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Plant species are basic component of agro-biodiversity and a complex situation created by their own ability to disperse; and the rapid changes of land use and climate is endangering their efficient conservation and use. In order to increase knowledge of bitter and sweet African bush mango trees (ABMTs) (Irvingia spp.: Irvingiaceae) and support small-scale farmers in establishing uniform plantations, germinability of earlier fruited trees was assessed. Germination rate and speed, from both systems were analysed in order to differentiate bitter and sweet bush mango trees and identify types of seeds suitable to overcome the climatic hindrances for uniform plantations establishment in the Dahomey Gap, a drier eco-region where savannah reach the sea coast including Benin and Togo. Fresh seeds of both bitter and sweet fruited trees, showed the highest growth performance (98 - 100%). Seed germination speed significantly depended on the drying level and the germination system (sunshine versus covered condition). The speed was higher for fresh seeds in closed condition, confirming bush mango seeds as typically recalcitrant, but not strictly photoblastic. Results also demonstrated that bush mango seeds do not require specific treatments for optimising germination. Germination did not depend on mango tree type (bitter or sweet) and fresh seeds were the best material for establishing viable and uniform plantations. Key Words: Benin, Dahomey gap, Irvingia
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African Crop Science Journal, Vol. 22, No. 4, pp. 291 - 301 ISSN 1021-9730/2014 $4.00
Printed in Uganda. All rights reserved © 2014, African Crop Science Society
GERMINATION OF SEEDS FROM EARLIER FRUITS OF BITTER AND SWEET
AFRICAN BUSH MANGO TREES
R. VIHOTOGBÉ, L.G. HOUESSOU, A.E. ASSOGBADJO and B. SINSIN
Laboratory of Applied Ecology, Faculty of Agricultural Sciences, University of Abomey-Calavi, 01 BP 526,
Calavi, Benin
Corresponding author: rlvihotogbe@gmail.com
(Received 7 May, 2014; accepted 18 November, 2014)
ABSTRACT
Plant species are basic component of agro-biodiversity and a complex situation created by their own ability to
disperse; and the rapid changes of land use and climate is endangering their efficient conservation and use. In order
to increase knowledge of bitter and sweet African bush mango trees (ABMTs) (Irvingia spp.: Irvingiaceae) and
support small-scale farmers in establishing uniform plantations, germinability of earlier fruited trees was assessed.
Germination rate and speed, from both systems were analysed in order to differentiate bitter and sweet bush
mango trees and identify types of seeds suitable to overcome the climatic hindrances for uniform plantations
establishment in the Dahomey Gap, a drier eco-region where savannah reach the sea coast including Benin and
Togo. Fresh seeds of both bitter and sweet fruited trees, showed the highest growth performance (98 - 100%).
Seed germination speed significantly depended on the drying level and the germination system (sunshine versus
covered condition). The speed was higher for fresh seeds in closed condition, confirming bush mango seeds as
typically recalcitrant, but not strictly photoblastic. Results also demonstrated that bush mango seeds do not
require specific treatments for optimising germination. Germination did not depend on mango tree type (bitter or
sweet) and fresh seeds were the best material for establishing viable and uniform plantations.
Key Words: Benin, Dahomey gap, Irvingia
RÉSUMÉ
Les plantes sont une composante essentielle de l’agrobiodiversité; leur habileté à se propager crée une situation
complexe et le changement rapide des systèmes d’utilisation des terres menace leur conservation et utilisation.
