Discrimination and selection of exploitable genetic diversity from a mixed population of cocoyam (Colocasia esculenta and Xanthosoma sagittifolium) germplasm based on agro- morphological traits and potential for addressing food and nutritional security in sub- Saharan Africa

doi:10.21203/rs.3.rs-4839792/v1

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Discrimination and selection of exploitable genetic diversity from a mixed population of cocoyam (Colocasia esculenta and Xanthosoma sagittifolium) germplasm based on agro- morphological traits and potential for addressing food and nutritional security in sub- Saharan Africa

https://doi.org/10.21203/rs.3.rs-4839792/v1

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Colocasia esculenta (L.) Schott and Xanthosoma sagittifolium (L.) Schott, collectively referred to as cocoyam, are two starchy edible tuber crops. In spite of their great nutritional, medicinal and industrial potential, a major setback in their commercial utilization is the lack of consistent genetic information on several traits of economic and dietary importance as such data is presently unavailable due to lack of commercial varieties, in situ reserves, and genetic and nutritional characterization of most accessions. To better understand the spectrum of divergence as well as identify suitable genetic resources, as precursors to enhancing systematic improvement and selection programmes that would support efficient conservation strategies and industrial utilization of the crop, twenty C. esculenta and thirty X. sagittifolium accessions were evaluated using taxonomic and agronomic fingerprinting based on twenty qualitative and eight quantitative descriptor states and the data derived were analyzed using appropriate statistical packages. Twelve of the qualitative descriptors found useful in differentiating and grouping the cocoyam accessions into clusters revealed 83.58% and 79.49%, respectively, of total variations. Analysis of variance test equally demonstrated significant variations (p ≤ 0.05) among the quantitative traits, with those showing discrimination appraised from factor analysis being plant span, plant height, number of suckers, corm weight, number of cormels, weight of cormels and yield, which accounted for 82.41% and 78.01%, respectively. In addition, variations were observed in the profiles of the two cocoyam species when standard analytical techniques were used for evaluation on the basis of their nutritional signatures, with significantly higher levels of nutrients detected among the representative accessions of X. sagittifolium than those of C. esculenta. Collectively, these findings demonstrate a genetically diverse cocoyam population, which implies that the accessions are derived from several lineages, having been introduced at different times through many different routes and, possibly, by multiple agents. Together, this preliminary information is highly informative and would serve as useful guide for tracing the evolutionary relationships among the two species of cocoyam. It can also be used for creating awareness on the high dietary value of the crop to increase its utilization for food and nutritional security in sub-Saharan Africa.

Agronomic traits

cocoyam

food security

genetic diversity

morphological descriptors

multivariate data analysis

In many parts of the world, the term cocoyam is used to describe two herbaceous perennial edible aroid species, Colocasia esculenta (L.) Schott (or taro) and Xanthosoma sagittifolium (L.) Schott (or tannia) (Ramanatha Rao et al., 2010; Opara, 2003), which belongs to the monocotyledonous Araceae family. Cocoyam is among the six most important root and tuber crops worldwide (FAOSTAT, 2014) with diverse food, feed, medicinal and industrial applications (Perez, 2010; Ndabikunze et al., 2011). The crop is nutrient-dense and has been cultivated on a subsistence basis for several millennia in most parts of the Caribbean Islands, Central and South America, Southeast Asia, South Pacific Islands, and West and Central Africa (Ramanatha Rao et al., 2010; Doungous et al., 2015), where it has contributed to the staple diets of over 400 million people (Onokpise et al., 1999; Vaneker and Slaats, 2012). It is most commonly cultivated for its starchy bland-tasting storage underground stems or corms as well as other parts including leaves, petioles, and inflorescence, which are all edible (Vaneker and Slaats, 2012) and are used in the preparation of different types of culturally acceptable traditional cuisines.

Arguably, the crop ranks third after yam and cassava in terms of consumption, land use and total production among known cultivated root and tuber crops in parts of West and Central Africa, which in fact harbors one of the greatest genetic resources and where the bulk of the world’s cocoyam is produced. The spread of the crop, often as mixed genetic populations rather than pure lines, in diverse geographical locations with diverse cultures has also led to its being used for different purposes (Vaneker and Slaats, 2012; Fufa et al., 2021). For example, the rhizomatous storage tuber of the crop is primarily used as a source of carbohydrate-rich human food while its foliage and floral components including the leaves, with significant levels of protein, antioxidants, dietary fiber and vitamins (Sukamoto, 2004), petioles, inflorescence, and cormels are often utilized as feed and fodder for hogs and goats kept in homesteads in most farming communities. Its organoleptic properties, which are related to its nutritional benefits, could make the crop a valid option for technological improvement in the development of specialty food products such as high energy infant weaning diets (Onwulata and Konstance, 2002), drum-dried flakes, ready-to-eat snacks, and baking powders (Eddy, 2012) in many areas within the tropics. In addition to its high nutritional qualities, cocoyam also has great industrial potential as it is progressively becoming an important cash crop, especially in its use as an excipient in pharmaceutical engineering on account of its high mucilage content and binding power (Andrade et al., 2015). In fact, once considered a subsistence crop consumed mainly by resource-poor farming households, cocoyam has lately become a commodity with many uses, supporting rural development, food security, and value addition with several macroeconomic benefits (Doungous et al., 2015).

