West African Yam Research Articles

The biocultural heritage and changing role of indigenous yams in the Republic of Guinea, West Africa

Societal impact statementGlobal yam production is centred on West Africa, but there are significant knowledge gaps about farm‐level diversity across much of the region, and especially in Guinea. Although yam production is increasing in Guinea, in the longer term, varietal diversity and the sustainability of agri‐systems are at risk. Documentation of local crop diversity is essential as a baseline to understand trajectories of past and future varietal loss. This study utilises interdisciplinary approaches, which are needed to help understand the ways historic crop diversity is created and maintained within indigenous agricultural and food heritage systems, as well as the reasons for its loss over time.Summary Yams are important staple foods in many tropical and sub‐Saharan countries. The ‘yam belt’ extends from Guinea to western central Africa. However, yam cultivation is comparatively little researched or documented in Guinea, and the country is commonly not included within descriptions of key yam growing areas in Africa. Our study utilises ethnobotanic methods and plant specimen collections to fill these gaps in West African yam research. Interviews with over 70 farmers from six villages across the Kankan region of Guinea provide information on yam cultivation and diversity, and changes in living memory over the past 40 years. We present the analyses of ethnobotanical data from Haute‐Guinée on yam cuisine, and commercialisation, and on changes to the range of varieties grown over time. The annual cycle of indigenous yam agri‐systems remains a key part of rural life, food systems and economy. However, interviews revealed dramatic temporal changes over the last 40 years. Although yam production has increased, a narrow range of commercial cultivars is currently displacing the historically‐rooted local diversity. The expansion of yam cultivation is regarded locally as having relied on a shift to more unsustainable and extensive land use, and with herbicides and chemical fertilisers replacing intensive organic soil management. This has implications for research and development in sub‐Saharan agriculture for yams and other important native West African crops in the future.

Pigeonpea (Cajanus cajan) and white yam (Dioscorea rotundata) cropping system: Improved resource use and productivity in Ghana

Yam production along the West African yam belt is challenged with deteriorating soil fertility and unavailability of stakes, resulting in decreased yam productivity, and farmers' livelihood. This study evaluated resource use and yam productivity in pigeonpea-yam cropping systems in Ghana's forest and forest-savannah transition zones from 2017 through 2019 cropping seasons. Pigeonpea was established either in an alley or as a border during the 2017 cropping season, while yam was cultivated in 2018 and 2019. A split-plot design of cropping system (yam planted in alleys of pigeonpea—PA; yam planted with pigeonpea as a border—PB and sole yam) as main-plot treatments and chemical fertilizer (0–0–0; 23–23–30; 45–45–60 N-P2O5-K2O kg ha−1) as subplot treatments were used for the study. Productivity data on pigeonpea and yam were collected. The results revealed significantly higher leafy biomass and correspondent higher N content and N due to fixation in PA fields than PB fields in both locations and seasons. The presence of the pigeonpea and its biomass resulted in a significant suppression of ridge erosion and weeds, while soil moisture and nutrients improved, resulting in increased yam tuber productivity than in sole yam production. Further, planting yam with pigeonpea and half (23–23–30 N-P2O5-K2O kg ha−1) the recommended fertilizer rate significantly improved tuber yield and productivity than planting sole yam with full recommended fertilizer level in both locations and seasons. Promoting and adopting the pigeonpea-yam cropping system could sustain soil fertility, provide readily available stakes to address the constraint of deforestation and land degradation associated with yam production.

Water Yam (Dioscorea alata L.) Growth and Tuber Yield as Affected by Rotation and Fertilization Regimes across an Environmental Gradient in West Africa