Afin d’améliorer l’état de nos connaissances sur les types amer et sucré de manguiers sauvages d’Afrique
(Irvingia spp.: Irvingiaceae) et aider les petits paysans à disposer de plantations uniformes, la germination des
fruits précoces de Irvingia a été évaluée en systèmes ouvert et fermé. Le taux et la vitesse de germination dans ces
deux milieux ont été analysés afin de différencier les arbres producteurs de fruits versus sucrés et d’identifier les
types de semences qui sont appropriés pour réduire les contraintes climatiques pour l’établissement des plantations
uniformes dans le Dahomey-Gap, une écorégion sèche où la savane atteint la côte, beaucoup plus au Benin et au
Togo. Seul le niveau de séchage influence significativement le taux de germination. Les graines fraiches des deux
types d’arbres ont le taux de germination le plus élevé (98 - 100 %). La vitesse de germination dépend à la fois du
taux de séchage et du système de germination. La vitesse est plus élevée pour les graines fraiches en système
d’ombrage, confirmant les graines de mangues sauvages comme étant typiquement récalcitrantes, mais non pas
photoblastique strictes. Les résultats ont aussi montré que les graines de mangues sauvages ne nécessitent aucun
traitement spécifique pour optimiser leur germination. Enfin l’étude démontre que la germination ne dépend pas
du type d’arbre de manguier (arbres à fruits amers de ceux à fruits sucrés) et que les graines fraiches sont les
meilleurs matériels pour l’établissement des plantations viables et uniformes.
Mots Clés: Benin, Dahomey gap, Irvingia
R. VIHOTOGBÉ et al.
292
INTRODUCTION
Plant species are basic component of agro-
biodiversity and a complex situation created by
their own ability to disperse; and the rapid
changes of land use and climate is endangering
their efficient conservation and use (Trakhtenbrot
and Perry, 2005; Fuller and Allaby 2010). The over-
exploitation of plant non-timber forest products
(NTFPs), in natural ecosystems, is causing their
scarcity and growing economic value in sub-
Saharan Africa (Ticktin, 2004). This is extremely
risky for seed crop species, since humans limit
their (non-timber products) dispersal and
evolution in an increasingly severe physical
environment. In a global context of faster
growing human population, handling valuable
tree species reproductive biology and adaptation
capacity for establishing promising
domestication and large plantation programmes
(Simons and Leakey, 2004; Leakey et al., 2005) is
essential for planning and responding to future
increasing demands, as well as for maintaining
the complex co-evolution in which human and
plant species are engaged (Trakhtenbrot and
Perry, 2005; Tchoundjeu et al., 2006; Scheldeman
et al., 2007). This is important in transforming
landscape, by establishing agricultural systems
that can mitigate and adapt to climate change in
the benefit of future generations livelihood
improvement.
Tropical trees germinability revealed their
recalcitrance, high sensitivity to conservation and
significant differences regarding rate and speed
of germination between and within species under
different environments (Lindgren and Wei, 1994;
Kyerehet al., 2001). In addition, light intensity
and temperature are key influential factors for
tropical trees germination (de Souza and Valio,
2001). Furthermore, fruit maturation is an
important physiological parameter that might
determine extents of germination in any plant
species. Even though farmers in SSA successfully
regenerate plant biodiversity, the heterogeneity
of traditional agroforestry systems demonstrates
lack of handling of fruits tree species’
reproductive biology.
In lowlands of West and Central Africa,
African bush mangoes trees are the top priority
food tree species abundant in traditional land use
systems (Shiembo et al., 1996; Tchoundjeu et al.,
2006). They are the most economically important
trees among the seven species of the Irvingiaceae
family that occur in Africa, and are under two
decades-long domestication programme led by
the World Agroforestry Centre. We can clearly
distinguish sweet and bitter fruited bush mango
trees. Okafor (1975) presented those two types
as varieties of a unique species Irvingia
gabonensis: I. gabonensis var gabonensis and I.
gabonensis var excels Okafor, respectively.
However, they were raised up to species level: I.
gabonensis and I. wombolu, respectively (Harris,
1996). Consequently, the taxonomic status within
bush mango trees remains a big issue to be
address (National Research Council, 2006). The
mesocarp of the sweet bush mangoes are edible;
while the endocarp of both bitter and sweet fruits
are important part of African communities’ diets
and is marketed all over the world (Lowe et al.,
2000; Tabuna, 2000; Ekpeet al. 2008).
The Dahomey Gap (the broad savannah inside
the West African forest block) is characterised
by increasing water scarcity with high
temperature and bush mango trees widely found
with a very high variation in the spatial density
pattern (Vihotogbé et al., Unpublished). Farmers
in the region are challenged with: (i) optimising
earlier seeds germination, (ii) obtaining drought-
resistant samplings to face short rain seasons
and increasingly longer dry seasons, and (iii)
establishing uniform and viable plantations.
