Morphological and biochemical characterization of Dioscorea alata L. landraces from Northern Eastern region, India

Introduction

The genus Dioscorea contains about 644 species that are distributed throughout the tropics out of which eight species are under cultivation (Govaerts et al., 2007; Asiedu and Sartie, 2010) [16,5] .Yams originated primarily from Africa and Asia then gradually spreading to other tropical or subtropical areas of America and Polynesia, where some Dioscorea species represent economically important tuber crops (Coursey, 1967). Now the crop is of major importance in the diet and economic life of more than 100 million people especially in West Africa, the Caribbean islands, parts of Asia, and Oceania (Ravindran and Wanasundera, 1992; Girardin et al., 1998) [37,15] . Yams are quite rich in starch, accounting for 60 to 89 per cent of yam carbohydrates (Degras, 1986) [10] as well as protein, vitamins, potassium, sodium, magnesium, copper and zinc (Baah et al., 2009) [7] . Yam flour had higher levels of fibre than potato flour, refined wheat flour, maize and rice (Woolfe, 1987) [45] . Besides, it also has medicinal properties such as the steroidal sapogenins, used in the production of cortisone and synthetic steroid hormones, hence gaining popularity in industrial and medicinal sector. Yam is the fourth most important tropical root and tuber crop after potato, cassava and sweet potato (Fu et al., 2005) [14] . Over the decades, the area under yam production in India has increased from 1.15 million hectare in 1961 to 5.05 million hectare in 2013 with an increasing yield from 72.35 thousand metric tons in 1961 to 119.12 thousand metric tons in 2013 (FAO, 2014). As compared to other cultivated species of yam, D. alata is superior to Performance of individual accession was recorded and composite sample from the three replications of each accessions were collected for further chemical analysis. The quantitative data were analyzed for analysis of variance (ANOVA) using RBD to identify genotypes that were significantly different from each other. Principal component analysis (PCA) was done using R software using factorMinor package (Le et al., 2002). The PCA data was used to generate eigenvalues, cumulative variability and load coefficient values. The principal component with eigenvalues >1.0 were selected and those traits that had load coefficients > 0.6 were considered relevant scores for the PC and considered as valuable traits for distinguishing between the genotypes (Jeffers, 1967) [22] . Cluster analysis was used to estimate pair-wise genetic similarity values based on qualitative traits among accessions using Jaccard's dissimilarity index. The similarity matrix base on Jaccard's coefficient of dissimilarity then used to construct a dendrogram using Ward's linkage method with IBM SPSS software.

Results and Discussion

Variation in qualitative traits assessed [3] where out of 49 D. alata accessions used for the study, 71.4 per cent had green leaf margin colour which was the majority over purple and yellow leaf margin colours. [3] and Munaweera et al. (2020) [31] both found comparable outcomes, indicating that the cordate leaf shape was the predominant shape observed during their examination of morphological variations in Dioscorea spp. in Ghana and Sri Lanka, respectively. Regarding matured leaf margin colour, majority of the germplasm under study (32 germplasm) possessed green coloured leaf blade margin and only few (5 germplasm) had purple margin. The existence of two types of petiole wing colour i.e. purple and green had similarly been reported by Islam et al. (2011) [21] with purple petiole wing colour being the dominant in D. alata accessions. However, in our study, green petiole wing colour is the most dominant (25 germplasm) followed by green with purple edges (9 germplasm) and purple (3 germplasm). Our study is in line with the findings of Anokye et al. (2014) [3] . Majority of the germplasm (29 germplasm) under study exhibited intermediate distance between lobes followed by no measurable distance between lobes (5 germplasm) and only three germplasm exhibited very distant distance between leaf lobes. With respect to leaf tip shape, the germplasm fell into two categories as acute (32 germplasm) which was the majority and obtuse (5 germplasm). The flowering aspect is advantageous to breeders since genotypes that flower can produce seeds for sexual reproduction to enhance genetic improvements (Denadi et al., 2020) [12] . In our study, only D. alata accessions flowered. Type of inflorescence among the germplasm were found to be male which further categorized under three groups i.e. spike (5 germplasm), raceme (4 germplasm) and panicle (5 germplasm). Wu-Wenqiang et al. (2019) [46] reported about 41.5 per cent flowering in D. alata genotypes in China. Sartie and Asiedu (2014) [39] observed that about 45 per cent of D. alata genotypes produced flowers with the male being 49 per cent and female 19.9 per cent. No variation was observed among the Dioscorea alata germplasm regarding colour of inflorescence. All the inflorescence bearing germplasm manifested yellow colour of inflorescences under investigation. The formation of aerial bulbils in D. alata has been reported (Sanada at el., 2018; Wu et al., 2019) [38,46] .In our study, majority of germplasm produced no areal tubers (26 germplasm) while only germplasm produced aerial tuber. Variation in the shape of aerial tubers was found among the germplasm under evaluation. Aerial tubers manifested most variable shapes i.e. round, oval, elongated and irregular. Jyothy et al. (2017) [23] also reported variations in aerial tubers among D. alata accessions from India in which oval, round, cylindrical and irregular tuber shapes were observed.

