On the development of descriptors in small bitter gourd (Momordica charantia L. var. muricata (Willd.) Chakrav)

Small bitter gourd (Momordica charantia L. var. muricata (Willd.) Chakrav.) is an under-utilized vegetable with ethnobotanical and therapeutic values, under domestication interphase. This taxon deserves a distinct crop status, owing to the existence of well-marked distinguishing morphological traits, unique flavour and taste apart from M. charantia var. charantia, wide variability within the taxon, preferential cultivation by the traditional farmers in Indian subcontinent and occurrence of distinct, named landraces. However, presently this crop is facing a threat of genetic erosion, restricting mostly at the homestead level throughout the region. In the present study, seventy-four small bitter gourd collections from across India were assessed for understanding morphological variability and their classification, and further devising key descriptors helpful for effective characterization and utilization. In order to describe it under a distinct taxon level, botanical description is also worked out. Morphological traits such as shape and lobing of leaf, shape and size of fruit, nature of tubercles and ridges, and size, shape and colour of seeds, besides robustness and extended growth period, showed remarkable character differentiation from cultivated bitter gourd. A set of 45 characterization and 35 evaluation descriptors have been developed, out of which 27 characterization and 26 evaluation descriptors are categorized under the minimum descriptor list.


Introduction
Vegetables are an important component of the human diet as they provide vitamins and minerals crucial for our metabolic functions. There are more than 60 cultivated and about 30 lesser-known vegetable crops grown in India due to the diverse agro-climatic conditions that existed in the region (Jena et al. 2018). Crop wild relatives have the ability to supply beneficial genetic traits important for crop improvement, thus playing a central role in breeding activities (Harlan and de Wet 1971;Maxted et al. 2006Maxted et al. , 2010. Recently wild edible forms are sought-after crops owing to their nutritional and therapeutic properties (Flyman and Afolayan 2007;Jiji 2014;Naik et al. 2017), and/ or as the source of tolerance to biotic and abiotic stresses (Asna et al. 2018;Sharma et al. 2019;Simon et al. 2021). The traditional farming community conserve them in the wild habitat, and always maintains these crops in their home garden or being grown as Abstract Small bitter gourd (Momordica charantia L. var. muricata (Willd.) Chakrav.) is an under-utilized vegetable with ethnobotanical and therapeutic values, under domestication interphase. This taxon deserves a distinct crop status, owing to the existence of well-marked distinguishing morphological traits, unique flavour and taste apart from M. charantia var. charantia, wide variability within the taxon, preferential cultivation by the traditional farmers in Indian subcontinent and occurrence of distinct, named landraces. However, presently this crop is facing a threat of genetic erosion, restricting mostly at the homestead level throughout the region. In the present study, seventy-four small bitter gourd collections from across India were assessed for understanding morphological variability and their classification, and further devising key descriptors helpful for effective characterization and utilization. In order to describe it under a distinct taxon level, botanical description is also worked out. Morphological traits such as shape and lobing of leaf, shape and size of fruit, nature of tubercles and ridges, and size, shape and colour of seeds, besides an orphan crop in the semi-disturbed to un-disturbed areas or as a sole crop in few pockets. Domestication, involved in bringing these lesser-known crops to mainstream agriculture, which in general, has led to changes in fruit morphology, reduction of bitterness, loss of dormancy, uniform seed germination, determinant growth habit, yield increase, etc. in various crops according to the need of the humans. As a result of this, a number of crop wild relatives have undergone domestication, thus attaining the status of cultivation either locally or on a wider scale over the years. Teasel gourd (Momordica subangulata subsp. renigera (Wall. ex G.Don) W.J.de Wilde) in North-Eastern states (Bharati and John 2013), Tuber cowpea (Vigna vexillata (L.) A. Rich.) in tribal pockets of Indian hills (Arora and Pandey 1996;Tripathi et al. 2021), V. stipulacea (Lam.) Kuntz. in Tamil Nadu state, (Tomooka et al. 2011), Kokum (Garcinia indica (Thouars) Choisy) in North Western Ghats and Konkan region (Devi et al. 2012;Singh 2014), Malabar tamarind [Garcinia cambogia (Gaertn.) Desr.] in Kerala part of Western Ghats ( Abraham et al. 2006) and Clove bean (Ipomoea muricata (L.) Jacq.) in Western Ghats (Parisa et al. 2019), etc., are some of them to cite, which have now attained cultivation status in various pockets in India.