Germinability of bush mango trees has intensively
been investigated in the eastern part of their
distribution range (Omokaro et al., 1999; Nya et
al., 2000; Nzekwe et al., 2002; Mbakwe, 2004;
Dolor, 2011). Even though Ladipo et al. (1996)
identified easy-to-crack (or self-cracked) seed,
fresh seeds displayed low germinability (Nya et
al., 2000) due to the long dormancy induced by
the highly fibrous seeds (Lesley and Brown,
2004). In general, these studies lack a comparative
basis between bitter and sweet fruited trees, while
seed dormancy and germinability have strong
genetic basis (Fuller and Allaby, 2010).
In the Dahomey Gap where little research is
undertaken on bush mango trees, preservation
of spontaneous trees and transplantation of
spontaneous seedlings remain important
strategies for enriching traditional agroforestry
Germination of seeds from earlier fruits 293
systems. As a solution for intensive cultivation
of bush mango trees, direct sawing of 2 - 3 seeds
in 5 - 10 cm depth holes is adopted. This technique
guarantees high seed germination, though with
evidence of waste of seeds. Hypothesizing earlier
fruits to have low viability, locals establish
plantations in July, while the small rainy season
(September - October) is not sufficient for
guaranteeing viable plantations throughout the
long November - March dry period. Therefore,
non-uniform plantations are common, adversely
affecting small scale farmers’ motivation for
intensive bush trees’ cultivation. Thus,
interventions to optimise early fruit germination
and early establishment of seedlings is
necessary. The objective of this study was to
assess differences in germinability between bitter
and sweet trees and to hypothesize possible
taxonomic implications.
MATERIALS AND METHODS
Study area. This study was conducted in the
Guinean-Congolian rainforest in West Africa. The
Dahomey Gap refers to the mosaic of savannah,
drier type of lowland, fields and fallows found
from Badagry (Nigeria) through Benin and Togo
to Accra in Ghana (Sowunmi, 2007). This low
rainfall and high temperature area splits the West
African Forest into Upper and Lower Guinean
blocks (Salzmann and Hoelzmann, 2005). Among
the tropical rainforest trees that survive this
climatic anomaly, bush mango trees are the most
economically important of the six Irvingiaceae
species occurring in Africa (Asaah et al., 2003).
The study covered the South of Benin and Togo,
the two countries that mostly contribute to the
Dahomey Gap. In this area, bush mango trees are
found in natural forests, forest gardens (in the
Volta Forest Region, South-western Togo) and
most abundantly in intensive cultivation systems:
home gardens, orchards, agroforestry parks all
over in the Dahomey Gap (Vihotogbé et al., 2014).
In general, the peak of fruit production in this
region occurs in March and June, for the bitter
and the sweet fruited trees, respectively
(Vihotogbé et al., 2014). Earlier fruits fall in January
- February and are suspected to have low
germination rate by local farmers who practice
intensive cultivation.
Sampling design and germination
experimentation. Five provenances from
different ecological areas were tested. The
provenances were defined based on their
geographic origin, type of bush mango trees,
current domestication process and type of
agroforestry systems from which the trees were
sampled (Fig. 1, Table 1). Seven to eleven healthy
trees that fruited earlier (in February - March)
were randomly sampled in each provenance. For
each provenance, seven hundred and twenty
healthy and mature fruits of 148 ± 51 g that had
freely fallen down were collected under the
sample trees. The fruit mesocarp was removed
and the seeds sun-dried outside in open air. The
weather parameters recorded by the Beninese
Institute for Agronomic Research indicated 9.7
mm, 35.3 ± 2.16°C and 66 ± 5.2% for total rainfall,
mean temperature and moisture, respectively.
Three replications (each made of a cluster of
30 seeds) of each provenance were sown fresh
(just after removal of mesocarp) and after seven,
fourteen and twenty-one days of sun-drying (Fig.
2). The three replications for all provenances at
each drying level were sown in a completely
randomised design with regard to each
germination condition (sunshine and covered).