Regarding skin colour of aerial tubers, three main colours were observed viz., greyish, dark brown and light brown whereas, the study on the surface texture of aerial tubers revealed three predominant textures among the Dioscorea alata germplasm under investigation i.e. smooth (7 germplasm), rough (3 germplasm) and wrinkled (1 germplasm).

With respect to number of underground tubers per hill, the greater yam germplasm were grouped into two categories of which 24 germplasm were under few (2-5 numbers per hill) group and remaining 13 germplasm were categorized under several (>5 numbers per hill) group. Based on relationship of tubers, three types of variations were observed among the germplasm, of which 8 of them were completely separate but closed together, germplasm were fused at neck region and majority of germplasm (17 germplasm) were completely separate and distant.Three predominant places for roots were found on tuber surface among the Dioscorea alata germplasm under investigation i.e. entire tuber (19 germplasm), upper surface (17 germplasm) and lower surface (1 germplasm). Roots on tuber surface classified the germplasm into two groups viz., few and many. Among them, majority of germplasm (30 germplasm) produced few roots on tuber surface while 7 germplasm produced many roots on tuber surface. Tuber skin texture varied from wrinkled (20 germplasm), which was the majority, followed by smooth (11 germplasm) and rough (9 germplasm). Similar observations were reported by Jyothy et al. (2017) [23] regarding tuber skin texture where out of 45 accessions conserved in CTCRI, Kerala, majority of the germplasm had warty skin texture while others have moderately smooth and smooth skin texture of underground tubers of D alata. Regarding tuber shape, a wide range of morphological variation was observed among the accessions. The observed tuber shapes were oval, cylindrical, digitate, oval-oblong and irregular. In the diversity pattern study of D. alata in Brazil (Siqueria et al., 2014) [41] three tuber shapes, namely irregular, oblong and round were found. But in the present study more than three shapes were found indicating more variability of this crop in India. Our observations are in line of research with Jyothi et al. (2017) in CTCRI, Kerala. Regarding tuber skin thickness, germplasm possessed skin thickness <1mm while 22 germplasm produced skin with >1mm thickness. Wide variation existed among the germplasm pertaining to skin colour beneath the bark. There was an equal percentage of yellowish white (45.95%) and light purple (45.95%) which was followed by dark purple to cream colour. Anokey et al. (2014) reported that light maroon tuber skin colour beneath the bark were in the majority among the 49 accessions of D. alata studied in Ghana, followed by dark maroon with only few collections having greyish tuber skin colour beneath the bark. Regarding skin colour at head portion of the tubers, variation was observed under evaluation. Tubers manifested variable colours at head portion i.e. yellowish white was in majority (23 germplasm), followed by purple with white (6 germplasm), light purple (4 germplasm), purple (3 germplasm) and white (1 germplasm). Four distinct flesh colours were observed among the germplasm under investigation. Majority of the germplasm (19 germplasm) exhibited white flesh colour, 10 germplasm produced yellow flesh colour, 5 germplasm expressed light purple flesh colour and remaining 3 germplasm manifested yellowish white flesh colour at central transverse cross section of underground tubers. Similar observations have been reported in the studies done by Demuyakor et al.

(2013) [11] and Anokye et al. (2014) [3] . Among the genotypes evaluated, yellowish white / off-white emerged at the pre dominant character among the flesh colour of the accessions. The sequence that ensued was purple, purple with white and yellow having the least relative frequency. Jyothy et al. (2017) [23] observed that 86.7 per cent of yam accessions had off-white while 2.2 per cent were purple flesh colour. Cream and purple flesh colours were also dominant colours in D. alata reported by Trimanto and Hapsari (2015) [43] .

Variations were observed regarding tuber flesh oxidation colour of the germplasm under study. Four main colours were observed viz., yellowish brown, brown, light brown and light purple of which yellowish brown colour was the majority (17 germplasm), followed by light brown (10 germplasm), light purple (7 germplasm) and brown (3 germplasm). Regarding tuber flesh texture, majority of the germplasm had smooth texture (18 germplasm), followed by smooth (13 germplasm) and very grainy (6 germplasm). Accessing 49 D. alata accessions from Ghana, Anokye et al.