In spite of these high-market value prospects and soaring demand in recent times on account of its economic and social significance as an important crop (Ramanatha Rao et al., 2010; Quaye et al., 2010; Fujimoto, 2009; Onwueme, 1978) for the production of specialty foods and industrial raw material, it has continually remained a grossly neglected, marginalized, and often under-exploited food and nutritional resource that tends to receive very little interventions in agricultural research and development initiatives (Falade and Okafor, 2013; Lebot et al., 2009; Watanabe, 2002). For example, in the three major cocoyam producing countries of West and Central Africa including Nigeria, Ghana and Cameroon (Onyeka, 2014), its yield potential is seldom realized due to several limitations including both abiotic and biotic factors such as the scarcity of clean and/or high-yielding planting materials, low input utilization, and high vulnerability to root rot and taro leaf blight diseases, which have the potential of depleting traditional and allelic diversity in the already narrow genetic base within the existing cocoyam germplasm (Okereke, 2021).

As with the other major tropical root and tuber crops, cocoyam species show considerable variations for several agronomic traits such that different criteria including pedigree records, phenotypic descriptors and DNA-marker technology including RAPDs, simple sequence repeats (or microsatellites) and retrotransposons (Offei et al., 2004; Cathebras et al., 2014; Doungous et al., 2015; Wang et al., 2017; Wada et al., 2021) have been used in the past to differentiate cultivars from one another as well as estimate genetic diversity prevalent within the species. For many years, agro-morphological traits have been regarded and used as an essential step for effective description, classification, and utilization of many crop species because this offers a useful approach for evaluation of the extent of genetic diversity present (Brisibe et al., 2016; Sinha and Kumaravadivel, 2016; Assar et al., 2009). These criteria are relevant for cocoyam even today as morphological descriptors have remained the major indices for varietal selection by farmers in village gardens. Despite this, however, there is still general paucity of information on genetic diversity among cocoyam species cultivated in most parts of West and Central Africa, especially Nigeria. For example, up until now, improvement of the crop through conventional breeding has been hindered due to lack of genetic and genomic information, having lost the ability to reproduce sexually because of its erratic flowering nature, poor seed setting, vegetative propagation through head-setts (or suckers), and heterozygous genetic background. These constraints, especially the narrow genetic base of the local population predominantly maintained by farmers in the field, therefore, calls for robust selection of the best-performing cultivars which are commensurate with the numerous economic and sociocultural values of the crop for food, nutritional and industrial purposes, than it has been accorded up until now.

Building on this and, especially, driven by the desire to assess the traits of significant agronomic and industrial interests in cocoyam cultivars for exploration of their cytogenetic background in designing appropriate germplasm exploration, protection and systemic conservation strategies in the future, there is an urgent need for selection of accessions with a wider diversity of genetically improved traits with more robust disease resistance, increased productivity and better adaptability to a wider range of growing conditions, particularly in terms of resource capture and conversion into economic yield that would enhance the tuber metabolome and large foliage as potential renewable sources of high-value compounds, as a basis for future improvement of the crop. The current study, which is part of a multi-dimensional investigation was, therefore, intended to provide details on the genetic diversity available in cocoyam accessions, based on information generated through variations in agronomic and morphological traits within the large germplasm collection, as a foundation for selection of genetically superior cultivars that would support efficient management and utilization of field gene banks for development of high yielding cultivars through conventional breeding and biotechnology in the future.

Collection of cocoyam germplasm - Plant material for this study consisted of twenty (20) Colocasia esculenta and thirty (30) Xanthosoma sagittifolium accessions, which were randomly collected from various cocoyam growing regions of Nigeria. Fifty to sixty corms of each sample were collected, assigned an accession number and maintained in a cool shady ex situ gene bank prior to evaluation. The local vernacular names provided by the residents of the localities from where the samples were collected were adapted for differentiation of the individual accessions. A Global Positioning System (GPS Etrex Legend, Garmin model) was catalogued to identify the coordinates of all the sample collection sites with geographical details, as presented in Tables 1 and 2, respectively.

Field layout - All trials were conducted within the same location at the Botanical Garden of the Department of Plant Science & Biotechnology, Abia State University, Uturu, Nigeria (N 5⁰42’ and E 6⁰48’). The experimental plots were situated in an area within the tropical rainforest zone, which is characterized by two distinct dry (November-February) and wet (March-October) seasons, with an annual rainfall of 2000–2500 mm that is distributed from early March to late October, having minimum and maximum daily temperatures that range between 23°C and 36°C, respectively, and a mean relative humidity of 84–86%.

Experimental design and crop management - For the agronomic measurements, trials were laid out in a randomized complete block design (RCBD) with six replications using a planting distance of 0.5 meters between rows and 0.5 meters between plants in a row while adjacent plots were separated by 1 m alleys.

The plants were established with pre-sprouted corms derived from accessions of both C. esculenta and X. sagittifolium, respectively, which were raised under similar agronomic practices in all the experimental plots, examined at regular intervals and data on morphological and yield-related traits recorded at the same time.

Characterization of morphological and yield-related traits - Visual observations indicated that there were significant differences between the twenty C. esculenta and thirty X. sagittifolium accessions in terms of morphotaxonomic attributes. Consequently, twenty eight (28) major agronomic and morphological descriptors based on twenty (20) qualitative and eight (8) quantitative traits that were taxonomically most informative for characterization of the plant, leaf, petiole, and corms following procedures outlined for sampling in the International Board for Plant Genetic Resources (1999) descriptor list for cocoyam species were used for data collection. Six replicates were taken to represent an accession for key morphological attributes and yield-related traits. While the aboveground qualitative traits including leaf and petiole characters, were taken 5 months after planting (MAP), those for the underground yield-related traits such as number of corms, corm fresh weight per plant (kg), number of cormels per plant, and cormel fresh weight per plant (kg), were recorded on a quantitative scale from an average of three individuals at the end of 6 and 9 months after planting for both the C. esculenta and X. sagittifolium accessions respectively.