Yam (Dioscorea spp.) is a staple food crop and a source of income for millions of people in West Africa. Traditionally, in West Africa, yams are grown without any external inputs, leading to low tuber yields. The rapid decrease of tuber yield observed after the first yam cropping season has been ascribed to nutrient depletion and/or to the accumulation of yam-specific pests and diseases. This has led farmers to grow yam on new surfaces under fallow each year. Using a transdisciplinary approach, we identified different yam-based rotations and fertilization regimes that could stabilize yam production in rotational cropping systems and improve water yam (D. alata) productivity. These innovations were tested in researcher-managed field trials established along an environmental gradient crossing four yam growing zones spanning from the Humid Forest (Liliyo in Côte d’Ivoire) to the Derived Savanna/Forest Transition (Tiéningboué in Côte d’Ivoire), the Southern Guinean Savanna (Midebdo in Burkina Faso), and the Northern Guinean Savanna (Léo in Burkina Faso) between 2016 and 2018. The fertilization factor implemented at each site included a control with no fertilization (NON), sole mineral fertilization as NPK (MIN), combined organic and mineral fertilization (MINORG) and sole organic fertilization as manure (ORG), while the rotation factor included water yam in rotation with cereal (YamCer), legume (YamLeg), and white yam (YamYam). The average water yam tuber yields were 32.8, 20.3, 2.7, and 2.5 t fresh matter ha−1 in 2016, and 16.4, 10.7, 8.9, and 5.2 t fresh matter ha−1 in 2018 in Liliyo, Tiéningboué, Midebdo, and Léo, respectively. The most important determinants of tuber yields were the total amount of rainfall recorded during the yam growing period and between tuber initiation and maximum canopy development, and the soil carbon stocks in the 0–30-cm layer. We confirmed in this study that soil surface coverage measured between 70 and 98 days after planting was an early indicator of tuber yield. Fertilization impacted positively the soil surface cover but had a weak impact on tuber yields. Rotation had no impact on either the soil surface cover or tuber yields. This lack of observable impacts was partly due to the very large variability of tuber yields, to the variable rainfall, and to an anthracnose attack in two sites in 2018. The impacts of fertilization and rotation on yam yields should be studied over longer periods. This is, to our knowledge, the first publication showing the relative impacts of site-specific properties (rainfall and soil carbon stocks) versus management practices on water yam yield along an environmental gradient going across the West African yam belt.

Genetic parameter estimation and selection in advanced breeding population of white Guinea yam

ABSTRACT White Guinea yam (Dioscorea rotundata Poir.) is an important tuber crop grown extensively in tropical regions of West African yam belt. Tuber yield, dry matter content, and tolerance to yam mosaic virus are key traits used for identification and selection of superior varieties for commercial deployment. In this study, we estimated genetic parameters for fresh tuber yield, tuber dry matter content, and quantitative field tolerance to yam mosaic virus in 49 clones grown in multi-environment trials (METs). We conducted genomic prediction involving 6337 single nucleotide polymorphisms (SNPs) and phenotypic field evaluation of data collected on the three traits from four sites. Additive genetic and non-genetic factors contributed significantly to phenotypic variation of studied yam traits in METs but to varying degrees. The non-genetic effects were relatively high for most of the measured traits. Narrow-sense heritability values were low (<0.30) for all studied traits. Further analysis of the performance of the clones at test sites with additive main effects and multiplicative interaction (AMMI) analysis exhibited significant genotype by environment interactions (GEI) for the three traits. The AMMI identified TDr10/00412, TDr11/00055, and TDr09/00135 clones with lowest mean trait stability index and outstanding performance for fresh tuber yield (t ha−1), tuber dry matter, and mosaic virus resistance across sites. The elite clones identified could serve as useful source of alleles for the genetic improvement of the crop and possibly considered for release to farmers.

English

Yam (Dioscorea spp.) cultivation has the potentials to greatly contribute to poverty alleviation and food security, in Cameroon. The full production potentials of yams have not been exploited, leaving Cameroon with an annual production of 648,407 metric tons (MT) at the sixth position, among the six countries of the West African yam zone, with 67.3 million MT. This review highlights research gaps in the yam production chain, which can be exploited to enhance production in the country. Subsistent yam cultivation takes place in all five agro-ecological zones of the country. Although with many fluctuations, yield and production quantities have recorded a marginal net increase, since 1961. Cameroon has nine cultivated and 17 wild species, exploited by Baka pigmies for food, but there is no established genebank, thereby exposing the genotypes to genetic erosion. Cultivated species are both indigenous and exotic, and traditional seed systems (sorting, junking, and milking) are exploited for seed procurement. Minisett technology is also gaining grounds. Yam processing is very limited, and, coupled with poor conservation facilities, contributes to elevated post-harvest losses. The yam marketing system is poorly organized, and hinders farmers from reaping optimum benefit from the activity. Other major constraints to yam production include high labour demand, pests and diseases, absence of improved seeds and research neglect. There is the need for concerted efforts involving all stake holders in the yam production chain to enhance yam production in Cameroon. Key words: Review, yam (Dioscorea spp.), production, Cameroon. &nbsp

PCR-DGGE Analysis: Unravelling Complex Mixtures of Badnavirus Sequences Present in Yam Germplasm.

