Morphological evaluation of some indigenous cultivars of greater yam (Dioscorea alata L.) of North East India

Introduction

Greater yam (Dioscorea alata L.) is a monocotyledonous tuber crop, herbaceous climbing plant, belonging to the family of Dioscoreaceae and it is reported to be an old crop species native to South East Asia (Burkill, 1951 and Coursey, 1967) [9,10] . It is also known as 'winged yam' or 'purple yam'. It is the most important tuber crop due to its wider distribution, adaptability, usefulness and greater yield potential. It is the third most important tropical root and tuber crop after cassava and sweet potato (Fu et al., 2005) [14] . It is also an important staple food in many parts of the world (Nedunchezhiyan et al., 2002) [19] . Based on data provided by All India Coordinated Research Project on Tuber Crops, the gross cultivated area under different yams come to approximately 27000 ha with an average productivity ranging between 15-30 t ha-1 in India (AICRPTC, 2016) [2] . Dioscorea alata is cultivated in Assam, Arunachal Pradesh, Meghalaya, Mizoram and other parts of North-East, Kerala, Andhra Pradesh, West Bengal, Bihar, Odissa, Uttar Pradesh, Tamil Nadu, Gujarat, Maharashtra, Andaman and Nicobar Islands. The yams are commonly grown in multiple cropping systems in homestead garden and in tribal areas. In India freshly harvested yam tubers are consumed as boiled, baked or fried vegetable, D. alata is also known for its high nutritional content, with crude protein content of 7.4 percent, starch content of 75-84 percent and Vitamin C content ranging from 13 to 24.7 mg per 100 gm and steroids which have pharmaceutical value. A few yam species are grown and used as health food and for medicinal purposes (Farombi et al., 1997; Albrecht and McCarthy, 2006) [13,3] . Dioscorea alata is the most polymorphic species in the genus Dioscoreaceae (Martin and Rhodes, 1973, Martin and Delpin, 1978) [16,17] . The cultivars of D. alata are known to vary in shape, colour of stem, leaves and tubers (Purseglove, 1972) [22] . Dioscorea alata tubers have variable shapes, the majority being cylindrical. Its tubers vary in number from one to five. The colour of the flesh of the tuber ranges from white to purplish (FAO, 1994) [12] . Typically, Dioscorea alata can be easily distinguished from the other species in the genus by its quadrangular winged stems. However, much confusion on identification exists among the cultivated varieties of the species (Onwueme and Ganga, 1996) [20] due to the presence of hundreds of variants carrying numerous complexes and overlapping characteristics. For proper cataloguing, characterization and evaluation of plant cultivars, knowledge of the gross aspect of plant morphology is the basic requirement of a biologist. Study of morphological traits helps the plant breeder to eliminate duplication of varieties and also identification of genotypes. The study of external morphology and plant structure has also become an integral part of crop improvement programme particularly in producing genotypes more responsive to better management. The present work will also contribute in increasing the knowledge regarding the genetic resources of Dioscorea alata available in North East India as in many cases the resources are threatened due to genetic erosion which is comparatively faster due to changing cropping pattern, crop preferences and changing food habits of human all over. So, the detailed study of morphological characterisation of Dioscorea alata cultivar was considered as the objectives in the present investigation. Most of the researchers have classified the species based on different morphological characteristics. The characters contributing largely to the intra and inter-specific variability are related to the shape, size, flesh colour of underground tubers, shape and colour of aerial tubers; position, shape, size and vein colour of the leaves, petiole colour, shoot growth rate, number of days for shoot to germinate, tuber quality parameters and the cooking quality. The success of any breeding program depends to a large extent on the amount of genetic variability present in the population. Several attempts have been made to classify and catalogue the immense genetic variability encompassed by the species (Burkill, 1917; Gooding, 1960; Bourret, 1973) [8,15,7] . The study of the extent of genetic variation and genetic parameters would help in formulating effective breeding programme. Keeping all these in view, the present investigation was carried out.

