and shoot biomass. Possibly the light crown of these host species and the nitrogen fixing ability of C. equisetifolia played a significant role in conferring this advantage. The species are thus recommended as appropriate host plants when raising O. lanceolata seedlings for planting. However, a decision on whether these hosts will support the growth of O. lanceolata at a later stage is subject to further experimentation as they may only be serving as initial or intermediate hosts as reported in a related species Santalum album.
Southern African Forestry Journal – No. 203, March 2005 55
Research Note
Host plants of Osyris lanceolata (African
Sandalwood) and their influence on its early
growth performance in Tanzania
P.L. Mwang’ingo1, Z. Teklehaimanot2, L.L. Lulandala3 and
S.T. Mwihomeke4
1Department of Biological Sciences, Faculty of Science, Sokoine University of Agriculture,
P.O.Box 3038, Morogoro, Tanzania
2School of Agricultural and Forest Sciences, University of Wales Bangor,
Gwynedd LL57 2UW, United Kingdom
3Department of Forest Biology, Faculty of Forestry and Nature Conservation,
Sokoine University of Agriculture, P.O.Box 3010, Morogoro, Tanzania
4Department of Forestry, University of Venda, Private Bag X5050, Thohoyandou,
Republic of South Africa
Corresponding author: pmwangingo@yahoo.com
SYNOPSIS
Identification of the host plants of the hemi-parasitic African sandalwood (Osyris lanceolata) and the
influence of some on its early growth performance was investigated at Image, Nundu, Sao Hill and Iringa
in the southern highlands of Tanzania. The aim was to identify host plants that support the growth of
O. lanceolata, and to evaluate the potential of some in promoting its early growth under artificial
establishment. The results revealed that O. lanceolata parasitises a wide range of hosts although some were
preferred. The preferred hosts were Rhus natalensis, Dodonaea viscosa, Tecomaria capensis, Catha edulis,
Apodytes dimidiata, Brachystegia spiciformis, Maytenus acuminatus and Aphloia theiformis. Of the
preferred hosts, Brachytegia spiciformis, Rhus natalensis and Casuarina equisetifolia promoted most
effectively the early growth of O. lanceolata in terms of height, diameter and overall root and shoot biomass.
Possibly the light crown of these host species and the nitrogen fixing ability of C. equisetifolia played a
significant role in conferring this advantage. The species are thus recommended as appropriate host plants
when raising O. lanceolata seedlings for planting. However, a decision on whether these hosts will support
the growth of O. lanceolata at a later stage is subject to further experimentation as they may only be serving
as initial or intermediate hosts as reported in a related species Santalum album.
Keywords: African sandalwood, Osyris lanceolata, parasitism, host plants, tree growth, host selectivity
INTRODUCTION
Osyris lanceolata Hochst & Steudel. commonly known
as African Sandalwood (Dale and Greenway, 1961;
Breintenbach, 1963) is a an evergreen dioecious shrub
or small tree growing to a height of 1-7 m depending
on soils, climate and genetics of the tree (Breintenbach,
1963; Beentje, 1994; Palmer and Pitman, 1972). It is
a highly branched tree with branches sometimes
pendant. Leaves are bluish green or yellow green
being simple and arranged in an alternate manner.
Flowers are small, pale yellow green inconspicuously
borne in an inflorescence that develop from the axil of
leaves. Female flowers develop to fleshy single seeded
fruits (Miller, 1989). For detailed description of the
O. lanceolata see a review of the species by Mwang’ingo
(2002).
Osyris lanceolata is among the sandalwood species
known to produce fragrant-scented wood from which
sandalwood essential oil is extracted (Walker, 1966;
Iyenga, 1968, Srinivasan et al., 1992; Mbuya et al.,
1994). Sandalwood oil is used in the production of
various luxurious cosmetics, perfumes and fragrances.
The excellent blending and antiseptic properties of the
oil makes it also valuable as a fixative for other
fragrances (Iyenga 1968; Srinivasan et al., 1992;
Coppen, 1995). The oil is popular in oriental medicine
as it has a chemo-preventive impact on inflammatory
and eruptive skin diseases, bronchitis, dysuria,
gonorrhoea and urinary infections (Okugawa et al.,
1995; Dwivedi and Zhang, 1999).