Badnaviruses (family Caulimoviridae, genus Badnavirus) have emerged as serious pathogens especially affecting the cultivation of tropical crops. Badnavirus sequences can be integrated in host genomes, complicating the detection of episomal infections and the assessment of viral genetic diversity in samples containing a complex mixture of sequences. Yam (Dioscorea spp.) plants are hosts to a diverse range of badnavirus species, and recent findings have suggested that mixed infections occur frequently in West African yam germplasm. Historically, the determination of the diversity of badnaviruses present in yam breeding lines has been achieved by cloning and sequencing of polymerase chain reaction (PCR) products. In this study, the molecular diversity of partial reverse transcriptase (RT)-ribonuclease H (RNaseH) sequences from yam badnaviruses was analysed using PCR-dependent denaturing gradient gel electrophoresis (PCR-DGGE). This resulted in the identification of complex ‘fingerprints’ composed of multiple sequences of Dioscorea bacilliform viruses (DBVs). Many of these sequences show high nucleotide identities to endogenous DBV (eDBV) sequences deposited in GenBank, and fall into six monophyletic species groups. Our findings highlight PCR-DGGE as a powerful tool in badnavirus diversity studies enabling a rapid indication of sequence diversity as well as potential candidate integrated sequences revealed by their conserved nature across germplasm.

West African yam food technologies: prospects and impediments to change

Yam is an important commodity for more than 350 million people in West Africa because of its food, monetary and cultural values. The commodity is expensive compared with alternative starchy staples such as maize and cassava because of low technologies used in its production and postharvest handling. Yam storage techniques are rudimentary and redundant but vary from place to place depending on a range of circumstances including agro-ecology. Yam food is prepared mostly from fresh tuber by simple methods that have not changed over time and consumption is also simple because those yam foods can be eaten straight without a condiment. Compared with cassava the relative long postharvest shelf life; relative ease of food preparation from fresh tuber; and non-conventional uses of yam constitute disincentives for yam processing. Processed yam food is inferior substitute for the yam’s fresh alternative; at the same time processed yam product is more expensive than processed cassava and grain products that are its substitutes.

A Sequence-Independent Strategy for Amplification and Characterisation of Episomal Badnavirus Sequences Reveals Three Previously Uncharacterised Yam Badnaviruses.

Yam (Dioscorea spp.) plants are potentially hosts to a diverse range of badnavirus species (genus Badnavirus, family Caulimoviridae), but their detection is complicated by the existence of integrated badnavirus sequences in some yam genomes. To date, only two badnavirus genomes have been characterised, namely, Dioscorea bacilliform AL virus (DBALV) and Dioscorea bacilliform SN virus (DBSNV). A further 10 tentative species in yam have been described based on their partial reverse transcriptase (RT)-ribonuclease H (RNaseH) sequences, generically referred to here as Dioscorea bacilliform viruses (DBVs). Further characterisation of DBV species is necessary to determine which represent episomal viruses and which are only present as integrated badnavirus sequences in some yam genomes. In this study, a sequence-independent multiply-primed rolling circle amplification (RCA) method was evaluated for selective amplification of episomal DBV genomes. This resulted in the identification and characterisation of nine complete genomic sequences (7.4–7.7 kbp) of existing and previously undescribed DBV phylogenetic groups from Dioscorea alata and Dioscorea rotundata accessions. These new yam badnavirus genomes expand our understanding of the diversity and genomic organisation of DBVs, and assist the development of improved diagnostic tools. Our findings also suggest that mixed badnavirus infections occur relatively often in West African yam germplasm.