Proximate composition analysis and profiling of anti-nutritional factors - Six accessions comprising of three each from C. esculenta and X. sagittifolium, respectively, were randomly selected and used for nutritional fingerprinting based on standard analytical methods.

All assays of the proximate composition of the edible aroid samples were undertaken at the Biochemistry Laboratory, Department of Biochemistry, Abia State University, Uturu, Nigeria. While the moisture, ash and protein contents were determined in consonance with the methodology described in the Association of Official Analytical Chemists (2005) and the crude fat was extracted from about 4 g of each sample using the Soxhlet method with petroleum ether (40–60ºC) for 8 h and determined according to AOAC (2005), the crude fibre and total non-structural carbohydrate contents were analyzed according to Denison et al., 1992).

The contents of anti-nutritional factors including phytates, oxalates and hydrogen cyanide were equally evaluated using different sampling methodologies. For example, while phytates were determined based on the bi-pyridine colorimetric method according to Onwuka (2018), oxalates was evaluated based on the method described by Okwu (2005). The content of hydrogen cyanide was determined by the alkaline picrate colorimetric method as described by Onwuka (2008). All determinations were undertaken in triplicates and results expressed as averages of percent values on a dry weight basis.

Mineral diversity - For analysis of mineral elemental composition, a microwave accelerated reaction system, MARSXPress model 907501 (CEM, Matthews, NC, USA) was used for sample digestion and set at 1600W (80% power) with an extraction time of 15 min, at 200ºC. A Genesis OES-ICP spectrometer (Spectro, Germany) with axial plasma configuration was used for elemental determination with a plasma power of 1400 W, coolant flow at 13.5 l/min, auxiliary flow of 1.2 l/min and a nebulizer flow of 0.8 l/min. The plasma was operated with a cross-flow nebulizer attached to a Scott spray chamber. Microwave assisted acid decomposition of the samples before elemental analysis was performed.

Dried and pulverized samples of both C. esculenta and X. sagittifolium corm pulp (or flesh) weighing 100mg were transferred to Teflon vessels. Two millilitres of concentrated HNO₃ were then added to each of these vessels. The samples were allowed to predigest for a minimum of 15 min in the vessels. Predigested sample vessels were capped, placed in the CEM, and subjected to the microwave operational parameters. Upon completion of the digestion programme, the vessels were cooled and the digests were transferred to 10ml volumetric flasks and brought to volume with distilled deionised water. Elemental concentrations in the samples were then determined by ICP with Analytes’ spectral lines based on their sensitivity and spectral interference according to Brisibe et al. (2009).

Phytochemical screening - For the determination of total flavonoids and alkaloids, the ethyl acetate precipitation and the alkaline precipitation gravimetric methods both described by Osuagwu and Ihenwosu (2014) were used. Tannins were determined as reported by Onwuka (2018). On the other hand, saponins were determined using the solvent extraction gravimetric method while the determination of phenols was undertaken based on the Folin – Ciocalteau colorimetric method.

Principal component and cluster analyses - Principal component and cluster analyses were undertaken using SPSS (2019) version 26.0 software to evaluate the accessions in terms of the most differentiating character; hierarchical cluster analysis (HCA) based on Ward’s method was also carried out to ascertain the diversity between accessions for the measured characters.

A dendrogram was constructed by using Unweighted Pair Group Method with Arithmetic mean (UPGMA) to group individuals into discrete clusters.

Data analysis - Frequency distribution of qualitative traits was presented as percentages while basic statistical parameters based on quantitative traits were subjected to analysis of variance test using Genstat Discovery Edition 4 (GenStat, 2007) Software. The least significant difference (LSD) test was used to identify significance among treatment means at 5% probability level (p ≤ 0.05), as outlined by Obi (Obi, 2002).

All data collected from the nutritional analyses were equally subjected to analysis of variance using SPSS (2019) version 26.0 software. At p < 0.05, differences in means were regarded as significant.

Analysis of agro-morphological characteristics

The field data generated across all 20 qualitative traits from the collection of genetic material consisting of twenty C. esculenta and thirty X. sagittifolium accessions were analyzed to characterize the cocoyam population used in the current study. High variability was observed for the morphological descriptor traits measured on a qualitative scale, with shape of the leaf base being the most striking morphological trait that differentiated the two cocoyam species from each other (Fig. 1).

Differences in leaf characters

As indicated in Table 3, variations were detected in five out of the nine leaf-associated (phenotypic) traits that were used for the assessment of the twenty C. esculenta accessions. In majority of the accessions, the leaf blade margin was predominantly undulate (80%) while it was entire for the remaining 20%. Overall, there were about 75% of the accessions where leaf blade colour was green while that of all others was either yellow or slightly yellowish-green (25%). Invariably, the leaf blade margin colour delineated these accessions into five groups including purple (35%), orange (25%), yellow (20%), green (15%) and pink (5%) while that of the leaf main vein was either yellow (55%) or green (45%). Similar percentages were equally detected regarding the level of wax in the leaves.

For the thirty X. sagittifolium accessions, six out of the nine leaf-associated traits revealed variations while there were striking similarities among the remaining three (Table 4). Unlike that of C. esculenta where it was similar for all accessions, predominant shape of the leaf lamina varied in the X. sagittifolium accessions and grouped them into two categories. The more abundant was cup-shaped and was observed in 86.67% of the accessions while the remaining 13.33% had an erect-apex down. For leaf blade margin, three variants were detected with 73.34% of the accessions being undulate, 13.33% sinuate and the remaining 13.33% were entire. The leaf blade colour grouped the X. sagittifolium accessions into either yellow or yellowish-green (60%) and green (40%), respectively.