The utilization of O. lanceolata in perfumery and
fragrance industry in the early 1900’s followed a
decline in the resource base of Indian sandalwood
56 Southern African Forestry Journal – No. 203, March 2005
(Santalum album) (Walker 1966; Iyenga, 1968;
Srinivasan et al., 1992). Since this time, natural
populations of O. lanceolata have been a major source
of the raw material supply to the perfumery industry
and general use. Consequently the species have been
over-exploited, with even the roots being intensively
harvested (Ruffo et al., 2002; Burgess et al., 1998).
Already some populations are considered to have
disappeared while others have deteriorated (Mbuya et
al., 1994; Mwang’ingo and Mwihomeke, 1997).
However, there is no sign of a decline in demand for
sandalwood oil and its related products (Rai and
Sarma, 1990; Srinivasan et al., 1992). The high prices
of oil and wood products (Nasi and Ehrhart, 1996), and
the increasing demand have prompted further
harvesting of O. lanceolata, imposing more threat on
the natural populations (Burgess et al., 1998).
Having realised the exiting market potential, the
rapid decline in the resource base and the importance
of the species as a source of income for some rural
communities, Tanzania has encouraged farmers and
those involved in sandalwood industry to domesticate
the species (Mwang’ingo and Mwihomeke, 1997; Ruffo
et al., 2002;). Domestication is aimed at increasing
the overall production of the species, providing
alternative sources and safeguarding the natural
populations against further depletion. This is
important as some populations in Tanzania are
considered to contain superior genotypes (Eggling and
Dale, 1966; Walker, 1966).
To date, domestication and creation of woodlots of
sandalwood have not been successful due to lack of
basic ecological information relevant to its silviculture.
Of special importance is the lack of understanding
about its parasitic behaviour. O. lanceolata is a hemi-
parasitic plant that requires the presence of a host
plant to support its normal growth and survival
(Kuijt, 1969; Herrera, 1988; Mbuya et al., 1994; Ruffo
et al., 2002). Seed germination can take place without
the influence of a host (Kuijt 1969), but the further
development and survival of the seedling is not possible
without a host (Rao, 1942; Metcalfe and Chalk, 1950;
Kuijt 1969; Herrera, 1988; Mbuya et al., 1994). The
hemi-parasitic nature of O. lanceolata is believed to be
a survival strategy to acquire water and minerals in
a dry environment where it is mostly found. It assists
in accessing the most important requirements of
water and minerals. The main nutrients acquired
through parasite/host relationship include phos-
phorus, potassium and magnesium, which are
essential for plant growth (Rao, 1942; Metcalfe and
Chalk, 1950; Niranjana and Shivamurthy, 1987; Rai
and Sarma, 1990).
Although a range of Mediterranean and India species
are reported to be hosts for sandalwood (Rao, 1942;
Niranjana and Shivamurthy, 1987; Rai and Sarma,
1990; Herrera, 1988), few studies have reported on the
hosts of O. lanceolata and no study has been reported
from tropical Africa. Thus nothing is known about the
potential of local host plants to successfully support
the growth of O. lanceolata. It is known from parasitic
plants that there can be alternative hosts but not all
can be considered as desirable hosts (Holmes, 1979).
Host suitability tends to vary between species, making
the identification of the best host for O. lanceolata of
great importance for its domestication and
establishment (Mbuya et al., 1994; Ruffo et al., 2002).
This study investigated the potential host species of
O. lanceolata in Tanzania through soil excavation in
natural populations, and nursery tests of the potential
of different hosts to support its early growth.
MATERIALS AND METHODS
Description of the study sites
Identification of host plants was carried out in the three
Tanzania natural forest populations of Image, Nundu
and Sao Hill, while the study on the influence of host
plants on the early performance of O. lanceolata was
carried out at Iringa Zonal Tree Seed Centre, Tanzania.
The location, climate, soil and vegetation composition
of the study sites are presented in Table 1. The study
sites were chosen because of the occurrence of
O. lanceolata in good proportion compared to others,
based on the survey that was conducted between 1999
and 2000 (Mwang’ingo, 2002).