THE CIVILIZATIONS OF THE YAM: INTERRELATIONSHIPS OF MAN AND YAMS IN AFRICA AND THE INDO‐PACIFIC REGION

Τ Τ appears at first sight remarkable that the term le civilization de Tignarne -*should have been used independently by different authors to describe two farseparated areas of human culture. In West Africa the term was first used by Miege (1954) to describe the more easterly part of the forest zone of West Africa, between the Bandama River in central Ivory Coast and the Cameroon Mountain chain ; and between the drier parts of the savanna, around 8°-io° N. and the sea with its bordering swamps and lagoons. This area, also described as the West African Yam Zone (Coursey, 1965, 1967) depends for its staple food supplies on root and tuber crops, especially yams (Dioscorea spp.), in contrast to the areas to the north, where sorghums and millets provide the major staples, and to the west, where upland rice (Oryza glaberrima Stapf.) is dominant. The identical term has been used in the Pacific Ocean context (Haudricourt, 1964) to refer to at least parts of that area, based on Papuo-Melanesia but extending far out into the Pacific Ocean, where yams are major staple foods. The concept has been developed by Barrau (1963, 1965), the delimitations of this area of the Indo-Pacific being essentially as indicated by him. There are many differences between these two areas of the world, which outweigh the similarities. Nevertheless, on closer examination the application of this term to both is less surprising than it seems at first ; for in both the basis of the civilization is, in fact, the cultivation of yams. The introduction of new crops, especially sweet potato, Ipomoea batatas (L.) Lam. and cassava (Manihot esculenta Cranz.), and others of American origin, in recent times, has reduced the dominance of yams as the material basis of the subsistence of the people, but in former times in some places within living memory yams provided substantially the greater part of the staple food. This was reflected in both these areas in the large volume of ritual and magico-religious practices associated with almost every aspect of yam cultivation and storage. These civilizations were then based on the yam not only in a material sense ; the crop was also fundamental to the cultural life of the people, to at least as great a degree as were the grain crops in Mediterranean and Asiatic cultures. To those who have received their scientific education within the framework of the cultural heritage of Europe and the Mediterranean world as have virtually all

The prevalence of badnaviruses in West African yams (Dioscorea cayenensis-rotundata) and evidence of endogenous pararetrovirus sequences in their genomes

Yam (Dioscorea spp.) is an important vegetatively-propagated staple crop in West Africa. Viruses are pervasive in yam worldwide, decreasing growth and yield, as well as hindering the international movement of germplasm. Badnaviruses have been reported to be the most prevalent in yam, and genomes of some other badnaviruses are known to be integrated in their host plant species. However, it was not clear if a similar scenario occurs in Dioscorea yam. This study was conducted to verify the prevalence of badnaviruses, and determine if badnavirus genomes are integrated in the yam genome.Leaf samples (n=58) representing eight species of yam from global yam collections kept at CIRAD, France, and 127 samples of D. rotundata breeding lines (n=112) and landraces (n=15) at IITA, Nigeria, were screened using generic badnavirus PCR primers. Positive amplification of an expected ca. 579bp fragment, corresponding to a partial RT-RNaseH region, was detected in 47 (81%) of 58 samples analysed from CIRAD collections, and 100% of the 127 IITA D. rotundata samples. All the D. cayenensis and D. rotundata samples from the CIRAD and IITA collections tested PCR-positive, and sequencing of a selection of the PCR products confirmed they were typical of the genus Badnavirus. A comparison of serological and nucleic acid techniques was used to investigate whether the PCR-positives were sequences amplified from badnavirus particles or putative endogenous badnavirus sequences in the yam genome. Protein A sandwich-enzyme-linked immunosorbent assay (PAS-ELISA) with badnavirus polyclonal antisera detected cross-reacting viral particles in only 60% (92 of 153) of the CIRAD collection samples analysed, in contrast to the aforementioned 81% by PCR. Immunosorbent electron microscopy (ISEM) of virus preparations of a select set of 16 samples, representing different combinations of positive and negative PCR and PAS-ELISA results, identified bacilliform particles in 11 of these samples. Three PCR-positive yam samples from Burkina Faso (cv. Pilimpikou) were identified in which no viral particles were detected by either PAS-ELISA or ISEM. Southern hybridisation results using a yam badnavirus RT-RNaseH sequence (Gn155Dr) as probe, supported a lack of badnavirus particles in the cv. Pilimpikou and identified their equivalent sequences to be of plant genome origin. Probe Gn155Dr, however, hybridised to viral particles and plant genomic DNA in three D. rotundata samples from Guinea. These results represent the first data demonstrating the presence of integrated sequences of badnaviruses in yam. The implications of this for virus-indexing, breeding and multiplication of seed yams are discussed.