Regarding the colour of the leaf blade margin, all the thirty accessions of this species could be grouped into four categories including yellow (60%), orange (20%), green (10%), and purple (10%), respectively. For the leaf main vein colour, 10% of the accessions were yellow while the remaining 90% were green. As for the leaf waxiness, 30% of all the accessions were detected to be low while the remaining 70% were considered to be medium.

Differences in petiole characters

Seven traits associated with the petiole were equally examined in the two cocoyam species. Significant variations were again detected among the twenty C. esculenta accessions (Table 3). For example, a petiole junction was detected amongst all of these accessions and the profile clearly delineated them into two groups, either as small (80%) or medium (20%), respectively. On the other hand, the colour of the petiole junction demonstrated that for 75% of the accessions, this was yellow while for the remaining 25% it was purple. For the colour of top-third of the petiole, 75% were purple while 15% were green and the remaining 10% were light green. Furthermore, the petiole colour of middle third was detected to be green (55%), purple (40%), or light green (5%), respectively. For petiole colour of the basal third, all the C. esculenta accessions were categorized into two groups, either as green (75%) or purple (25%). Three groups including green (70%), purple (25%), and pink (5%) were observed for the colour of the petiole basal-ring. Furthermore, the cross-section of the lower part of the petiole demonstrated that 80% of these accessions were closed while that of 20% was open.

Conversely, for the thirty accessions of X. sagittifolium variations were detected only in five out of the seven petiole-associated characters (Table 4). Unlike those of C. esculenta, the petiole junction was characteristically absent amongst all the thirty accessions of this species. For the petiole colour of top third, 13.33% of the accessions were purple, 16.67% were green and the remaining 70% were light green. The petiole colour of middle third was detected to be purple (3.33%), green (76.67%), and light green (20%) for the accessions. On the other hand, for petiole colour of basal third, the X. sagittifolium accessions were categorized into three groups as purple (13.33%), green (70%), and light green (16.67%), respectively, while the petiole basal-ring colour delineated all these accessions into four groups as purple (14%), green (78%), white (2%), and pink (6%), respectively. Furthermore, our evaluations equally revealed that the cross-section of the lower part of the petiole presented two variants. It was closed amongst 86% of the accessions while only that for 14% was open.

Differences in corm-associated traits

Like with the qualitative traits, significant variability was also detected in four agronomic (or yield-related) traits including corm shape, flesh colour of the central part of the corm, flesh colour of cormel and shape of cormel, which were used for evaluation of the performance of the two cocoyam species in this investigation. For example, 5% of the C. esculenta accessions (Table 3) had a corm that was conical in shape while that of 10% was round and for the remaining 85% of the accessions, corm shape was dumb-bell. In addition, an examination of the corm flesh colour of central part as well as that of the flesh colour of cormels demonstrated that for 70% of the accessions this was white, while for 15% it was pink and for the remaining 15% it was purple. For shape of the cormels, those from 5% of the accessions were conical, 90% were round and the remaining 5% were cylindrical.

On the other hand, for the corm shape there were five different presentations among the X. sagittifolium accessions (Table 4). These included round (10%), cylindrical (20%), elliptical (20%), dumb-bell (40%) and elongated (10%). For the corm flesh colour of central part as well as that of the flesh colour of cormel, the data recorded in this study demonstrated that for 50% of the accessions this was white, that for 23.33% was yellow, for 10% it was pink and that for the remaining 16.67% was purple. For shape of the cormels, those derived from 6.67% of the accessions were conical, 66.67% were round, 13.33% were elliptical and the remaining 13.33% were elongated.

Intriguingly, none of the accessions of both cocoyam species flowered during the course of the current investigation, which implies that flowering is not common amongst the major cocoyam species cultivated in Nigeria.

Principal component analysis of qualitative characters in cocoyam accessions

Standardization of the principal component analysis using data derived from the qualitative descriptors for both C. esculenta and X. sagittifolium, respectively, in a second series of experiments is presented in Tables 5 and 6. Principal component analysis for the characters in the C. esculenta accessions demonstrated that the first six principal components (PCs) with Eigen values greater than 1.00 were important and accounted for 83.58% of all variations. Factor loadings of different variables in principal component one (PC1) showed an Eigen value of 3.65 and contributed 22.82% of the total variability observed. The high loading values for this principal component were contributed by such traits as the cross-section of lower part of petiole, flesh color of the central part of the corm and flesh color of cormels. Principal component two (PC2) showed an Eigen value of 3.132 and accounted for 19.578% of the total variation observed. Petiole colour of basal third was detected as the most important trait contributing to total variability in this component. Principal component three (PC3) had an Eigen value of 2.17, which accounted for 13.57% of the total variation with petiole basal-ring colour as the important contributing variable. Principal component four (PC4) showed an Eigen value of 1.731 and accounted for 10.82% of the total variation observed. Petiole junction colour was detected as the most important trait contributing to total variability in this component. The fifth and sixth principal components (PC5 and PC6) presented Eigen values of 1.485 and 1.203, which accounted for 9.28 and 7.52%, respectively, of the variations (Table 5).