Field identification of host species of
O. lanceolata
The field study was carried out between June and
September 2000. At each of the three study sites
(Image, Nundu and Sao Hill), trees of O. lanceolata of
uniform height and diameter, ten from each sex, were
selected. The mean diameter and height of trees at
Image, Nundu and Sao Hill were 3.5 cm and 4.2 m,
6.2 cm and 10.4 m, 4.8 cm and 6.3 m respectively.
The distance between two individuals was at least
20 m.
The soils up to a distance of 3 m in radius from the
base of each tree was excavated, exposing roots of both
host plants and the parasite as described by Herrera
(1988). Haustoria connections between roots of the
hosts and the parasites were inferred through visual
inspection of their proximity and by the difficult of
detaching the roots by pulling at the point of
attachment. No structural investigation of the
haustoria was possible, unfortunately.
Records were made of the sex of the parasite, the
frequency of haustoria connecting with each host species
at each site. To compare the occurrence of parasitism
for each host species, Sorenson Index (Ss) of similarity
was used between tree sexes and sites as described by
Krebs (1998). The index was calculated as:
Ss = 2a/(2a+b+c)
Where,
a = The number of host species occurring in sites A
and B (common occurrence) or the number of
hosts associated with both female and male in
each site
Southern African Forestry Journal – No. 203, March 2005 57
Location, climate or
soil property
TABLE 1. Location, climate, soil and vegetation composition of the study populations in Tanzania
Nundu
9o26’ S
34o50’ E
1500
19.7
7.9
999
1900
Humic red earths, being
partially deteriorated in
the upland forests.
Thickets, secondary to
upland humid evergreen
and dry upland sclero-
phyll forest.
Image
7o32’ S
36o10’ E
630
26.5
14
1400
1900
Predominantly laterized
low-humic red earths,
fertile initially, but loose
fertility rapidly with
frequent cultivation.
Miombo woodland (main-
ly, Brachystegia and
Isoberlinia sp.).
Sao Hill
8o26’ S
35o13’ E
950
21.7
10.8
1280
1900
Granitic in origin, deep and
relatively uniform in physical
structure. Mostly of sandy clay
loam texture. Low in nitrogen,
exchangeable bases, organic
carbon, and available phos-
phorus.
Scattered trees and shrub
vegetation occurring in clumps
or individually around rock
knolls or gullies. Common
species are Albizia sp., Parinari
curatellifolia, O. lanceolata,
Tecomaria capensis, Dodonaea
viscosa Rhus natalensis.
Climate
Mean annual rainfall
(mm)
Maximum monthly
temperature (oC)
Minimum monthly
temperature (oC)
Mean annual evapo
transpiration (mm)
Altitude (m.a.s.l)
Soil
Vegetation
Study site/population
Location
b = The number of host species that occurs in site B
only or the number of host species associated with
only male parasites in each site
c = The number of host species that occurs in site A
only or the number of host species associated with
only female parasites in each site.
If the index between sexes or sites was less than 0.5,
it was inferred that male and female O. lanceolata
trees had no host preference, or that sites were
dissimilar in host species composition.
Evaluation of some host plants in promoting
the early growth of O. lanceolata
To study the effects of different host plants from the
three sites on the growth of O. lanceolata, the four
most common and ubiquitous host species (Brachy-
stegia spiciformis (Caesalpiniaceae), Dodonaea viscosa
(Sapindaceae), Rhus natalensis (Anacadiaceae), and
Tecomaria capensis (Bignoniaceae) were selected. This
choice was also influenced by the availability of seeds
of the host species. Casuariana equisetifolia
(Casuarinaceae), an exotic species, was also included
in the study as it is one of the most effective hosts of
members of family Santalaceae (Srinivasan et al.,
1992; Rai, 1990; Taide et al., 1994). It is known to grow
and survive well in most semi arid areas of Tanzania
where O. lanceolata occurs (Mbuya et al., 1994).