In total, three thousand and six hundred seeds
were germinated in polybags filled with compost,
in closed/covered and sun-shine germination
conditions. The germination condition totally
exposed the polybags to sunshine, while in the
closed/covered one, were covered with vegetable
mats, preventing them from intensive light of
February - March 2011. However, this covering
system did not prevent the experiment from
accessing rainfall.
Seeds were sown in 1 cm soil depth in the
polybags watered with 261.7 mm of water (12 mm
every two days until the fortieth day in addition
to the 9.7 mm rain fall during the essay).
Daily observations were made to register the
total number of seeds that germinated within each
replication (cluster of 30 seeds) up to forty days
after sowing (number of day after which no new
germination was obtained). Here, we considered
germination to be the complete emergence of the
seedling, since this is an indicator of successful
plantation establishment.
R. VIHOTOGBÉ et al.
294
Figure 1. Germination rate of bush mango seeds in sun-shining germination condition in Benin. SwS = seeds of sweet
bush mangoes; Bts = seeds of bitter bush mangoes; P= from Pobè; L = from Lomé; C = from Couffo; B = from Badou;
O = germinated in sun-shining condition; second C = covered condition; 0 = fresh; 1, 2, 3 = dried for 1, 2, 3 weeks.
Figure 2. Germination rate of African bush mango trees in close germination condition in Benin. SwS = seeds of sweet bush
mangoes; Bts = seeds of bitter bush mangoes; P= from Pobè; L = from Lomé; C = from Couffo; B = from Badou; O = germinated
in sun-shining condition; second C = covered condition; 0 = fresh; 1, 2, 3 = dried for 1, 2, 3 weeks.
Data analysis. The following computations were
made:
(i) The germination rate (GRr) which is the final
germination percentage until the fortieth day
after sowing:
(1)
Where:
Ngr = total number of seed that germinated in each
replication
Percentage in germination
Germination of seeds from earlier fruits 295
TABLE1. Characteristics of mango provenances tested in the Dahomey Gap in Benin
Type of ABMTsGeographic areas and corresponding GPS position around which Description of the sampled provenances
sampling was done
Sweet trees Couffo: South West BeninLong = 1.77106Lat = 6.75011P1 = Only cultivated trees(in orchards and agroforestry parks) selected throughout decades
in mass selection process for endocarp commercialization
Pobè: South East BeninLong = 2.68083Lat = 6.7896 P2 = Spontaneous trees from thrown seeds after the mesocarp consumption on fields and
home gardens.
Lomé: South TogoLong = 0.91072Lat = 6.53256 P3 = Only cultivated trees on farm without selection process
Badou: South West Togo in the Volta Forest RegionLong = 0.56461Lat = 7.60055 P4 = only spontaneous trees in forest gardens without any selection process
Bitter trees Badou: South West Togo in the Volta Forest RegionLong = 0.57450; Lat = 7.45222 P5 = Only native trees sampled natural forest
Nt= 30 (number of seeds in the replication);
(ii) Germination speed at different germination
rates up to the highest one reached in the
replication: GSi:
........................................................ (2)
Where:
NGi = number of seed corresponding to the ith
considered germination rate;
NDi= number of days required for the ith
germination percent to be reached in the
replication. In this study, the germination
rates considered
Where:
T = 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100%.
For each cluster of the three replications
representing each combination of the different
levels of the four factors studied (type of bush
mango trees, drying level, provenance and
germination condition), we calculated:
(iii) Mean germination rate (mGRrj) which is the
average of the GRr values for the three
replications at the jth combination of the
different levels of the three factors:
.......................... (3)
Where:
GRrjz = GRr value of the zth (z = 1...3) replication in
the jth combination the levels of factors.
(iv) Mean germination speeds at the ten
potential germination rates considered
(mGSij), that is, the average of the GSi values
considering the three replications at the jth
combination of the different levels of the
three factors:
............................. (4)
Where:
R. VIHOTOGBÉ et al.
296
GSijz germination speed at the ith germination rate
in the zth (z = 1...3) replication of the jth
combination the levels of factors. The germination
speed indices, GSi and GSijz, depict the dynamic
aspect in the germination process and as such,
are useful for identifying both categories of seeds
based on means of the germination speeds as
well as uniformity of the germination inside each
provenance.