English

Implementation of pollen genebanks allows the conservation of plant genetic resources at the haploid level, pollen genetic manipulations, scheduling of hybrid seed production and improvement of breeding efficiency. To establish pollen storage protocols&nbsp;for various genotypes of West African yams,&nbsp;laboratory experiments were conducted on fresh pollen and pollen stored under various conditions&nbsp;at the Genetic Resources Unit International Institute of Tropical Agriculture Ibadan, Nigeria (IITA). The storage treatments examined were air-dried storage, freeze-drying followed by storage in liquid nitrogen and hermetic cold storage without previous drying (&ldquo;wet-cold&rdquo; storage). Pollen maintained under dry conditions (dry-air and freeze-dried) maintained aceto-carmine stainability up to 400 days but drastically lost germination capacity, most notably under dry-air storage. But pollen samples maintained at &ldquo;wet-cold&rdquo; conditions under &minus;80&deg;C retained germination capacity after 2 years. The results provide evidence to suggest recalcitrance of yam pollen grains in view of poor pollen survival with drying. Therefore, the &ldquo;wet-cold&rdquo; storage procedure appears to be the most promising method for the sustainable implementation of yam pollen genebanks. Pollen storage protocols for active and base collections were proposed, based on these findings. &nbsp; Key words:&nbsp;Yams, pollen genebank, preservation protocols.

Sequence diversity among badnavirus isolates infecting yam (Dioscorea spp.) in Ghana, Togo, Benin and Nigeria

We analysed the sequence diversity in the reverse transcriptase (RT)/ribonuclease H (RNaseH) coding region of 19 badnavirus isolates infecting yam (Dioscorea spp.) in Ghana, Togo, Benin, and Nigeria. Phylogenetic analysis of the deduced amino acid sequences revealed that the isolates are broadly divided into two distinct species, each clustering with Dioscorea alata bacilliform virus (DaBV) and Dioscorea sansibarensis bacilliform virus (DsBV). Fourteen isolates had 90-96% amino acid identity with DaBV, while four isolates had 83-84% amino acid identity with DsBV. One isolate from Benin, BN4Dr, was distinct and had 77 and 75% amino acid identity with DaBV and DsBV, respectively, and may be a member of a new badnavirus species infecting yam in West Africa. Viruses of the two main species were present in Ghana, Togo and Benin and were observed to infect both D. alata and D. rotundata indiscriminately. This is the first confirmed report of DsBV infection in yam in Ghana and Togo. The results of this study demonstrate that members of two distinct species of badnaviruses infect yam in the West African yam zone and suggest a putative new species, BN4Dr. We also conclude that these species are not confined to limited geographic regions or specific for yam host species. However, the three badnavirus species are serologically related. The sequence information obtained from this study can be used to develop PCR-based diagnostics to detect members of the various species and/or strains of badnaviruses infecting yam in West Africa.

First Report of Cucumber mosaic virus in Yams (Dioscorea spp.) in Ghana, Togo, and Republic of Benin in West Africa.