Materials and Methods

The experimental material comprised of 27 cultivars of Dioscorea alata (L.) collected from Assam, Arunachal Pradesh, Manipur, Meghalaya, Mizoram, Nagaland and Tripura. Each cultivar was given a code no. for easy identification. The codes were CV1, CV2, CV3 etc. The detail list of cultivar's name, Genotype collected from and place of collection are presented in Table 1. The experiment was conducted under the agro-climatic condition of Jorhat (Assam) in the Experimental farm of the Department of Horticulture, Assam Agricultural University, Jorhat during during 2020-21 and 2021-22.The geographical location of the farm is 26°47N Latitude and 94°12E longitude having an elevation of 86.6 metres above the mean sea level. The experimental material was analyzed in Randomized Block Design with two replications. The plot size was 3.6 m x1.2 m with spacing of 120 cm x120 cm. Yam setts or cut pieces weighing 200-250 g each were planted in pits of size 45cm x 45cm x 60 cm. The collections were characterized and evaluated using plant descriptor published by IPGRI (International Plant Genetic Resource Institute), 1997. Morphological characters of mature leaf and underground tubers were studied to establish the varietal differences on various aspects. All these traits were recorded through visual observation from each of the sampled plants. Mature leaf characters were assessed at maturity stage.

Results

Qualitative characters are considered as important markers in describing the physical appearance of a plant. In this study the characters of young stem, mature stem and mature leaf and underground tuber were evaluated to know the morphological variability of the cultivar.

Young stem character

The young stem was evaluated for four qualitative characters viz. stem colour, presence or absence of wings, wing colour and presence/absence of spines at the base (Table 2). In respect to stem colour, three classes of variation were observed among the cultivars studied. Most of the cultivars (CV3, CV4, CV5, CV7, CV8, CV9, CV10, CV11, CV14, CV16, CV18, CV19, CV20,CV24, CV25, CV26, CV27) had purplish green colour stem, few (CV1, CV2, CV6, CV12, CV13, CV15,CV21, CV22, CV23) had green colour and only one cultivar, CV17 had purple colour stem. All the cultivars were found to possess wing except CV22 and based on the intensity of wing colour, cultivars were characterized into green with purple edges group (16 cultivars), purple (7 cultivars) and green (3 cultivars). Cultivar CV1, CV11, CV17, CV19 and CV22 had spines at the stem base whereas all the other cultivars had no spines at the stem base.

Mature stem character

Stem colour, stem cross section shape and absence or presence of ridges was considered as important characters of mature stem for characterization of the greater yam cultivars (Table 2). Two distinct types, green colour (11 cultivars) and purplish green (16 cultivars) were observed with respect to matured stem colour. Based on the stem cross section at the base all the cultivars had square stem cross section and all the cultivars had ridges.

Mature leaf Characters

In matured leaf, characters like leaf lobation, leaf colour, leaf vein colour, leaf margin colour, waxiness, leaf shape, leaf apex shape, distance between lobes, petiole colour and petiole wing colour were studied (Table 3).With respect to leaf lobation, the greater yam germplasm were grouped into shallowly lobed (6 cultivars) and deeply lobed (20 cultivars). Regarding leaf colour three types of variation were observed among the cultivars, of which, 4 cultivars were pale green in leaf colour, 16 cultivars were green in colour and the remaining 7 cultivars were dark green. Based on the leaf vein colour, the most common group was green (26 cultivars) and only 1 cultivar (CV 17) had purple leaf vein. Two types of variation were observed for leaf margin colour, of which, 6 cultivars had purple margin colour and the remaining 21 cultivars had green margin colour. The cultivars were divided into two group with respect to waxiness of leaf, of which, 16 cultivars exhibited waxiness on the upper surface of leaf and the remaining 11 cultivars exhibited waxiness on both upper and lower surface of leaf. Four types of variation were observed regarding the leaf shape, of which, 14 cultivars were cordate shape, 6 cultivars were cordate long, 4 cultivars were cordate broad and 3 cultivars had sagittate long leaf shape. Most common group of leaf apex shape were of acute type (25 cultivars) and only two cultivars showed obtuse apex shape viz., CV 7 and CV10. Based on the distance between lobes, the cultivars were classified into three groups of which 14 cultivars were of no measurable distance, 9 cultivars were intermediate, and the remaining 3 cultivars were grouped under very distant category. Based on the intensity of petiole colour, cultivars were categorized into green, green with purple ends, purplish green with purple base, green with purple base, and green with purple leaf junction. Among them, 2 cultivars (CV6 and CV23) were under green group, 10 cultivars were fall under green with purple ends, 4 cultivars in purplish green with purple base, 7 cultivars in green with purple base, 3 cultivars were under green with purple leaf junction group. Variation were observed for petiole wing colour, three classes of variation were observe, 2 cultivars had green petiole wing colour, 10 cultivars had purple colour and 14 cultivars had green with purple edges except CV22 which had no wing.