The influence of the main effects, i.e. the type
of bush mango tree, provenance, drying level and
germination condition on the germination rate,
as well as on the germination speed were assessed
using Analysis of Variance (ANOVA) in a
Generalised Linear Model carried out in Statistica
6 (StatSoft, 2001). Using PAST package (Hammer
et al., 2001), a Principal Component Analysis
(PCA) was carried out on the correlation matrix
of the speed at the 10 considered germination
rates for categorising seeds and identifying the
type of seed that could meet small farmers’
expectations in the Dahomey Gap. The trend in
the germination (integrating the beginning and
total days necessary to accomplish their
germination process) of the different categories
of seed was compared by plotting the first two
axes of the PCA.
RESULTS
Only the drying level showed a strong significant
(P<0.0001) influence on the germination rate.
Fresh seeds had the highest percentages of
germination (98 – 100%). This steadily decreased
when the drying duration increases: (82, 49 and
11% for 7 days, 14-15 days and 21 days of drying,
respectively; Figs. 1 and 2).
Germination speeds significantly depended
on the drying level (P<0.0001) and on the
germination condition (P= 0.001). In the closed
condition, germination started earlier (13-14 days
after sowing) with higher speeds (mean= 1.7± 1.24
seeds day-1; max = 4.5 seeds day-1), than in the
sun-shining condition (mean = 1.32 ± 0.93 seeds
day-1; max = 3.5 seeds day-1) for which the earlier
germination occurred 21 days after sowing. In
addition, the highest speed was found in fresh
seeds (2.9 ± 1.02; max = 4.5) and this decreased
with the increase of the drying duration (1.8 ±
0.62, 1.1 ± 0.3, 0.23 ± 0.08 for 7; 14 and 21 days,
Figure 3. Germination speed of bush mango seeds at different percentages of germination in the sun-shining condition in Benin.
SwS = seeds of sweet bush mangoes; Bts = seeds of bitter bush mangoes; P= from Pobè; L = from Lomé; C = from Couffo; B
= from Badou; O = germinated in sun-shining condition; second C = covered condition; 0 = fresh; 1, 2, 3 = dried for 1, 2, 3 weeks.
Germination of seeds from earlier fruits 297
Figure 4. Speed of germination of bush mango seeds at different percentages of germination in the close condition in Benin. SwS
= seeds of sweet bush mangoes; Bts = seeds of bitter bush mangoes; P= from Pobè; L = from Lomé; C = from Couffo; B = from
Badou; O = germinated in sun-shining condition; second C = covered condition; 0 = fresh; 1, 2, 3 = dried for 1, 2, 3 weeks.
Figure 5. PCA Analysis based on the speeds of germination at different percentages of germination of the different types of seeds:
SwS = seeds of sweet bush mangoes; Bts = seeds of bitter bush mangoes; P= from Pobè; L = from Lomé; C = from Couffo; B
= from Badou; O = germinated in sun-shining condition; second C = covered condition; 0 = fresh; 1, 2, 3 = dried for 1, 2, 3 weeks.
R. VIHOTOGBÉ et al.
298
respectively). No significance effect was found
for provenance and type of bush mango trees
(Figs. 3 and 4).
The first two axis of the PCA accounted for
about 88% of the variations of the germination
speeds of the different categories of seeds
through the 10 different germination rates
considered (Fig. 5). The first axis of the PCA (80%)
presented the highest and positive correlations
with all percentages of germination (Fig. 5). Using
the position of the different types of seed on this
first PCA, we defined groups of seeds
characterised by the quick start as well as the
speed of their germination. The general trend
showed that the position of the types of seed of
the first PCA decreased with the drying duration.
Thus, 5 groups were clearly defined in Figure 5.
The first group (cluster G1: made up of all fresh
seeds of bitter and sweet trees in the close
germination system) started their germination
very quickly and maintained higher germination
speed (3.5 - 4.5 seed per day) to quickly reach the
maximum of germination (95 - 100% of
percentage). The second group (cluster G2) was
composed of all fresh seeds of bitter and sweet
trees, germinated in the sun-shine germination
system. This group started germination and
completed it with high speed; but, lower than the
first group (2.5 - 3.5 seed per day to reach 90 -
100% of percentage). The other three groups
(clusters G3, G4 and G5) started germination late
and completed with lower germination speeds.