(2014) [3] observed that majority of the accessions had smooth texture while others had grainy tuber flesh. Variation in quantitative characters (Morphological characters) Several factors influence quantitative traits and hence are not stable as qualitative traits. Among the 37 D. alata accessions, data was recorded for eighteen quantitative characters and performed ANOVA (Analysis of Variance). [21] .

Their study presented that the length of main tuber ranged between 5.4 to 66.4 cm and width was 2.5 to 15. [29] . Lebot et al.

(2005) [25] observed that D. alata varieties with good eating quality are characterized by high dry matter, starch and amylase contents.

Starch is known to account for about 80 per cent on a dry weight basis of yam carbohydrate, which is also a dominant factor in determining the physicochemical, rheological and textural characteristics of yam food products (Moorthy, 1994) [30] . There is a significant range of starch content in white yam (Dioscorea rotundata), with values ranging from 33.9 to 75.7 per cent (Oladeji et al., 2014) [34] and 93.67 per cent (Adegunwa et al., 2011) [1] . On the other hand, Dioscorea alata has a starch content of 62.94 per cent (Adegunwa et al., 2011) [1] , which is comparable to the findings of Baah et al. (2009) [7] with a range of 16. [25] . In present study, percentage of sugar content ranged between 4.48 per cent for CVand 11.60 per cent for CVwith a mean of 8.04 per cent. A comparable range of 0.62 to 15.8 per cent sugar had been reported for D. alata varieties by Maziya-Dixon and Asiedu (2003) [29] . A wide range of soluble sugar in D. alata had also been detected earlier by Oladeji et al. (2004). Germplasm like CV29 and CVwere recorded with the highest ash content where as lowest ash content was found in germplasm CV23 (Table.9). The ash content serves as an indicator of the mineral status, despite the fact that contamination may suggest a high concentration in a given sample (Baah, 2009) [7] . Values ranged between 4.78 per cent to 2.53 per cent with a mean of 3.66 per cent. Comparable ash content (2.50 to 4.90%) had been reported for D. alata tubers (Labot et al., 2005). Crude fiber content ranged between [1] .39 percent to 4.58 per cent with a mean of 2.99 per cent. The presence of crude fiber in our study was found to be significantly higher compared to previous reports in False Yam flour (Icacina trichantha) (1.06 to 1.51%) (Umoh and Iwe, 2014) [44] , greater yam (Bhandari et al., 2003) (0.6 to 1.5%), and elephant foot yam (Lenka and Nedunchezhiyan, 2013). Crude fiber plays a crucial role in preventing obesity by promoting food bowel movement and also contributes to the prevention of diabetes, colon cancer, and other gastrointestinal tract ailments (Topping and Clifton, 2001) [42] . Comparable range of 1.10 to 4.09 per cent crude fiber content for Dioscorea alata had been reported by Patel et al.

(2019) [35] . In the present study, CV21, CVand CV32 genotypes with higher crude fiber content can be selected and cultivated specially for high crude fiber content. The total phenol content in yam has been positively correlated with the rate of browning as browning is attributed to enzymatic oxidation of polyphenols in yam (Asemota et al., 1992; Muzac-Tucker et al., 1993) [4,32] . In present study, total phenol content of D. alata germplasm ranged between 0.46 to 4.69 g/100g. Muzac-Tucker et al.

(1993) [32] reported a range of 0.061 to 10.50 g/100g of total phenol on dry weight basis in D. alata germplasm while 0.023 to 0.034 g/100g was obtained for D. rotundata germplasm. [23] . Haydar et al. (2007) [18] also stated that diversity among genetic materials were contributed by the traits that have the greatest value and positive feature vector. [8] . The first six principal components contributed 49.15 per cent of the total variation and the results are in line with the work done by Jyothy et al. (2017) [23] with 50.12 per cent and was slightly lower than variations observed by Norman et al. (2011) [33] and Agre et al. (2019) [2] with 74.3 per cent and 71.44 per cent of the total variation, respectively. Among the 57 morphological and biochemical characters used in this study, 16 (28.07%) were the most discriminating characters for the D. alata for the yam genotypes evaluated. Our results are in confirmatory with the findings of Anokye et al. (2014) [3] who reported 23 characters (21.5%) out of the 107 characters as the most discriminating traits. In another study, Hasan et al. (2008) [17] reported 25 characters (53.2%) out of 47 characters were the most discriminating. Despite using many characters, only a few characters might be necessary to discriminate among genotypes of different yam species. Our study revealed that, the most discriminating characters such as underground tuber size, shape and skin colour, young stem and leaf colour, leaf number, crude fiber, ash content and sugar content can be used by the farmers and breeders to preliminary identify and distinguish D. alata genotypes of North-Eastern region of India.