Seeds of D. viscosa, R. natalensis, and O. lanceolata
were collected from Sao Hill forest while those of B.
spiciformis were collected from Image. Seeds of T.
capensis were collected at Nundu forest while those of
C. equisetifolia were purchased from Tanzanian
National Tree Seed Program. Seeds were sown at
Iringa Zonal Tree Seed nursery in November 2000,
using a soil mixture ratio of 2:1 composed of forest soil
and sand. After germination the host seedlings were
pricked out in mid December 2000 into containers
measuring 25 cm in diameter and 30 cm deep filled
with mixture of forest soil, sand and cow manure at
the ratio of (2:1:1) each containing a O. lanceolata
seedlings. The forest soil was obtained from O.
lanceolata stand in Nundu forest. O. lanceolata (alone)
58 Southern African Forestry Journal – No. 203, March 2005
were also grown alone as a control. Thus, there were
six treatments of 12 containers each and replicated
three times, making a total of 216 containers. These
were then laid out in a randomised complete block
design. This experimental layout was used before in
the investigation of the intermediate host influence in
Santalum album (Radomiljac et al., 1999).
The seedlings were grown and monitored for one
year. Destructive assessments were made at three
months intervals on three containers, one from each
treatment and block. These were randomly selected
and measured for height, root collar diameter (using
a veneer calliper) and leaf area (using a planimeter).
Since it was difficult to measure the leaf area of C.
equisetifolia needles, the volume of 100 needles,
harvested from the mid point from each plant bound
together were measured through water displacement
method. Plants were thereafter harvested, separating
the shoot and root portions. These were oven dried at
80oC for 48 hours and then weighed.
ANOVA of the MINITAB statistical package was
used to determine significant differences between
treatments at every three months of assessment.
Where significant differences were observed, the
Turkey’s Pair Wise comparison was used to tell the
differing means
RESULTS
Identification of host plants of O. lanceolata
Forty nine plant specis were identified as hosts of O.
lanceolata at Image, Sao Hill and Nundu forest
(Table 2-4). The species varied among sites (Image =
29, Sao hill = 16, Nundu = 18), although some species
were observed to be common at all three study sites.
The most frequent hosts at Image were B. utilis,
B. spiciformis, R. natalensis, D. viscosa, Ormocarpus
kirkii, Combretum zeyheri and Clerodendron
myricoides while at Sao Hill forest, R. natalensis,
T. capensis, M. acuminatus, A. dimidiata and
D. viscose were common. A. theiformis, R. natalensis,
Catha edulis, Jasminum odoratissimum, Myrsine
melanophloeos and T. capensis were preferred at
Nundu. Host species found to be common in at least
two sites included R. natalensis, D. viscosa,
T. capensis, C. edulis and A. dimidiata.
TABLE 2. Host plants of O. lanceolata at Image Forest Reserve, Tanzania: number of O. lanceolata trees observed
to utilised the host and number of host trees (frequency) that made parasitic attachment with the parasite.
Host plant identity Number of O. lanceolata trees Number of host trees
observed to utilize the host (frequency) that made parasitic
attachment
Brachystegia spiciformis 12 72
Dodonaea viscosa 11 16
Brachystegia utilis 10 34
Rhus natalensis 915
Ormocarpum kirkii 811
Clerodendron myricoides 810
Combretum zeyheri 69
Maprounea africana 517
Aeschynomene abyssinica 510
Indigofera rhynchocarpa 57
Senna singueana 57
Faurea saligna 56
Dichrostachys cinerea 45
Diospyros whyteana 45
Albizia glaberrima 35
Mundulea sericea 35
Acacia hockii 34
Euclea natalensis 34
Flacourtia indica 34
Schrebera alata 34
Maytenus mossambicensis 33
Ochna holstii 33
Tarenna neurophylla 26
Rytigynia uhligii 23
Tapiphyllum cinerascens 23
Dombeya shupangae 22
Securidaca longepedunculata 22
Hymenodictyon floribundum 14
Markhamia obtusifolia 13
Southern African Forestry Journal – No. 203, March 2005 59
TABLE 3. Host plants of O. lanceolata at Sao Hill Forest Reserve, Tanzania: number of O. lanceolata trees observed
to utilised the host and number of host trees (frequency) that made parasitic attachment with the parasite.