The three weeks dried seeds of every provenance
represented the group with worst germination
speed (Fig. 5). Thus, fresh seeds, regardless of
germination condition, type of bush mango trees
and the provenance, had the highest positions
on the first axe indicating that they germinated
faster than any other category of seeds.
DISCUSSION
This study highlighted no valuable discriminative
power of both the germination rate and speed to
distinguish provenances and types of bush
mango trees (Table 1). Therefore, in addition to
the morphological high similarity of bitter and
sweet fruited trees (Harris, 1996), similarity in their
germinability strengthens the uncertainty
regarding their species integrity (National
Research Council, 2006). Because morphological
data available (Harris, 1996; Vihotogbé et al.,
2013) did not cover the total life cycle of bush
mango trees, we suggest a broader and detailed
vegetative evaluation that includes seedling
characterisation, as well as evaluation of earlier
adaptation of seedlings/samplings in different
ecological conditions in order to strengthen
conclusions regarding the taxonomic distinction
of bitter and sweet trees.
Local assumption on the questionability of
the viability for earlier bush mango seeds was
not confirmed in this study. The low germination
percentages obtained by locals using earlier fresh
seeds also appeared in the results of Ewedjê et
al. (2007), who germinated seeds collected during
the pick fruiting time. The sowing depth of more
than 1 cm might presumably be excessive,
jeopardising the completion of the germination
process in both situations. Indeed, direct sowing
in heavy clayey soils, common over the entire
distribution range of bush mango trees, by
farmers might not guarantee high percentage of
germination. As a solution to this, Nzekwe et al.
(2002) recommended a mixture of soil and sawdust
to predict high percentage of germination (80%)
and vigorous seedling. Therefore, sowing fresh
seeds in less depth than 1 cm hole guarantees
better performances (germination rate of 98 -
100%) for any provenance and for both bitter
and sweet trees. Thus, our result demonstrated
that no particular treatment of bush mango seeds;
drying plus rehydration or scarification or
particular chemical-based pre-treatment (Omokaro
et al., 1999; Nya et al., 2006) was necessary to
optimise the germination rate of bush mango
trees. We, therefore, conclude that these usual
dormancy suppression techniques often applied
to recalcitrant seeds are not always necessary,
most importantly when they cannot be promoted
in low income Sub-Saharan African farmers’
condition. In the case of bush mango trees, just
compost or forest soil (affordable by local
nurserymen) holds sufficient water to allow the
highest germination rate of fresh seeds with
healthy and vigorous seedlings for both bitter
and sweet trees under the close germination
condition. In contrast, attempting to dry these
high oil content seeds (up to 70%; Joseph, 1995)
in a tropical high moisture environmental
Germination of seeds from earlier fruits 299
condition exposes them to rapid moulds
colonisation, jeopardising the viability of seeds
after few days of conservation under ambient
conditions (Lowe et al., 2000). Thus, Okafor
(1999) already observed less germinability for pick
time fruits (80%) after two drying days.
Consequently, our results evidenced the
recalcitrance of bush mango seeds as suggested
for many tropical trees species (Kyereh et al.,
2001; Nya et al., 2006).
High temperature, intensive light and
fluctuations of moisture for breaking pioneer
recalcitrant tropical seeds dormancy (Kyereh et
al., 2001), was proven less efficient than steady
maintenance of low temperature, low light and of
humid substrate, regarding the speed of
germination. Both bitter and sweet trees are non-
strict photoblastic taxa and, therefore, seedlings
and samplings might germinate faster under
deeper shade conditions. This confirms the easy
recolonisation of bush mango trees in low land
natural forests (Van Dijk, 1996), as well as their
abundance in human made agrosystems even in
the Dahomey Gap (Shiembo et al., 1996).
Therefore, climate change plays fewer roles in
the low regeneration numerous valuable fruit tree
species in natural stands, over their entire
distribution range, that land use change and seed
or other body parts collection for marketing.
