Tigernut belongs to the family Cyperaceae, and produces rhizomes from the base  and  tubers  that  are  somewhat spherical (Cortés et al., 2005). The plant is not really a nut but a  tuber  which  was  first  discovered  some  4000 years ago (Lowe et al., 2000). It has other names like yellow nutsedge, chufa, flat sedge, rush nut, water grass, earth almond, northern nut grass and nut grass (Shilenko et al., 1979). The tubers are very nutritious, typically, hundred grams of the nuts contain 386 kcal (1635 kj) of energy, 7% proteins, 26% fats (oils), 31% starch, 21% glucose and 26% fibre of which 14% is non-soluble and 12% soluble (Burden, 2003). It also contains vitamins A, B1, D2 and E, while the minerals include, Calcium, Magnesium, Sodium, Potassium, Copper, Iron and other beneficial enzymes (Burden, 2003). Tigernut is used as a source of food, medicine and perfumes (De Vries, 1991). It can be eaten raw, roasted, dried, baked or made into a refreshing beverage called Horchata De Chufas or tigernut milk which is very nutritive.

Medically, the nuts are reported to be aphrodisiac, carminative, diuretic, stimulant and tonic which can be used in the treatment of constipation, high blood pressure and diarrhoea (Oladele and Aina, 2007). Economically tigernuts provides Ghana with foreign exchange through its exportation. In 2010 Ghana exported 63,462 tonnes of tigernut valued at US$ 25,130.82 to countries such as England, Japan and America (GEPC, 2010). Its cultivation also provides jobs to about 85% of the youth and women in the major growing areas of Ghana (Tetteh and Ofori, 1998). In spite of the nutritional, medicinal and economic value of tigernut, the crop still remains an orphan.

Research into the production and general improvement of tigernut through breeding has received very little attention and farmers still cultivate landraces which are low yielding and susceptible to diseases and pest. Available accessions of tigernut in Ghana have not yet been collected, characterized and conserved. This further exposes available landraces to erosion of their genetic resources and limits breeding of improved varieties. Germplasm characterization plays an important role in varietal development of crops as genotypes with desirable traits are identified and utilized in the crop improvement programmes. Knowledge on the genetic diversity and variation among available accessions is very important for pragmatic use of plant genetic resources and also to determine evolutionary relationships (Zada et al., 2013). It will also aid in the early identification and exploitation of desirable traits such as high yield and early maturity. The existence of genetic variation among accessions can be employed as the basis for improving yield and other potentials of crop plants (Makinde and Ariyo, 2013).

Morphological attributes of crops have been employed as characterization tools among crops such as clusterbean (Manivannan et al., 2016), groundnut (Makinde and Ariyo, 2013), cowpea (Manggoel et al., 2012). Morphological traits has also been used to determine the extent of genetic variation among purple and yellow nut sedge accessions (Cruz and Baltazar, 2001, Peña-Fronteras et al., 2009, Okoli et al., 1997, Casimero et al., 1999). Before any effective work  can  be done on tigernut, there is the need to collect and characterize the local accessions that are available in Ghana. The objective of this study was to collect and characterize the accessions of tigernuts available in Ghana to promote their conservation and utilization.

Study area

The study was conducted at the multipurpose nursery of the College of Agriculture, University of Education, Winneba Mampong-Ashanti, during the minor season growing season.  Mampong-Ashanti lies within longitude 0°05ʺW and 1°30ʺW and latitude 6°55ʺN and 7°30ʺN and altitude 395 m above sea level. The area has an average annual rainfall of 1270 mm in two seasons (March and September) and a mean daily temperature of 27°C (Metrological Service, Mampong, 2010).

Germplasm collection

Twenty-four accessions of tigernut were collected from six major tigernut growing regions in Ghana that is Eastern region (Asukese Donkokrom, Nkwakwa), Volta region (Krachi), Upper East region (Bawku), Upper West region (Wa), Central region (Kasoa, Badwiase, Gomoa Feteh, Twifo Praso) and Brong Ahafo (Techiman). Table 1 shows the names, colour and collection area of the accessions.  The accessions collected were kept in polyethylene bags and tagged with their names. The accessions were named using the first letters of the towns where they were collected and the colour of the nuts. Numbers were used to differentiate accessions from the same town which were having the same colour for example WY1 meaning Wa Yellow, first accession.  Figure 1 shows the map of Ghana showing the location of regions and the towns where the accessions were collected. Figure 4 shows the pictures of some of the accessions collected.