Host plant identity Number of O. lanceolata trees Number of host trees
observed to utilize the host (frequency) that made parasitic
attachment
Tecomaria capensis 13 59
Dodonaea viscosa 13 24
Rhus natalensis 12 30
Apodytes dimidiata 12 26
Maytenus acuminatus 11 45
Byrsocarpus orientalis 716
Myrsine africana 413
Dombeya shupangae 47
Flacourtia indica 46
Ochna holstii 45
Schrebera alata 45
Bersama abyssinica 37
Catha edulis 33
Vangueria infausta 22
Psychotria lauracea 17
Myrsine melanophloeos 14
TABLE 4. Host plants of O. lanceolata at Nundu Forest Reserve, Tanzania: number of O. lanceolata trees observed
to utilised the host and number of host trees (frequency) that made parasitic attachment with the parasite.
Host plant identity Number of O. lanceolata trees Number of host trees
observed to utilize the host (frequency) that made parasitic
attachment
Aphloia theiformis 19 144
Rhus natalensis 16 50
Catha edulis 13 19
Myrsine melanophloeos 12 27
Jasminum odoratissimum 12 16
Tecomaria capensis 11 20
Diospyros whyteana 11 17
Rhamnus prinoides 10 18
Heteromorpha trifoliate 10 11
Psychotria lauracea 711
Olinia rochetiana 613
Dais cotinifolia 69
Dodonaea viscosa 66
Myrica salicifolia 520
Erythroxylum fischeri 45
Apodytes dimidiata 35
Euclea divinorum 33
Agauria salicifolia 27
Preference of parasite’s sex to hosts and site
specificity
Sorenson’s index of similarity indicated that male and
female plants of O. lanceolata had no preference with
both sexes occurring in more than 80% of hosts. In
contrary, there were host x site specificities, with the
most species being found as hosts at image. Very few
species utilized at one site were also utilized at the
other as hosts. Of the host plants at Image and Sao
Hill, only 15% were utilized at both sites while at
Image and Nundu 7% were commonly utilized.
Likewise at Sao hill and Nundu only 26% of the hosts
were utilized at both sites. The Sorenson index of
similarity (Table 5) further clarifies the situation.
60 Southern African Forestry Journal – No. 203, March 2005
Growth performance of O. lanceolata under
different hosts
Plant height, root collar diameter, and root & shoot
biomasses in O. lanceolata were all significantly
affected by host species (Figure 1, 2, 3 and 4) in
similar way. Clear significant difference had developed
by nine months between B. spiciformis, R. natalensis,
TABLE 5. Similarity in the utilization of host plants by male and female O. lanceolata between Image, Sao Hill
and Nundu Forest reserves, Tanzania
Sites compared Number of species Sorensons index of
parasitized smimilarity
Image & Sao Hill
Species utilized at Image only 23
Species utilized at Sao Hill only 10 0.27
Species utilized at both sites 6
Image & Nundu
Species utilized at Image only 26
Species utilized at Nundu only 15 0.13
Species utilized at both sites 3
Numdu & Sao Hill
Species utilized at Nundu 11
Species utilized at Sao Hill 9 0.41
Species utilized at both sites 7
C. equisetifolia and the control, while T. capensis and
D. viscosa were not significant different from the
control. By the end of 12 months, O. lanceolata grown
with B. speciformis were the tallest (62.6 cm) while
large plants in terms of diameter were those grown
with C. equisetifolia (10.3 mm). O. lanceolata grown
with R. natalensis had the highest root biomass
(8.84 g) while shoot biomass was more in O. lanceolata
FIGURE 1. Height growth of O. lanceolata planted with various host plants at Iringa Zonal Tree Seed Centre
nursery, Tanzania
Southern African Forestry Journal – No. 203, March 2005 61
grown with C. equisetifolia (10.67 g). Statistically
height, root collar diameter, root and shoot biomass
were the same between B. speciformis, C. equisetifolia
and R. natalensis.
Although there were some differences in O.
lanceolata leaf area between species, this was much
less variable than the other trait (Figure 5) and only
O. lanceolata plants on B. spiciformis had large
leaves than the control. The root/shoot ratio also
showed a significant difference (P < 0.05) between
treatments at a later stage. The highest ratio (1.1)
was in O. lanceolata grown with D. viscosa at an age
of 12 months. This ratio was significantly higher
compared to what was noted in the control (0.74 ±
0.08) and the rest of the treatment except in O.
lanceolata grown with T. capensis.
