Quantitative descriptors of variation in the fruits and seedsof
R. R. B. LEAKEY1, J.-M. FONDOUN2, A. ATANGANA3 andZ. TCHOUNDJEU41
Centre for Ecology and Hydrology (CEH), Bush Estate, Penicuik, Midlothian, Scotland, EH260QB, UK (E-mail: RRBL@ceh.ac.uk); 2 Institut de la Recherche Agricole pour le Dveloppement(IRAD), BP 2123, Yaound, Cameroon; 3 Department of Plant Biology and Physiology,University of Yaound I, BP 812, Yaound, Cameroon; 4 International Centre for Research inAgroforestry (ICRAF), BP 2123, Yaound, Cameroon (E-mail: Z.Tchoundjeu@camnet.cm)
Key words: Bush mango, Dika nut, domestication, ideotype
Abstract. Methods were developed to quantify variation in the fruit, nut and kernel traits usingthe fruits from four trees of Irvingia gabonensis, an indigenous fruit tree of west and centralAfrica. The measurement of 18 characteristics of 1632 fruits per tree identified significantvariation in fruit, nut and kernel size and weight, and flesh depth. Differences were also iden-tified in shell weight and brittleness, fruit taste, fibrosity and flesh colour. Relationships betweenfruit size and weight with nut and kernel size and weight were found to be very weak,indicating that it is not possible to accurately predict the traits of the commercially-importantkernel from fruit traits. Seven key qualitative traits are recommended for future assessments ofthe levels of genetic variation in fruits and kernels. These traits describe ideotypes for fresh fruitand kernel production.
Recent work in the humid lowlands of West Africa has established that thereare substantial local and regional markets for the non-timber forest products(NTFP) of certain indigenous trees, such as Irvingia gabonensis (Aubry-Lecomte ex ORorke) (Ndoye et al., 1997). Some NTFPs from Cameroon (e.g.Gnetum leaves, Irvingia kernels) have even entered the international marketand a recent market survey in Europe has emphasised their high value.
Studies on the biological variability of indigenous fruit tree species, theirpropagation using cheap and simple methods appropriate for rural develop-ment projects, and their suitability for domestication have been progressivelyincreasing in West Africa over the last 20 years (reviewed by Leakey et al.,1990; Okafor and Lamb, 1994). In the last five years, the International Centrefor Research in Agroforestry (ICRAF) initiated a coordinated initiative acrossthe Region, based on the priorities of subsistence farmers and national researchscientists (Franzel et al., 1996). This species prioritisation activity involvedfield activities such as farmer preference, product ranking, and the valuationand ranking of priority species (Franzel et al., 1996). From this process, thetop priority species identified for humid West Africa was Irvingia gabonensis
Agroforestry Systems 50: 4758, 2000. 2000 Kluwer Academic Publishers. Printed in the Netherlands.
(Bush mango/Dika nut). Domestication of indigenous fruits through agro-forestry is seen as one of three important issues in the transformation of landuse in Africa (Sanchez and Leakey, 1997), through the establishment of abetter balance between food security and natural resource utilisation. It shouldalso help to alleviate the poverty that drives deforestation (Leakey and Simons,1998) and is an important element of the Woody Plant Revolution (Leakey,and Newton, 1994). To be successful in these terms, domestication has to belinked to commercialisation and market expansion. This has to be done inways that will provide smallholder farmers with socio-economic (Leakey andIzac, 1996), policy (Leakey and Tomich, 1999), and marketing benefits(Leakey, 1999).
In Nigeria, Okafor (1974) identified two varieties of Irvingia gabonensis,one with sweet, edible fruits and the other with bitter fruits. The seed cotyle-dons (kernels) of both varieties are, however, used as a food thickening agent,although those of the latter are more important commercially. Harris (1996)subsequently revised the taxonomy calling the bitter variety Irvingia womboluVermoesen. In Cameroon, the trade in these kernels to Gabon, Nigeria,Equatorial Guinea and the Central African Republic has been valued atUS$260,000 per annum (Ndoye et al., 1997). The kernels are an importantsource of both a polysaccharide, which forms the glutinaceous thickeningagent, and an oil. The oil content of these kernels, however, varies from 51%to 72%, depending on the geographic origin of the fruits (reviewed by Leakey,1999). More knowledge of this sort of variability is needed if marketopportunities are to be expanded in support of domestication.
The geographic range of I. gabonensis is from Nigeria to Congo, while thatof I. wombolu is much greater: Senegal to Uganda (Harris, 1996). In order tocapture much of the genetic variation, germplasm collections were imple-mented during the fruiting seasons of 1994/95 (Ladipo et al., 1996). Thesecollections which were made in collaboration with the authorities and farmersin the host countries, were targeted at visible variation and farmers percep-tions of superiority. They were made at 1030 locations per country in thehumid forest belt of Ghana, Nigeria, Cameroon and Gabon, exchangedbetween countries and established as living genebanks at three sites (Ibadanand Onne in Nigeria; Mbalmayo in Cameroon) in 1995. Each genebankcontains approximately 60 accessions (single progenies).