Principal component analysis for the qualitative traits in the thirty X. sagittifolium accessions evaluated demonstrated that the first six principal components (PCs) with an Eigen value greater than 1.00 were important and accounted for 79.49% of total variation. Principal component one (PC1) with an Eigen value of 3.91 accounted for 26.07% of the total variation. Essentially, petiole colour of top third, petiole basal ring colour, corm flesh colour of central part and flesh colour of cormel were the important contributing variables to this principal component. Principal component two (PC2), on the other hand, had an Eigen value of 2.25 and accounted for 14.98% of the total variation with shape of cormel as the major contributing variable. Principal component three (PC3) had an Eigen value of 2.01 and accounted for 13.40% of the total variation with leaf blade margin as the highest contributor. Principal component four (PC4) with an Eigen value of 1.42 accounted for 9.45% of total variation with leaf waxiness as the important trait that contributed to total variability. Principal component five (PC5) with an Eigen value of 1.28 accounted for 8.52% of the total variation. The major loading value for this component was contributed by leaf main vein colour. Principal component six (PC6) had an Eigan value of 1.06, which accounted for 7.08% of the total variation with predominant position of lamina surface as the only trait that accounted for the loading value for this component (Table 6).

Cluster analysis of qualitative characters in cocoyam

Given the high genetic variability in some of the morphological traits that were detected in the C. esculenta and X. sagittifolium species above, a significant but not perfect classification of the two groups was expected. In order to classify them on the basis of similarity indices, a single linkage cluster analysis based on Ward’s method was performed. Using the phenotypic descriptors, it was possible to delineate the genetic landscape of the two cocoyam species on the basis of their relationships.

In the resulting dendogram, the twenty C. esculenta accessions were grouped into four major (I – IV) clusters with cluster 1 represented by 3 accessions that all had pink corm flesh colour of central part and flesh colour of cormel while clusters II and III had 7 accessions that were represented by two sub-clusters, with the corm flesh colour of central part and flesh colour of cormel being white. Cluster IV had 3 accessions with the colour of the central part of the corm and cormel being purple (Fig. 2). This is an indication that colour of the flesh (or pulp) of corm and cormel are important traits in the characterization of C. esculenta accessions. On the other hand, the thirty X. sagittifolium accessions were divided into two major clusters. Cluster I had two well-supported sub-clusters that were represented by 20 accessions in sub-cluster IA, which consisted of plants with yellow corm flesh colour of central part and majority of white corm flesh colour of central part. Cluster IB was composed of accessions with yellow leaf blade colour, green leaf main vein colour, green petiole colour of middle third and a majority with pink corm flesh colour of central part. Cluster II on the other hand, had only 5 accessions, all with purple corm flesh colour of central part (Fig. 3).

Mean values and analysis of variance for quantitative characters in cocoyam accessions

The mean values of the eight quantitative parameters were analysed in the two cocoyam species evaluated in the current study. The data for the twenty C. esculenta accessions are presented in Table 7. Analysis of variance of the traits demonstrated that significant (p < 0.05) variations were observed among these accessions. For example, accession BND02 had the highest mean plant span (70.47cm) while accessions BND04 and UMA04 had the lowest (17.33cm). The highest mean plant height (36.10cm) was recorded in ORS03 while the lowest (9.67cm) was recorded in BND04 and UMA04, respectively. The mean number of suckers ranged from 1.00 in OWN05 to 6.00 in OWN06. Accession BND03 and UMA03 had the highest mean corm length (7.73cm) while OWN05 had the lowest (3.87cm). The mean corm weight ranged between 36.00g in BND04 and UMA04 and 116.00g in BND02. While the mean number of cormels ranged between 2.13 in ORS01 and 7.67 in ABS01 and UMA02, the mean weight of cormels ranged from 30.53g in ORS01 to 167.7g in OWN05. The highest mean yield value (6680.00kg/ha) was recorded for OWN05 while the lowest yield value (1392.00 kg/ha) was recorded for BND04 and UMA04.

The mean values of the eight quantitative traits equally evaluated in the thirty X. sagittifolium accessions are presented in Table 8. Analysis of variance showed that all the quantitative characters varied significantly (p < 0.05). Accession IFE01 had the highest mean plant span (76.50cm) while IBM01 had the lowest plant span (14.67cm). The highest mean plant height (51.17cm) was recorded in IFE01 while the lowest (8.50cm) was recorded in NWI01. The mean number of suckers ranged from 1.33 in ABK01 to 4.50 in OWN01. Accession ABS03 had the highest mean corm length (10.77cm) while ABK02 had the lowest (3.40cm). The mean corm weight ranged from 24.90g in NWI01 to 183.00g in ORA01 and UYO01, respectively. While the mean number of cormels ranged from 0.00 in eight of the accessions including UMN01, ABS02, ABS03, BND01, BND05, ORN01, OWN01 and UMA01 to 4.67 in AGT01, the mean weight of cormels ranged from 0.00g in eight accessions including UMN01, ABS02, ABS03, BND01, BND05, UMA01, ORN01 and OWN01 to 51.10g in ABK02. The highest mean yield of 4655.33kg/ha was recorded for ORA01 while the lowest yield value of 743.33kg/ha was recorded for NWI01.

Principal component analysis of quantitative characters in fifty cocoyam accessions

A combination of the field data derived from the eight quantitative traits subjected to principal component analysis demonstrated the presence of diversity among the C. esculenta accessions evaluated in this study. The top three principal components had Eigen values above 1.00 and accounted for 82.41% of variation when ordered among the accessions. The first principal component (PC1) with an Eigen value of 3.660 accounted for 45.75% of the total variation and was associated with characters such as plant span, plant height, corm weight, weight of cormel and corm yield. The second principal component (PC2) with an Eigen value of 1.835 explained 22.94% of variation and was mainly associated with the number of suckers and corm length. The third principal component (PC3) with an Eigen value of 1.097 contributed 13.71% of the total variation (as presented in Table 9).