The genus Momordica comprising about 45 species (Mabberley 2017), is distributed chiefly in Africa while eight occur in India, including recently added new species M. janarthanamii Gosavi, Gholave, Madhav & Kambale (Gosavi et al. 2022), from North-Western Ghats of India. De Wilde and Duyfjes (2002) gave a detailed taxonomic treatment of the genus Momordica in South and South-East Asia and for India by Joseph and Antony (2010). M. charantia var. muricata is acknowledged as the progenitor species of bitter gourd (Degner 1947;Chakravarty 1982;Joseph andAntony 2009, 2010). M. charantia var. muricata, was first described by Willdenow (1805; page 602) based on the illustrations and descriptions provided in the seventeenth-century publication Hortus Malabaricus (van Rheede 1688; Plate no. 10). According to him, it was described as: Affinis praecedenti sed fructus tuberculis densis crassis muricatus, non sulcatus et multo minor, nempe sesquipollicaris meaning 'closely related to proceding, but the fruit thick with tubercles, muricated, not-grooved, and much smaller, namely only one and half inches'. Roxburgh (1832) designated them as M. muricata Willd. and later, Chakravarty (1990) treated M. muricata (Willd.) as a varietal entity, with small and round or spindleshaped fruits and M. charantia var. charantia, those producing fairly large fusiform fruits. Of late, Prasad (2016) has assigned M. charantia var. muricata to a forma status ie, M. charantia f. muricata. Owing to its distinct nature, this taxon is variously assigned with different ranks i.e., spp., var., forma status. It seems that Momordica charantia f. abbreviata (Ser.) W.J.de Wilde & Duyfjes, described as delicate plant, fruit ± fusiform 2-5 cm long, with 6 rows of few, low, broad-based soft prickles with acute apex-growing wild, maybe a synonym of M. charantia var. muricata. World Checklist of Vascular plants (https:// wcvp. scien ce. kew. org/ taxon/ 77161 687-1 accessed on 23 May 2022) has treated this taxon as a synonym of Momordica charantia subsp. charantia. However, our observations suggest that this taxon requires a distinct taxonomic status at the variety level, supporting the view of Chakravarty (1990).
The domestication process of wild bitter gourds is evident by the occurrence of semi-domesticated (naturally introgressed) forms in considerable frequency in the homesteads of Peninsular and North-Eastern hills of India. It was also observed that in few pockets, it is grown as a sole crop. Recently, Matsumura et al. (2020) also reported the occurrence of wild types and their designation as cultivars in the new world, presumably migration from the old-world countries. The domesticated material was believed to arrive from Africa to Brazil with the slave trade, then dispersed to Central America (Ames 1939;Marr et al. 2004). The domestication process of cucurbits in general includes non-bitter fruits (Marr et al. 2004), increased fruit size, higher sugar content, decreased spininess, increased biochemical quality attributes like carotenoid content and more compact and less branched growth with apical dominance (Chomicki et al. 2020). Nevertheless, fruit bitterness was always been accepted as a desirable trait in wild and cultivated bitter gourd. However, it is unclear whether the bitter fruits were the deliberate choice of humans or due to the fact that a non-bitter variant has not been found (Marr et al. 2004). Momordica charantia is native to Africa (Marr et al. 2004;Schaefer and Renner 2010) and the centre of diversity is ascribed to be South India by Joseph and Antony (2008). Diversity analysis based on simple sequence repeat markers among Asian collections of bitter gourd landraces and hybrids showed that South Asia is the centre of domestication of bitter gourd (Dhillon et al. 2016) and the abundant occurrence of wild types in the fields in North-eastern India is also reported by Gaikwad et al. (2008). As per our observations during the exploration and collections missions across the country, varied morphological forms of M. charantia var. muricata are available in the Indian subcontinent.
To meet the changing climate and other challenges, breeders need to have access to a wide range of plant genetic resources with relevant information to facilitate their use. For effective cataloguing, and exchange of data, a compatible documentation system which follows a common standard of description is needed, and the descriptor lists provide such well-established standards for cataloguing (Bioversity International 2007). Crop descriptor lists have already been published for 102 agri-horticultural crops (Bioversity International 2007); however, a number of crops were overseen by the crop curators to be included in crop descriptor lists, which were otherwise being cultivated locally and often not commercially important. To date, crop descriptors have already been developed for under-utilized crops like dioecious Momordica species (Joseph and Antony 2011) and tuber cowpea (Tripathi et al. 2022), both minimal descriptors (NBPGR 2001) and DUS (Distinctness, Uniformity, Stability) descriptors (https:// plant autho rity. gov. in/ sites/ defau lt/ files/ dbitt ergou rd. pdf) available for the cultivated (large fruited) bitter gourd, however, less attention was given to characterize the wild and semi-domesticated small bitter gourds coming under the taxon var. muricata. Though muricata has close resemblance to M. charantia, they can be easily identified by the smaller size of fruits and seeds (Bharathi and John 2013) and Ghosh et al. (2018) while studying the karyotypes of var. charantia and var. muricata populations, found significant differences in genome size between the two, with slightly greater chromosome size in var. muricata (1.32-3.24 µm) than var. charantia (1.27-3.07 µm).