Yam (Dioscorea spp., family Dioscoreaceae) is one of the most important food crops cultivated in the West African yam zone comprising the forest and savannah areas of Nigeria, Ghana, Côte d'Ivoire, Republic of Benin, and Togo, which account for more than 90% of the 4.59 million ha of yam cultivation worldwide (1). A survey was conducted in 2005 to document viruses in yams in Ghana, Togo, and the Republic of Benin. Samples (1,405) from five species of yam showing mosaic, chlorosis, and stunting as well as asymptomatic plants were tested for Dioscorea bacilliform virus (DBV, genus Badnavirus), Yam mosaic virus (YMV, genus Potyvirus), and Yam mild mosaic virus (YMMV, genus Potyvirus), the three most common viruses infecting yams. In addition, samples were tested for Cucumber mosaic virus (CMV), since CMV was previously reported to infect yams in Côte d'Ivoire (2) and Nigeria (3). In protein-A sandwich-ELISA with polyclonal antibodies to a cowpea isolate of CMV, 23 of the 1,405 samples (6 of 218 samples from Togo, 13 of 628 samples from Ghana, and 4 of 559 samples from Republic of Benin) tested positive for CMV. The CMV-positive samples were from D. alata (N = 16) and D. rotundata (N = 7), whereas all samples from D. cayenensis, D. dumetorum, and D. bulbifera tested negative. CMV was detected as mixed infections with DBV, YMV, or YMMV in 21 of 23 samples. Some of these samples showed puckering, chlorosis, mottling, and crinkling, whereas some plants infected by two or more viruses were asymptomatic. Only two samples from D. rotundata had a single infection of CMV and they showed mild chlorotic symptoms in young leaves that were inconspicuous in mature leaves. In sap inoculations, the virus induced systemic mosaic in Nicotiana glutinosa. The presence of CMV in ELISA-positive yam samples was further confirmed by immunocapture-reverse transcription (IC-RT)-PCR using CMV antibodies as trapping antibody and oligonucleotide primers specific for a 485 nt corresponding to 3' end of the coat protein gene and C-terminal noncoding region of RNA-3 (4). To confirm the specificity of IC-RT-PCR, the 485-bp amplicons from an isolate from the Republic of Benin was cloned into pCR2.1 (Invitrogen, Carlsbad, CA) and three independent clones were sequenced from both orientations. Pairwise comparison of a consensus sequence (Accession No. EU274471) with corresponding sequences of other CMV isolates deposited in GenBank showed 99% identity at the nucleotide sequence level (Accession No. U22821) and revealed that the CMV isolate from yam belongs to sub-Group IA. To our knowledge, this is the first report of CMV infection in yams (D. alata and D. rotundata) in Ghana, Togo, and the Republic of Benin. Together with a previous documentation of CMV in D. alata and D. trifida in Côte d'Ivoire and Nigeria (2,3), this report adds to existing knowledge on distribution of CMV in yams with implications for yam production and germplasm distribution in the West Africa Region.

Development of West African yam Dioscorea spp. core collection

Yams (Dioscorea spp.) are important crops in some West African locations. The West African yam collections held at the IITA were characterized using the standard descriptor list for this crop (IPGRI/IITA. 1997. Descriptors for yam (Dioscorea spp.). International Institute of Tropical Agriculture, Ibadan, Nigeria/International Plant Genetic Resources Institute, Rome, Italy) to assess the extent of diversity and develop a core collection employing 77 out of 86 descriptors and the Shannon–Weaver diversity index. The core collection consists of 391 accessions (13% of entire collection). It represents all the six cultivated and two wild Dioscorea taxa. The appropriateness of the procedure was confirmed by comparing the mean and diversity distributions of 11 (out of 13) quantitative traits. This article explains the relevance of this core collection of yams for West Africa yam cropping and improvement.

Development of a West African yam Dioscorea spp. core collection

Development of a West African yam Dioscorea spp. core collection

A comparative assessment of molecular marker assays (AFLP, RAPD and SSR) for white yam (Dioscorea rotundata) germplasm characterization

SummarySeveral DNA‐based marker systems are available for genetic fingerprinting of plants but information on their relative usefulness for yam germplasm characterisation is lacking. The efficiency of RAPD, AFLP and SSR markers for the assessment of genetic relationships, and for cultivar identification and discrimination among 45 West and Central African white yam cultivars belonging to 22 morphotypes/cultivar groups was investigated. Dendrograms were produced based on band pattern scores using the UPGMA method. Results showed that each of the three techniques could unequivocably identify each cultivar, but that techniques differed in the mean number of profiles generated per primer (or primer pair) per cultivar, referred to as genotype index (GI). The order of merit based on this criterion in this study was AFLPs (GI = 2.56), SSRs (GI = 0.39) and RAPDs (GI = 0.35). Yam genotypes classified in the same cultivar group based on morphology were often genetically different, emphasising the need for molecular fingerprinting in yam germplasm characterisation. AFLPs showed the highest efficiency in detecting polymorphism and revealed genetic relationships that most closely reflected morphological classification.