For the X. sagittifolium accessions on the other hand, the first three principal components with Eigen values above 1.00 were significant, and accounted for 78.01% of the total variation. Principal component one (PC1) with an Eigen value of 2.869 accounted for 35.86% of the total variation with high loading values for plant span, plant height and corm weight. Principal component two (PC2) with an Eigen value of 2.268 accounted for 28.35% of total variation with high loading values for number of cormels, weight of cormels and yield. Principal component three (PC3) had an Eigen value of 1.104 and contributed 13.81% of the total variation with number of suckers as the main contributing variable (Table 10).

Cluster analyses of quantitative characters and outlier identification in cocoyam

To obtain a further overview of genetic dissimilarities between and within the two cocoyam species, phylogenetic relationships among the accessions were determined based on the eight quantitative (or agronomic) traits, which were generated separately for the C. esculenta and X. sagittifolium accessions, respectively. These led to a number of observations. In the resulting dendrograms following multivariate analysis, the two species each produced three major clusters. The twenty accessions of C. esculenta were grouped into three main clusters (Fig. 4). While cluster 1 had 10 accessions, cluster II had 9 and cluster III had only 1 accession (OWN05), which was identified as an outlier that was distinct and located farther away from the proximity of all other samples and recorded the highest mean values in such parameters as weight of cormel, and yield per hectare. Given these observations, this is an indication that this single outlying accession (OWN05) has good agro-morphological values and could be further selected for breeding purposes.

For the X. sagittifolium species, the multivariate analysis also grouped the thirty accessions into three (3) clusters as presented in Fig. 5. Cluster I was further characterized into two sub-clusters (IA and IB) with 11 accessions that recorded the highest mean value for plant height and corm weight. On the other hand, cluster II has only 2 accessions which recorded the highest plant span and weight of cormel while cluster III has two sub-clusters (IIIA and IIIB) with 17 accessions and recorded low values in plant span, plant height, corm weight, and weight of cormel, respectively. These observations overall support the notion that the accessions from both clusters I and II such as ABK02, ORA01, and several others, which recorded good agronomic characters could be maintained in an ex situ gene bank and used for future breeding purposes.

Nutritional profiles and anti-nutrient fingerprinting in cocoyam species

Cocoyam germplasm is a potentially rich source of natural variations and the genetic variability present in it can be exploited for the development of nutrient-rich food items. Building on this, the nutritional composition of each of the three representative accessions of C. esculenta and X. sagittifolium evaluated in the current study are presented in Table 11. While there were no significant (p > 0.05) differences detected in the moisture contents between and within the six accessions, there were differences in the ash contents with the maximum value (4.64%) detected in accession OWN02, which was statistically higher than the values recorded for all other accessions. Not surprisingly, the crude fibre content was not also significantly different among the accessions. For this parameter, values ranged between 1.51 in OWN06 and 2.43% in KET02. Generally, whilst the protein content ranged between 6.89% in OWN02 and 9.22% in KET02 with higher levels often recorded amongst the X. sagittifolium accessions than those of C. esculenta, the crude fat percentage recorded on the other hand ranged between 0.65 in UMA03 and 1.83 in OWN06 with no significant (p > 0.05) differences between the accessions of both cocoyam species evaluated.

Table 12 is a summary of the contents of various anti-nutritional factors including phytates, oxalates and hydrogen cyanide found in the six representative samples of cocoyam. It is striking that these components were comparatively higher in the three accessions of C. esculenta than in those of the X. sagittifolium species. While the phytate and oxalate contents of the C. esculenta accessions were significantly (p < 0.05) higher than those of the representative X. sagittifolium accessions, no significant differences were detected in the levels of hydrogen cyanide between both species. Collectively, it is of great interest and importance that the levels of these anti-nutrients are rather low and below the established toxic levels to significantly interfere with the utilization of other nutrients in cocoyam.

To further assess the nutritional quality of the cocoyam species, the phytochemicals present in the six representative accessions including alkaloids, flavonoids, saponins, tannins and phenols was equally evaluated through a series of independent investigations. As expected, the Colocasia accessions again had higher alkaloid contents than those of the Xanthosoma species with values that ranged between 0.51 in KET02 and 1.41% in UMA03 (Table 13). Regarding the content of flavonoids, the values differed significantly (P < 0.05) among the accessions examined in both species. Moreover, all the Xanthosoma accessions examined had higher levels of saponin than those of C. esculenta with values that ranged between 0.31% in OWN06 and 1.41% in UMA03 (Table 13).

Intriguingly, the contents of both tannins and phenols were significantly low in both species of cocoyam. For example, the tannin content ranged between 0.50 in OWN06 and 0.87 in KET02. Incidentally, two Colocasia accessions (OWN06 with 0.50% and UMA03 with 0.51%) had the lowest tannin contents while higher values were recorded in one of the Xanthosoma accessions (0.87%).

With respect to the content of phenols, the mean values ranged from 0.27 in OWN02 to 0.59 in BND05 (Table 13). Generally, the values were higher among the accessions of X. sagittifolium than in those of C. esculenta.