DISCUSSION
Host plant identification and their intensity of
utilization
The large number of hosts species utilized by O.
lanceolata in the three sites suggests that this
sandalwood species is not very selective in its choice of
hosts as earlier reported by Rao (1942). Interestingly
Image has the greatest host diversity and amount
(almost twice of what is observed either at Sao Hill or
Nundu). This is likely to be attributed to differences
in climatic and soil conditions between the sites.
Image receives less rainfall than the other two sites
and thus water deficit may be more critical at image
and may promote parasitism. Soils of Image are also
relatively poor in nutrient status especially nitrogen
(Gilchrist, 1952; FAO, 1984). This suggests that
parasitism is beneficial at sites where water and
nutrients are limiting in accordance with the findings
of Attsat (1983).
The high frequency of infection in some host species
suggest that although O. laceolata can utilize a range
of species, it has some preferences as in many other
parasitic plants (Holmes, 1979). Where suitable hosts
are available, O. lanceolata tend to utilize some of
them more frequently compared to others. For
example, at Image forest, B. spiciformis and B. utilis
were the most utilized while at Sao Hill the most
utilized species were T. capensis, M. acuminatus and
R. natalensis. At Nundu O. lanceolata utilized more
A. theiformis and R. natalensis. The reasons for such
preferences are still unknown. Possibly these hosts
provide more of the needed resources to the parasite
compared to other species. On the other hand, it has
been shown that host plants tend to release some
chemical exudates, which sensed by the parasites.
FIGURE 2. Diameter growth of O. lanceolata planted with various host plants at Iringa Zonal Tree Seed Centre
nursery, Tanzania
62 Southern African Forestry Journal – No. 203, March 2005
FIGURE 3. Leaf area growth of
O. lanceolata planted with
various host plants at Iringa
Zonal Tree Seed Centre nursery,
Tanzania
FIGURE 4. Root biomass
growth of O. lanceolata
grown with various host
plants at Iringa Zonal Tree
Seed Centre nursery,
Tanzania
Southern African Forestry Journal – No. 203, March 2005 63
Once the preferred compounds are sensed, roots of the
parasites tend to grow towards the hosts that have
released the signals (Kuijt, 1969; Musselman and
Press, 1995).
It seems clear that male and female sandalwood
trees use the same host species as reported earlier by
Herrera (1988) in the Mediterranean.
Potential of host plants to promote the growth
of O. lanceolata
The initial health and vigour of any plant is considered
important to ensure future success in the field (Evans,
1982). The variation in the growth of O. lanceolata
seedlings grown with the various hosts suggests that
although germination of O. lancelata seeds could start
without host support, selection of host plants is
important at a later stage of seedling growth if the
species has to be raised successful. The noted
differences in growth attributes after three months,
suggest that O. lanceolata could support itself at an
early stages of seed germination and growth as reported
by Kuijt (1969) from food reserve in its large seed.
Similar observations are reported in most large seeded
parasitic plants (Musselman and Press (1995). After
this initial period, the food reserve is depleted and the
plant becomes depended on photosynthesis, which in
parasitic plant is insufficient to support the growth of
a big plant (Press et al., 1988; Press, 1995). This
necessitates the introduction of a host to support
subsequent growth of the parasite.
After three months, B. spiciformis, R. natalensis
and C. equisetifolia promoted the growth of O. lanceolata,
while D. viscosa and T. capensis did not. The reasons
for these differences between are uncertain but the
comparably lighter crowns of the three successful
species may have allowed more light to be available to
the O. lanceolata for photosynthesis and growth. Rama
Rao (1911) has suggested a similar mechanism for the
hosts to S. album in India. In this way B. spiciformis,
C. equisetifolia and R. natalensis could perhaps promote
the growth of O. lanceolata by directing assimilates to
the root system so allowing the parasite to explore
greater volumes of soil to meet their water and nutrient
requirements, as well as to make additional parasitic
connections on the host plants.