Cluster analysis

Cluster analysis of thirty seven Dioscorea alata germplasm generated a dendrogram which divided the grmplasm into two main clusters viz., cluster I (CL-I) and cluster II (CL-II). Observations recorded for the morphological and biochemical characters of D. alata were used for clustering with hierarchical clustering using the ward's linkage. It revealed that the D. alata germplasm exhibited greater degree of variation for the 57 different morphological and biochemical traits observed and germplasm were clustered mainly by stem height, emergence habit, petiole length and skin colour at tuber head. The germplasm having skin colour white with purple at tuber head, light maroon cortex colour and light purple flesh oxidation colour (CV7, CV6, CV37, and CV2) were found to be highly divergent from other germplasm and grouped as a separate sub cluster under study. Similar results were also observed by Sheela et al.

Dendrogram using ward linkage

(2016) [40] and Jyothy et al. (2017) [23] where germplasm with purple flesh colour were found to be highly divergent from other germplasm. Mahalakshmi et al. (2007) [27] also reported the use of morphological traits to obtain a core collection for D. alata by evaluating a total of 772 accessions.

In the dendrogram, most of the germplasm clustered randomly among two groups regardless of geographical locations. It was also observed that geographically closer germplasm were grouped under different clusters. Germplasm like CV37 (check) from Kerala were grouped with the germplasm from Arunachal Pradesh and Assam in cluster I. These results suggested that geographical origin of the D. alata germplasm does not bear any relationship with morphological and biochemical characterization. This is confirmatory with other reports based on morphological analysis by Massawe and Temu (2022) [28] , Jyothy et al.

(2017) [23] and Anokye et al. (2014) [3] . The informal nature of yam trade and exchange of planting materials among farmers and states leads to duplication of planting materials and majority of the germplasm were most likely clones of a common source (Anokye et al., 2014) [3] . Furthermore, lack of breeding and conservation programmes could have contributed to the similarity of yam genotypes between the region (Massawe and Temu, 2022) [28] . Atieno et al. (2020) [6] also noted this phenomenon, where yam genotypes from various regions of Kenya were classified together, regardless of their geographical origins.

Conclusion

In the present study, 37 germplasm of greater yam collected were morphologically and biochemically analyzed using the descriptors of yam. According to the present study, there were no duplicate germplasm identified and it can be maintained as core collection because it depicts wide spectrum of variability and also represent landraces representing different yams growing regions of the North-Eastern states. Moreover, out of 37 germplasm studied, 10 germplasm viz. G7 (Dhubri), G16 (West Kameng), G5 (Bongaigaon2), G29 (Sivasagar), G13 (Barpeta), G14 (Goalpara1), G3 (Nagaon), G27 (Namphai), G2 (Mokokchung) and G12 (Tezpur2) showed highest values for desirable characteristics. Among these 10 germplasm, G7 (Dhubri) showed highest values for maximum 5 desirable characters followed by G16 (West Kameng1) for 4 desirable characters. Therefore these germplasm can be further evaluated and used in breeding programmes. Principal component analysis showed that the eigen values of the first six principal components of data accounted for 49.15% of the total variance among germplasm. Morphological traits like tuber width, tuber length, tuber skin colour, stem height, distance between lobes, leaf tip shape, young leaf margin colour, young leaf vein colour, young leaf petiole colour, internodal length, tuber shape, days to emergence, leaf number, tuber yield, stem colour and biochemical characters like crude fiber content, ash content and sugar content were all found to be of great importance in distinguishing the germplasm in greater yam. The 37 greater yam germplasm were grouped into two major clusters with Cluster I having maximum number of germplasm (13) and Cluster II with 24 germplasm. The cluster dendrogram based on morphological and biochemical data revealed that the D. alata germplasm exhibited very high level of variation and no duplicate accessions were observed among the genotypes studied. So, conservation of these genotypes and inclusion in food chain as an additional tuber crop next to potato, cassava and sweet potato will help to curb malnutrition & lower incidence of other diet related diseases particularly in weaker section of the society. This will broaden the range of uses for this underutilized tuber beyond its current utilization in the North-Eastern region of India.

Acknowledgement

AcknowledgementThe authors thank the Assam Agricultural University (AAU), Jorhat, Assam for allowing conducting the research work. They also acknowledge the whole staff members in the Department of Horticulture, Crop Physiology, Agricultural Biotechnology and Agricultural Statistics under AAU, Jorhat Assam and the farmers who provided planting materials for the study.

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