Mineral composition in different species of cocoyam

Comparison of the mean concentrations of five different mineral elements on a dry weight basis (mg/100g) between the three representative accessions of C. esculenta and those of X. sagittifolllium are presented in Table 14. There were considerable differences in the contents of the minerals, especially potassium and phosphorus, which varied significantly (p < 0.05) according to the cocoyam species. Potassium was the most abundant mineral in the accessions of both cocoyam species evaluated. The concentration of potassium was significantly very high, especially among the X. sagittifolium accessions, where it varied between 575.33 and 736.67mg/100g, thus confirming the assertion that cocoyam can be a good source of dietary potassium. Phosphorus content ranged from 74.32mg/100g in UMA03 to 310.25mg/100g in OWN02. As expected, all the six accessions also contained calcium and magnesium, albeit at varying concentrations and were the next most abundant minerals in both species of cocoyam. Sodium was the mineral element with the least value amongst all those evaluated in the current study. Its content was quite negligible in the accessions of both C. esculenta and X. sagittifolium, respectively. This is an assertion that has also been reported by Sefa-Dedeh and Agyir-Sackey (2002), and is particularly very noteworthy and fascinating viewed from the perspective of human nutrition. Overall, there were small but statistically significant differences between the three representative accessions of X. sagittifolium, which seemed to be nutritionally superior to those of C. esculenta in terms of the composition of mineral elements.

Progressive erosion of genetic diversity in cocoyam species on account of decreased population sizes resulting from high incidence of viral and fungal disease complexes, loss of traditional diversity compared to other tropical root and tuber crops such as yam, cassava and sweet potato and changes in local land use patterns and climate that may select against some genotypes, has become quite a serious threat to food security in many parts of the developing world (Brisibe et al., 2016). To prevent further escalation of genetic erosion in cocoyam, germplasm management is fundamental for conservation of important genetic resources while information on available diversity that would be helpful in germplasm utilization must also be put in place. In the present study, therefore, morphotaxonomic and agronomic descriptors were used to evaluate twenty C. esculenta and thirty X. sagittifolium accessions, respectively, for which very little is known about the extent of genetic diversity and preservation in any germplasm banks in Nigeria.

The morpho-agronomic characterizations employed in this study demonstrated that a large number of the phenotypic attributes including plant span, leaf shape, presence of petiole junction pattern, and flesh colour of corms and cormels, were informative for the purpose of discriminating the studied cocoyam accessions. For example, while the shape of the leaf in all the C. esculenta accessions were peltate with a predominant leaf lamina surface that was principally erect-apex down, those of the X. sagittifolium species were sagittate with a predominant position of leaf lamina surface that has majority of the accessions being cup-shaped, as also previously reported by Mwenye et al. 2016) and Ezeabara et al. (2015). Furthermore, while a petiole junction pattern was characteristically present in the plants of all C. esculenta accessions, this was absent among those of X. sagittifolium, tacitly implying that these two qualitative traits are paramount and could serve as strong factors for morphologically distinguishing the two cocoyam species in the country. In addition to these qualitative traits, variations were equally detected among the underground or yield-related traits. For example, the principal component analysis for these agronomic traits accounted for 82.41% and 78.01%, respectively, of the total variation among the C. esculenta and X. sagittifolium accessions, with plant span, plant height, number of suckers, corm weight, number of cormels, weight of cormels and yield per hectare that were detected as the key contributing factors. It is speculative from these observations, therefore, that these traits could be considered useful for major improvement programmes of this highly important but currently marginalized and underexploited food resource (Wada et al., 2021; Villavicencio et al., 2016; Mezhii et al., 2015).

At face value, these complex accession- and species-specific profiles which have shown a rich base of diversity that can inform breeding strategies in the crop may not seem to be very significant. However, on the basis of the large collection of data generated herein, a remarkable turnover in the genetic structure of the accessions during which the two species of cocoyam have established themselves in the country is demonstrated. It is suggestive from these data, therefore, that enough morphological and agronomic polymorphisms exist to discriminate between the individual cocoyam accessions evaluated in the current study, perhaps due to the multiplicity of lineages generated over the years arising from the introduction of the crop at different times through many different routes and by multiple agents. These findings, overall, support the notion that the cocoyam population in the country is genetically diverse, with clustering of the accessions based on mixed ancestry from different sources on account of multiple introductions or drift during introduction bottlenecks and, perhaps, even introgression from related exotic aroid species.

Cocoyam is a viable food commodity which is presumed to have a superior nutritional value over alternative staple foods of the developing world. It contains sufficient quantities of both macro- and micro-nutrients including complex carbohydrates such as amylose and amylopectin, protein, fat, minerals, and vitamins. The crop is also one of the best sources of dietary fibre, a nutrient linked to multiple health benefits in humans on account of its pivotal role in feeding and fueling the growth of the complex and balanced community of beneficial gut microbiota, which plays a well-established role in the maintenance of physical health, ranging from digestion and metabolism to promotion of immunity. Some of the data presented in the current study and several other reports have demonstrated that the crop has a highly desirable nutritional profile, considering the availability of significant levels of proteins (Owusu-Darko et al., 2014) with many essential amino acids, which are reportedly higher than those proposed by the FAO/WHO (FAO/WHO, 2002) and can satisfy the amino acid needs of an average adult human being. In addition, cocoyam also has very low contents of anti-nutritional factors including phytates, oxalates and hydrogen cyanide, which though present, are quite negligible. These results are of particular nutritional interest, as the levels of these anti-nutrients detected in most cocoyam accessions are far below established toxic levels, which is intriguing as their presence is thus not a cause for concern for human nutrition. More interestingly, cocoyam is considered to be a culinary wonder that has a rich repertoire of easily digestible carbohydrates (Owusu-Darko et al., 2014) and high levels of fatty acids and soluble fibres, which fits all types of diets and are suitable even for consumers suffering from food intolerances on account of its gluten-free status. Invariably, these constituents give cocoyam several nutritional advantages over the other more common tropical root and tuber crops including yam, cassava and sweet potato, which are abundant within the African agricultural landscape. Above all of these, however, cocoyam also contains moderate quantities of the valuable B-complex group of vitamins including thiamine (B₁), riboflavin (B₂), niacin (B₃), pantothenic acid (B₅), pyridoxine (B₆), and folic acid (B₉) (Sukamoto, 2004; Bradbury and Holloway, 1988; Sefa-Dedeh and Agyir-Sackey, 2004), as well as vitamin C and phenolic antioxidants, which in consonance with several important minerals including potassium, phosphorus, calcium, magnesium, copper, iron, zinc, and manganese, are highly appreciated for their long-term health benefits in humans. These vitamins, minerals, and antioxidants are essential for proper growth and development of several bodily systems in multiple ways including the significant roles they play in boosting immunity, building of strong bones, teeth and muscles, as well as in the regulation of blood pressure and heart functions, amongst several other physiological influences in mostly vulnerable groups such as preschool children, the elderly, pregnant women, and nursing mothers.