FIGURE 5. Shoot biomass growth of O. lanceolata planted with various host plants at Iringa Zonal Tree Seed
Centre nursery, Tanzania
64 Southern African Forestry Journal – No. 203, March 2005
Additionally, the good performance of O. lanceolata
on C. equisetifolia may be attributed to its nitrogen
fixing ability as reported in pot cultures of S. album
(Rai, 1990; Taide et al., 1994; Nasi and Ehrhart,
1996). In a similar way, Radomiljac et al. (1999) have
noted that nitrogen-fixing plants such as Sesbania
formosa, Acacia trachycarpa and Acacia ampliceps
were better hosts for S. album than the non-nitrogen
fixers. Tennakoon and Pate (1996) have reported
similar benefits of N-fixing hosts on Olax phyllanthi,
another parasitic plant. The enhanced performance of
parasites with nitrogen fixing hosts is thought to be
related to the reduced competition for nitrogen between
the hosts and the parasites (Radomiljac et al., 1999).
Conversely the growth retardation of O. lanceolata
with D. viscosa and T. capensis may be the result of
excessive competition for light between host and
parasite. These developed a large canopy biomass and
cast heavy shade, which as in S. album would retard
growth of the parasite (Radomiljac, 1998; Rama Rao,
1911). Poor growth performance of root hemi-parasites
due to inability of the hosts to supply enough nutrients
is also reported for O. phylanthi attached to
Amaranthus caudatus (Tennakoon and Pate, 1996)
and S. album on Acacia hemignosta and Crotolaria
retus (Radomiljac 1998). It has been suggested that
the low uptake of nitrogen from the host by the
parasite is caused by nitrate accumulation in the
host’s xylem sap, which reduces the intake of
heterotrophic carbon (Tennakoon and Pate, 1996).
This impairs the growth process.
CONCLUSION AND RECOMMENDATION
This study has identified many potential host species
for O. lanceolata with some specific host preferences.
Host species with light canopies were found to promote
better growth of O. lanceolata and this was attributed
to greater potential for photosynthesis. N-fixing hosts,
such as C. equisetifolia, also seem to confer benefits to
the parasite. Casuarina equisetifolia, B. spiciformis
and R. natalensis) are recommended as appropriate
host plants to support the early growth O. lanceolata.
Further research is required to determine whether
these hosts will continue to be effective longer-term
hosts for O. lanceolata
ACKNOWLEDGEMENTS
FINNIDA through Finnish Support to Forestry
Research in Tanzania (FORST) in collaboration with
Tanzania Forestry Research Institute (TAFORI)
funded this study. Many thanks are extended to the
Lushoto Silviculture Research Centre herbarium staff,
particularly the late Mr. C.K Mabula (the botanist)
and Mr. Timilai Sigara for assistance in identifying
plants both in the field and confirmation at the
herbarium. Nursery experiments were tended and
monitored by Ms. Fatuma Ramadhani of Iringa Zonal
Tree Seed Centre, Tanzania.
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66 Southern African Forestry Journal – No. 203, March 2005
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Heterotrophic gains of carbon from various host species by the root hemiparasitic shrub Olax phyllanthi (Labill) R.Br. were assessed using techniques based on carbon isotope discrimination (13C) on C3 and C4 hosts and C:N ratios of xylem sap and dry matter of host and parasite. Heterotrophic benefits (H) to Olax based on 13C values were 30% and 19% from two nonnative C4 hosts (Portulaca oleracea and Amaranthus caudatus respectively) compared with 13% and 15% from these hosts when computed on the basis of C:N ratios of host xylem sap and C and N increments of Olax dry matter. Nitrate was the source of N available to pot cultures of the above species and estimates based on C:N ratios assumed that all N accumulated by Olax had come from nitrate absorbed by the host. Equivalent estimates of H for Olax, grown in nitrogen-free pot culture with the native N2-fixing host Acacia littorea as its sole source of N, indicated 63% and 51% dependence on host carbon when assessed in terms of xylem sap composition of host parasite respectively. Comparisons of xylem sap solutes of Olax and a range of partner hosts indicated marked selectivity in haustorial uptake and transfer of nitrate, amino compounds, organic acids and sugars. Possible implications of variations between hosts in absolute levels of C and inorganic and organic forms of N in xylem are discussed in relation to evidence of much better growth performance of Olax on Acacia littorea and other N2-fixing legumes than on non-fixers.