Underground tuber characters:

The underground tubers characters were number of tubers per hill, relationship of tubers, tuber shape, roots on the tuber surface, tuber skin thickness, tuber skin colour beneath the bark, skin colour at the head of tuber constituted some distinguishing characters to know the variability of the germplasm (Table 4).With respect to number of tubers per hill, the greater yam cultivars were grouped into three categories of which 10 cultivars were under one (1) group, 8 cultivars were under few (2-5) and 9 cultivars were placed under group several (>5) number of tubers per hill. Based on the relationship of tubers three types of variation were observed among the cultivars, of which 9 of them were completely separate but close together, 12 cultivars were completely separate and distant and 6 cultivars were fused at the neck region. Tuber shape is an important character for varietal identification. In the present study, Seven types of variation was observed of which 5 cultivars were categorized under cylindrical group,8 cultivars under oblong, 7 cultivars under oval oblong, 2 cultivars each under oval, forked and flattened while only 1 cultivar under round shape. Roots on tuber surface classified the cultivars into two groups viz., few and many. Among them, 15 cultivars were placed under few group and 12 cultivars in the second group i.e. many group. Depending upon the skin thickness, cultivars were classified into two groups, <1 mm and >1 mm group of which 16 cultivars were grouped under <1 mm group and the remaining 11 cultivars were under >1 mm group. Tuber skin colour beneath the bark as a discriminating parameter divided the 27 cultivars into six groups which included purple, light purple, brown, light brown, offwhite and maroon colour where in 3 cultivars fall under purple group, 3 cultivars in light purple, 8 cultivars under brown group, 6 cultivars under light brown, 6 cultivars in offwhite group and only 1 cultivar fall under maroon group. For skin colour at head of the tuber, cultivars were grouped into yellowish white or offwhite (14 cultivars), white (12 cultivars) and only 1 cultivar (CV2) in purple group.

Discussion

Wide range of variation was observed in various morphological characters used in the characterization of the greater yam. If morphological observations differ consistently from all others then a cultivar is considered to be distinct. Athira et al., (2017) [5] evaluated 45 accessions of greater yam for various morphological traits like tuber shape, tuber flesh colour, tuber skin texture, tuber cortex colour, leaf shape and young leaf colour and found all these parameters to be of great importance in distinguishing the accessions. Morphological variation among 70 accessions of greater yam for shape, size and flesh colour of underground tubers, shape and colour of aerial tubers and position, shape, size and vein colour of the leaves were also reported by Sayed et al., (2008) [24] . Rao et al., (2019) [23] reported high variation among 27 genotypes of greater yam in terms of shape of leaf, position of leaf, distance between lobes, colour of wing, stem and leaf, pigmentation of petiole on leaf base, formation of aerial tuber, skin and flesh colour of underground tuber, shape of tuber, presence of hairiness on tuber and spines on stem. Significant variation for number of tubers per plant was also reported by Mulualem and Ayanew, (2012) [18] , Adigbe et al. (2015) [1] , Bassey and Akpan, (2015) [6] and Panja et al., (2016) [21] . In the diversity pattern study of D. alata in Brazil by Siqueria et al., (2014) [25] three tuber shapes, namely irregular, oblong and round were found. Here more than three number tuber shapes indicates more variability found in North East India. Similar observations have been reported in the studies done by Demuyakor et al., (2013) [11] , Anokye et al., (2014) [4] and Tiama et al., (2016) [26] . In this study the 27 cultivars exhibited wide variation for most of the morphological characters and were classified into distinct group. There were no duplicate accessions identified based on the morphological classification and it can be maintained as core collection because it depicts wide range of variability. Variation in morphological characters of greater yam cultivars could be used as an index in varietal identification of greater yam and to detect duplication of varieties. Present study provides a scope for the selection of desirable parents for hybridization programs.

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