Storage of West African yam (Dioscorea spp.) seeds: modelling seed survival under controlled storage environments

Survival data was collected from seeds of two yam species, D. dumetorum and D. togoensis stored at moisture contents ranging from 3% to 25% under temperatures ranging from 5°C to 55°C for 400 days (13 months). The data was subjected to probit analysis to estimate seed longevity under each of the seed storage treatment combinations. The relationship between seed longevity and storage environment was evaluated by a seed viability equation whose constants were determined by linear regression modelling. Seed viability constants estimated for D. dumetorum were K E = 5.859, C W = -3.060, C H = -0.0007, C Q = -0.0007 and for D. togoensis were K E = 4.505, C W = -1.646, C H = -0.020, C Q = -0.00011. R-values of the data fit to the equation were approximately 0.92 for D. dumetorum seeds and 0.87 for D. togoensis seeds. The estimated constants are recommended for monitoring the viability of seed lots of the species in gene banks operating controlled storage facilities within the range of the experimental storage conditions.

The Dagara and their Neighbors (Burkina Faso and Ghana)

Today, Dagara settlements can be found on both sides of the Black Volta River (Mouhoun), roughly between the 11°20' and 10° parallels. The international boundary between Ghana and Burkina Faso divides a Dagara-speaking population which should soon reach a million persons. However, given ethnic categories have been suppressed in the population censuses of both Burkina Faso and Ghana since the 1960s, this can only be a rough estimate. The region lies within the Sudanic vegetation belt of the West African savannah, where millet, sorghum, corn and yams are grown as the main staples. Migrant labour has been an important economic factor in the region from early colonial times up to the present day, with many Dagara working the gold mines and plantations of southern Ghana and the Ivory Coast. This bibliography was revised and updated in June 2000. While literature on the Dagara should be more or less complete, titles on the neighboring societies such as the Lobi, Birifor, Dyan, Phuo (Pougouli), Bwaba, and Sisala, may not be, as they merely reflect the specific research interests of the project members. The scope of the bibliography is not so much governed by ethnic criteria but rather by a more or less well defined regional frame. We do not aspire to compile an exhaustive bibliography on stateless gur- or voltaic-speaking societies; therefore one will not find - to cite just one example - Meyer Fortes’s writings on the Tallensi. On the other hand works on Kong, Wa, the Zaberma and the regional Dyula trade are included because these states or groups had some influence on the Dagara and their immediate neighbors. However, literature on regional powers such as the Mossi, Mamprusi or Gonja states are not included as they were of no or only very indirect relevance for the history of our research region.

Characterisation of starches from West African yams

Starches from four varieties of West African yams were extracted and characterised. The physicochemical properties investigated (granule size and morphology, amylose content, crystal form, gelatinisation and pasting behaviour) depended strongly on the yam variety. The starch granules extracted from water yam (Dioscorea alata), white yam (D rotundata) and yellow yam (D cayensis) varieties showed mononodal particle size distributions centred between 31 and 35 µm, while the bitter yam (D dumetorum) exhibited a binodal size distribution of starch granules centred at 4.5 and 9 µm. Light microscopy confirmed the variation in starch granule size and shape with yam variety. The X-ray diffractogram of yellow yam was of the B type, while bitter yam showed an A pattern. The starches extracted from the white and water yams were of the intermediate C-type patterns. The temperatures of onset of gelatinisation were derived from DSC and RVA measurements; values of 69.4 and 75.0 °C for the yellow yam, 71.5 and 78.2 °C for the white yam, 76.5 and 79.8 °C for the water yam and 78.1 and 83.1 °C for the bitter yam were obtained. © 1999 Society of Chemical Industry

Characterisation of starches from West African yams

Starches from four varieties of West African yams were extracted and characterised. The physicochemical properties investigated (granule size and morphology, amylose content, crystal form, gelatinisation and pasting behaviour) depended strongly on the yam variety. The starch granules extracted from water yam (Dioscorea alata), white yam (D rotundata) and yellow yam (D cayensis) varieties showed mononodal particle size distributions centred between 31 and 35 µm, while the bitter yam (D dumetorum) exhibited a binodal size distribution of starch granules centred at 4.5 and 9 µm. Light microscopy confirmed the variation in starch granule size and shape with yam variety. The X-ray diffractogram of yellow yam was of the B type, while bitter yam showed an A pattern. The starches extracted from the white and water yams were of the intermediate C-type patterns. The temperatures of onset of gelatinisation were derived from DSC and RVA measurements; values of 69.4 and 75.0 °C for the yellow yam, 71.5 and 78.2 °C for the white yam, 76.5 and 79.8 °C for the water yam and 78.1 and 83.1 °C for the bitter yam were obtained. © 1999 Society of Chemical Industry