Germplasm evaluation

The accessions were evaluated using RCBD with five replications in plastic buckets. The volume of the bucket was 12212 cm³ and was fully filled with heat sterilized sandy loam soil. The buckets were arranged 50 cm within rows and 100 cm between rows.  Each bucket contains five stands of tigernut per genotype. The five stands were arranged 5 cm within rows and 5 cm between rows in the buckets. The plants were raised under irrigation and manual weeding was done regularly in the buckets as well as between and within the rows of the arranged buckets.  Data was collected on all the five stands in the buckets. Data collection was started when the plants were a week old. There was no fertilizer and pesticide application. The following traits were evaluated; percentage germination, number of tillers/stand, number of attached nuts/stand, number of detached nuts / stand, number of good nuts/stand, number of bad nuts /stand, total nuts /stand, number of leaves/ plant, number of ridges /nuts, nut length and width (cm), hundred nut weight, economic yield, biological yield and Harvest index (%).

Statistical analysis

The data collected was subjected to Analysis of Variance (ANOVA) using the GenStat statistical software, version 11.1 (GenStat, 2008).

Dissimilarity matrix based on Euclidean distance was estimated using GenStat  11.1  version.  The  scores of the dissimilarity matrix were used to perform a hierarchical cluster analysis (Ward, 1963). Principal Component Analysis (PCA) based on the traits was performed to find out the relative contribution of the different traits to the total variation in tigernut.  A biplot was drawn to show the relationship between the accessions and the traits using the Eigen values associated with the components versus the number of the component.  Pearson (1901) Correlation coefficients was carried out for all the traits and a correlation matrix was prepared to understand the relationship among the different traits.

The significance differences (<0.05) among the accessions for the agronomic traits is a sign of the presence of high degree of genetic variation. This provides the plant breeder the opportunity to select the best accession for utilization in future breeding programs. Accessions such as BB, ADL, BY and BKB which had high good nut/stand, total nuts/stand, economic yield and harvest index respectively could be included in breeding programmes for varietal development of tigernut. The observed variability could be attributed to the genetic differences among the accessions. Variation in morphological traits among yellow nut sedge and purple nut sedge biotypes has been reported by Tayyar et al. (2003), Bhowmik (1997) and Wills (1998) also reported considerable heterogeneity in morphological among Cyperus rutundus populations from around the world.

The clustering of the accessions in the six major groups is an indication of diversity among the accessions of tigernut in Ghana.  The grouping of the accessions from same origin and colour into different clusters suggests diversity among accessions within a geographical origin and among accessions beyond geographical origin.

Tayyar et al. (2003), Okoli et al. (1997) and Abad et al. (1998), reported on similar clustering of nutsedge populations on the basis of morphological traits.

The biplot also shows relationship between the accessions and traits evaluated. The observation of the accessions in all the quarters of the biplot suggests a high level of genetic diversity in the accessions evaluated. Concentration should be on the traits that defined PC1 for varietal development of tigernut. Divergence among the purple nutsedge accessions for the morphological traits has been reported by Holt (1994) and Tayyar et al. (2003).

Correlation among traits provides information on the nature and level of association between two pairs of traits and it could be possible to improve a trait by the selection of the other pair. The correlation analysis shows significance association among the traits studied which suggest that they can be predicted by using the other. Therefore, traits that showed significance and positive correlation in this study could be improved simultaneously while those that showed negative association will have to be improved independently.  

The study was conducted to characterized twenty- four tigernut accessions based on yield and yield related traits. The  data  shows that there exists  a wide range of diversity among the accessions for the traits studied. This should help provide necessary information for the breeding of improved tigernut varieties in Ghana. Promising accessions such as TPY, CCB, BB, DY, ADL, KAY, WY1 AND BKB which recorded high values for the yield and yield related traits should be included in breeding programmes for varietal development of tigernut in Ghana. Also the diverse forms of the accession studied should be conserved at the gene bank in Ghana.

The   authors  have  not  declared  any  conflict  of interests.