The germplasm collections are being evaluated by ICRAF and partnersfor growth and phenological traits and will become part of a programme ofgenetic testing, with selection of superior individuals. Molecular studies ofthe extent of genetic diversity in these populations are underway to providea baseline for the domestication programme. These studies by Institute ofTerrestrial Ecology have determined that the centre of diversity for I. gabo-nensis is in southern Cameroon, while that of I. wombolu is in south eastCameroon and western Nigeria (Lowe et al., in press), areas postulated asforest refugia during the last Ice Age. For the latter species, the centre ofdiversity could extend either further east or west into areas not covered
by this study. As would be expected for outbreeding species, the geneticdiversity within progenies was high, while that between populations was low.This information provides a fundamentally important basis on which to builda farmer-oriented cultivar selection programme.
The second phase of the domestication programme is being initiated inNigeria and Cameroon. In these two areas, field teams are working withfarmers to identify, select and multiply superior trees. To date over 2000mature plus-trees have been identified and these are being propagated vege-tatively by air-layering (Tchoundjeu et al., in press). The first rooted marcottsfrom these selected trees are currently in nurseries and will become stock-plants for further multiplication and, subsequently, these cultivars will beestablished in on-farm trials. In addition, village nurseries are being estab-lished so that the participating farmers are the beneficiaries of the programmein accordance with the Convention of Biological Diversity.
Vegetative propagation techniques are being used for the capture andmultiplication of superior phenotypes as putative cultivars. Currently, studiesare in progress in Nigeria and Cameroon to improve existing methods of airlayering, grafting and budding, as these are all means of propagating fromthe mature crowns of selected trees. However, an attractive alternative wouldbe to propagate by cuttings as this avoids grafting incompatibilities andpotentially gives high multiplication rates. Simple, low technology methodshave been developed in Cameroon for juvenile shoots of tropical trees (Leakeyet al., 1990), including Irvingia gabonensis (Shiembo et al., 1996), but it isknown that mature shoots are more difficult as they become physiologicallyand ontogenetically old. Research is needed to try to rejuvenate mature crownshoots physiologically, while retaining the benefits of reproductive capacitydue to ontogenetic age.
ICRAFs tree domestication programme for I. gabonensis is currentlyfocussing much of its resources on the vegetative propagation of trees thatfarmers are identifying as superior. There is at present, however, little knowl-edge about farmers perception of genetic variation or their capacity to identifyand capture the potential for improvement through the formation of cultivarsbased on the propagation of elite trees in a wild population. Evidence fromfarmer interviews suggests that there is substantial variation in certain fruittraits, such as fruit size and the taste of fresh fruit pulp, as well as somevariation in the size of the commercially-important kernels. Attempts toidentify elite trees are, however, subject to both the cooperation of farmersand their willingness to disclose knowledge of local trees. The most commonlyavailable information is about variation in fruit sweetness, but this usuallylimited to the differentiation into two classes: sweet and bitter. Farmersindicate that kernel size is important on the market, although there is as yetno indication if this is recognized by price discrimination. It seems to begenerally considered that large kernels will be found in large fruits.
The purpose of the present study was to develop a methodology for thelater quantitative characterization of tree-to-tree variation in I. gabonensis
fruits in Cameroon and Nigeria. This assessment of genetic variation is aimedat the determination of: the level of diversity available to farmers within the area of their communal
ownership; how farmers evaluate superiority in their trees; farmers concepts of genetic variation in fruit traits and their improve-
ment; how farmers criteria for selection matches with recognized horticultural
traits of fruit quality, yield, and chemical composition; the levels of selection intensity being applied by farmers; and the relationships between the variability of fruit/kernel traits and market
Methods and materials
Fruits were collected from four I. gabonensis trees in August 1998 fromdifferent locations in Cameroon (Table 1). Fruits were scarce this year as manytrees were not fruiting or fruiting poorly. Consequently, a sampling proce-dure was only tested in the case of Tree 2 Ngalan, where the area below thetree crown was divided into four quadrants. Eight ripe, but undamaged, fruitswere collected from each quadrant at a point 2/3 of the distance from the trunkto the edge of the crown along transects at 90 from each other. The meanweight of 5, 10, 15, 20, 25, 30 fruits from this sample were compared withthat of a larger sample of 90 fruits in order to determine optimal sample size.