Indeed, the low densities of bush mango trees in
natural areas are typically human driven
consequence of the intensive collection of fruits
for marketing
ACKNOWLEDGEMENT
The International Foundation for Science entirely
funded this research (IFS / Grant No: D/4672-1,
Stockholm, Sweden to Romaric Vihotogbé).
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... The development of the domestication programme for many candidate species is constrained by the taxonomic confusion amongst existing varieties or species. In Irvingia spp., the morphological, agronomical, biochemical, ecological and molecular characterization undertaken in Benin, Nigeria, was mainly aimed at the taxonomic delimitation between the so-called "bitter" and "sweet" varieties for bush mangoes [295,296,315,316]. In the same way, in Cameroon, more than eight species belonging to the Irvingiaceae, including five from the Irvingia genus, were found to be grouped under the common name of "bush mango" and used by the Baka indigenous people [200]. ...
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The variation of the morphological characters of bitter and sweet African bush mango trees (Irvingia species) was investigated in the Dahomey Gap which is the West African savannah woodland area separating the Upper and the Lower Guinean rain forest blocks. African bush mangoes have been rated as the highest priority multi-purpose food trees species that need improvement research in West and Central Africa. A total of 128 trees from seven populations were characterized for their bark, fruits, mesocarp and seeds to assess the morphological differences between bitter and sweet trees and among populations. Multivariate analysis revealed that none of the variables: type of bark, mature fruit exocarp colour, fruit roughness and fresh mesocarp colour, could consistently distinguish bitter from sweet trees in the field. The analysis of the measurements of fruits, mesocarps and seeds demonstrated that bitter fruits have the heaviest seeds and this consistently distinguishes them from sweet fruits. However, the measurements of the fruit, mesocarp and seed did not have a joint effect in grouping types and populations of ABMTs. This indicates high diversity with a potential for selection existing across all phytogeographical regions investigated. The sweet trees of Couffo and those of Dassa in Benin are clearly different from all other populations. This can be attributed to traditional domestication (bringing into cultivation) and climate, respectively. The large fruits and the heavy seeds of the cultivated populations are evidence of successful on-going traditional selection of sweet trees in the Dahomey Gap.
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Seeds from freshly collected mature fruits of Irvingia gabonensis var. excelsa (Fam Irvingiaceae), with an average moisture content of 43%, failed to germinate within 60 days after sowing. Seeds which were slowly dried (under ambient conditions) for 72-84 hrs to a relative moisture content of 17.4-19.6% and subsequently rehydrated for 24 hrs to a relative moisture content of 22-26% gave 100% germination in 31-33 days. Rapid drying of seeds (at 35°C in an incubator) for short durations (up to 60 hrs) was less effective in improving germinability and if prolonged (72 hrs or more) was deleterious to germination. With the excised embryo maximum germinability was achieved within a few days (6-9 days) suggesting that the investing structures might be limiting germination. The technique of slow drying and subsequent rehydration could form the basis of a commercially viable seed-treatment process.
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An investigation on how to improve the germination of the seeds of the bush mango Irvingia gabonensis var. excelsa (= I. Wombulu) was carried out using various soil media in planting depulped and undepulped and, dried and non-dried fruits in polythene bags. The results showed that depulping of fruits, and drying for two days before planting in a 1:1 mixture of sawdust (SD) and forest top soil (TS) enhanced the production of a large quantity of uniform vigorously growing seedlings during the period of investigation. These methods significantly (P £ 0.05) increased seed germination and thus could be used to solve the age long problem of poor germination encountered by growers of the bush mango Irvingia gabonensis var. excelsa. Keywords: Irvingia gabonensis, depulping, germination media, sawdust, topsoil
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As a result of long-term field observations in Nigeria, the author distinguishes 2 varieties in Irvingia gabonensis (Aubry-Lecomte ex O-Rorke) Baill., namely var. gabonensis and var. excelsa (Mildbr.) Okafor comb. et stat. nov.
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A taxonomic revision of the Irvingiaceae is presented. Three genera Desbordesia (1 species), Klainedoxa (2 species) and Irvingia (6 species) are recognised. Standard herbarium characters are not very useful in the identification of the species of Irvingia. Additional field characters and in particular fruit characters facilitate identification.