Other than its unique medicinal and nutritional values, another important aspect of the novelty of cocoyam is its promise as a crop of the future with huge industrial potentials. However, these enormous opportunities for industrial utilization of the crop would depend, to a very large extent, on the selection of high yielding genotypes and development of cost-effective technological tools that will enhance its utility in the development of new products for different industries and market requirements such as the ever-increasing proprietary canned baby food, infant weaning (complementary) food and ready-to-eat snack segments (Boakye, 2018). Interestingly, all of these sectors are currently thriving markets in sub-Saharan Africa where cocoyam is produced through low technology farming systems. Consequently, the utilization of cocoyam for these and other purposes including its industrial exploitation for production of ready-to-use stable extruded food items such as noodles and its starch as raw material for food-grade ethanol (Opara, 2003), would go a long way to produce culturally acceptable and nutritious food products that would have potential to offer tangible contribution towards alleviation of food insecurity as well as the occasional food shortages experienced in cocoyam-producing communities, by promoting the production and consumption of the crop. However, for cocoyam to play such a sustainably significant role in contributing to food and nutritional security in many low income food deficit countries where it is produced, it would be necessary to broaden the genetic base of the crop through modern biotechnologies and the exchange of germplasm between these countries while also taking cognizance of the need to select for improved varieties with high yield, pest and disease tolerance, high post-harvest storability and good tuber quality traits.

Cocoyam has high yield and high industrial value potentials. Yet, breeding of the crop is currently hindered by the lack of germane genetic and genomic information as well as improved genetic resources. The current study, which is prelude to more robust molecular breeding, has identified and highlighted some phenotypic and agronomic traits that are largely responsible for the observed morphological plasticity and high genetic discrimination between and among the two edible cocoyam species, which could provide an excellent opportunity for utilization and improvement of this important but currently under-exploited food and potentially industrial crop that is globally widespread with a predominant tropical, subtropical and warm-temperate distribution. The current study, especially, identified six elite genetic resources including OWN05, ORA01, UYO01, IBM01, ABK02, and ORSO3 from among the large collection of cocoyam evaluated with reasonably high mean values for yield per hectare which could, therefore, be crucial to building a foundation for defining efforts in germplasm conservation in an ex situ gene bank. These six high-value accessions could also serve as candidates for future large-scale farming and further breeding or plant biotechnological means in the crop. Bearing this is mind, and especially as recent advances in the field of genomics and molecular genetics including molecular markers and single nucleotide polymorphisms, which are more informative for the discrimination of closely related genotypes and are considered to be of great value in fingerprinting, mapping, and genetic analysis of plants become increasingly available as tools for assessment of crop molecular data than ever before (Brisibe et al., 2019), there is obviously a need for characterization and separation of the C. esculenta from X. sagittifolium accessions used in this study with these tools. Moreover, these two edible cocoyam species could also be separated through an integrative multilevel omics molecular characterization using transcriptomic, proteomic, and metabolomic profiling, which have helped in transforming conventional breeding programmes with rapid genetic gains and shorter breeding cycles in other crops, to complement and confirm the highly diverse characteristics observed in both the Colocasia esculenta and Xanthosoma sagittifolium accessions studied here.

Availability of experimental materials

Plant materials used for the study are available in the ex situ gene bank at the Abia State University botanical garden, Oturu, Nigeria.

Competing interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. This article does not contain any studies involving animals or human participants performed by any of the authors.

Funding

No Open Access funding was received from any source

Author Contribution

All authors contributed substantially to the work presented here. EAB and NCE conceived the hypotheses in support and design of the study and conceptualized the experiment. NCE executed the experiments, wrote the first draft of the manuscript. NCE and EAU carried out the data analyses. EAU and VON were involved with methodology, and overall administration of the project. EAB contributed to the writing and editing of the final copy of the manuscript.

Acknowledgement

Not applicable

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Tables 1 to 14 are available in the Supplementary Files section.

No competing interests reported.

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Discrimination and selection of exploitable genetic diversity from a mixed population of cocoyam (Colocasia esculenta and Xanthosoma sagittifolium) germplasm based on agro- morphological traits and potential for addressing food and nutritional security in sub- Saharan Africa

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Abstract

Figures

Introduction

Materials and Methods

Results

Analysis of agro-morphological characteristics

Principal component analysis of qualitative characters in cocoyam accessions

Cluster analysis of qualitative characters in cocoyam

Mean values and analysis of variance for quantitative characters in cocoyam accessions

Principal component analysis of quantitative characters in fifty cocoyam accessions

Cluster analyses of quantitative characters and outlier identification in cocoyam

Nutritional profiles and anti-nutrient fingerprinting in cocoyam species

Mineral composition in different species of cocoyam

Discussion

Conclusions

Declarations

Availability of experimental materials

Competing interests

Funding

Author Contribution

Acknowledgement

References

Tables

Additional Declarations

Supplementary Files

Status:

Version 1