On the day of collection, the fruits were weighed using a small portablekitchen scales graduated to 2 g, and measured using calipers graduated to0.1 mm. Fruit flesh depth was measured by a spike attached to the samecalipers. The spike was inserted until it hit the hard nut at the centre of thefruit. The external measurements of the fruits were made in three dimensions(length, width and thickness) and the flesh depth measurements were madeon both sides of the fruit in these three dimensions (Figure 1).
The next day, the fruits were depulped to expose the endocarp (nut) and arecord made of fruit taste (scored 1[bitter]-5 [sweet]), colour (yellow ororange) and fibrosity (scored 1 [nonfibrous]-5 [fibrous]). The nuts were thenleft to dry for seven to 10 days until the remnants of flesh had dried. They
Table 1. Locations of I. gabonensis trees that provided fruits for this study.
Tree no. Village Farmer No. of fruits Latitude Longitude
1 Ngomedzap Market 16 316
11111 Nkoevos Deux Moise Nkomo 22 256 11261 Ngalan Odile Essong 26 254 11122 Ngalan Odile Essong 32 254 1112
were then weighed and measured using the calipers in the same three dimen-sions as the fruits. The nuts were then carefully broken open so that the seeds(kernels) could be extracted. The fresh kernels were then weighed using alaboratory balance (Mettler Toledo PB 3002) and measured in the same threedimensions using the calipers. The weight of the nutshell was determined bythe difference between the nut and kernel weights, and a shell brittlenessfactor derived as 50/shell weight. Statistical analyses of the weight and lengthdata were done using Microsoft Excel 97, and skewness was calculated usingGENSTAT 5 Version 4.1.
A comparison of the standard errors of the mean values of the first 5, 10, 15,20, 25, and 30 fruits from the 32 fruit sample from Tree 2 at Ngalan, foundthat the mean weight of a sample of 20 or more fruits differing by less thanthe standard error (Table 2). The weights of the 30 individual fruits forminga sample were normally distributed (skewness = 0.02).
Figure 1. Diagram of the positions of fruit, endocarp (nut) and seed (kernel) measurement(Lg = length [T= top, B= bottom]; W = width [L = left, R = right]; Th = Thickness [F = front,B = back]).
Fruit weight and size
Mean fruit weight, fruit length, fruit width and fruit thickness differed sig-nificantly between trees (Table 3). Fruits from Tree 1 at Ngomedzap weregreater in length than they were in width or thickness, while those of Tree 1at Nkoevos Deux were greater in width than they were in length or thickness(Table 3). The overall relationship between fruit weight and fruit lengthwas linear (r2 = 0.923), as it was in trees from Ngalan and Nkoevos Deux(Table 4). Similar relationships were found between fruit weight and fruitwidth or thickness.
Table 2. Effect of sample size on the mean weight and standard error of I. gabonensis fruitsfrom Tree 2 at Ngalan.
Sample size Mean fruit weight (g) Standard error Skewness
5 fruits 44.0 4.7 0.7610 fruits 44.7 2.5 0.8115 fruits 43.7 2.0 0.5020 fruits 42.7 1.8 0.2925 fruits 42.7 1.6 0.4430 fruits 43.9 1.6 0.02
Table 3. Tree-to-tree variation in fruit, nut and kernel weight (g) and size (mm) in Irvingiagabonensis.
Village Ngomedzap Nkoevos Deux Ngalan: Tree 1 Ngalan: Tree 2
Mean SE Mean SE Mean SE Mean SE
Fruit weight 103.8 1.40 58.2 2.88 36.3 2.29 43.3 1.62Fruit length 058.4 0.51 47.0 0.81 42.4 0.90 43.1 0.53Fruit width 056.6 0.30 55.6 0.99 44.2 1.09 45.4 0.53Fruit thickness 049.4 0.42 46.8 0.93 36.2 0.79 42.3 0.63Nut weight 011.9 0.55 07.7 0.42 06.0 0.36 06.0 0.20Nut length 042.0 0.61 40.2 0.86 31.7 0.71 31.7 0.39Nut width 033.3 0.74 36.9 0.74 30.0 0.94 27.9 0.38Nut thickness 022.2 0.35 19.3 0.46 19.0 0.63 19.0 0.28Kernel weight 003.5 0.14 03.1 0.19 03.3 0.30 01.8 0.12Kernel length 032.0 0.55 26.5 0.73 25.5 0.59 23.1 0.41Kernel width 021.4 0.72 22.0 0.68 21.8 1.06 18.7 0.45Kernel thickness 011.6 0.35 11.3 0.26 12.5 0.43 10.1 0.22Nut shell weight 008.5 0.19 04.6 0.24 02.6 0.53 04.2 0.14
Nut weight and size
Nut weight was significantly different between trees from the different villages(Table 3), with those from Ngomedzap being on average twice as he...