Understanding the importance of small bitter gourd as a crop, promoting its use as fresh vegetable, dried stuffed products, its nutraceutical and medicinal values and occurrence of wide variability between the landraces or primitive cultivars representing different agro-climatic regions, domestication potential as an independent crop, and being under the threat of genetic erosion; development of a separate descriptor list for small bitter gourds is of relevance. Systematic characterization and evaluation studies using the proposed descriptor states can lead to the identification of trait-specific germplasm for utilization in crop improvement programmes.

Materials and methods
The present study was done based on morphological variability observed in 74 live collections of small bitter gourds comprising both wild and semi-domesticates from nine Indian states (Table 1). In addition, published descriptions of this taxon in various flora, observations made during field trips, and the available virtual herbarium specimens housed at Kew Herbarium (K!), Edinburgh (E!), Paris (P!) and India (BSI! NHCP!) including the type specimens were also referred. The diverse accessions were raised in the field from June-September, 2020 at ICAR-NBPGR, Regional Station, Vellanikkara, Thrissur, located at 10.5480° N, 76.2830° E, to understand the variability pattern and to identify and validate key descriptors. The temperature during the crop period was between 22.4 and 31.1 °C, with a mean rainfall ranging from 427.20 to 607.7 mm and an average of 19.75 days of rainy days per month.
The recommended agricultural package of practices was followed for proper growth of the crop. Guidelines and definitions provided by Bioversity International (2007) Nayar and More (1998) and Beentje (2010), and modified accordingly. The illustrations of the plant parts i.e., leaves, fruits and seeds were drawn using isograph high precision technical pen and NikonD5600 camera was used to take the photographs. RHS colour chart developed by the Royal Horticultural Society, UK (http:// rhscf. orgfr ee. com/c. html) was referred for indicating fruit colour. Observations were documented for seven seedling, nine stem, 11 leaf, 16 flower, 19 fruit and 13 seed characters, along with preliminary observations on major biotic and abiotic stress tolerance and tests of organoleptic quality.

Variability pattern and descriptor development
The true wild types are characterized by green spindle shaped fruits with rostrate blossom end, sharp tubercles, mildly raised bumps and long peduncle in relation to the fruit size. The dimensions of the leaves, flowers, fruits will be slightly greater in semidomesticated/ introgressed genotypes compared to the true wild types. The semi-domesticated types from Madhya Pradesh and North Eastern states of India possessed green, rhomboid fruits with raised tubercles either ridged or discontinuous, but larger than the spindle shaped fruits. Fruits of Rudrakshahagali landrace are green, globose with both tubercles and bumps raised and blunt. Spindle to elliptical fruits with partially ridged or broken tubercles were the characters of Kuttathipaval/ Kattupaval of Kerala. However, fruits of Uchie were with variable shapes, but with comparatively thick fleshed pericarp. Figure 1 depicts the morphological diversity of landraces in small bitter gourd. Small bitter gourd is also shade tolerant, an ideal crop for semi-forest habitat and a candidate for food forest concept along with the trees. Also, wild types are reported to be tolerant to biotic and abiotic stresses (Asna et al. 2018). Dhillon et al. (2016) reported that these small-fruited landraces, locally named as Kee Nuk (''bird droppings'') are grown in home gardens in Thailand and also farmers select disease-resistant variants from these populations. The characterization and evaluation descriptors developed for small bitter gourds are given in Tables 2 and 3 respectively. Cotyledonary leaf characters, vinyness, nature of the sinus, leaf lobes and shape of the leaves were the significant variables among vegetative traits, as reported earlier by Asna et al. (2020). Thin stem was present in wild/semi-domesticated cultivars, whereas stem girth was high in the cultivated types (Asna 2018;Asna et al. 2020). Marr et al. (2004) noted that pubescence of stem tips and leaves, and deep lobing in leaves were characteristic of wild types, while the domesticates were glabrous or nearly so. The internodal length was more in cultivated types than the wild/semi-domesticated types, thus deciphering a more compact carpet-like growth in small bitter gourds. Variability in leaf shapes of small bitter gourd was reported by Asna (2018) and generally leaf shapes like ovate, obovate, reniform, orbicular and pedate were observed (Fig. 2), with acuminate, acute or obtuse terminal lobe apex (Fig. 3). Margin of the leaves also varied from undulate or dentate (Fig. 4). Shallow, medium and deep lobing (Fig. 5) with variable sinus nature at leaf base (Fig. 6) were the other key leaf characters.
Both male and female flower characters exhibited comparatively lesser variability. However, the flowering continued for about six months in cultivated types and 8-12 months in wild genotypes (Asna 2018). Fruits being the economic part of the plant, the colour, shape and size are the chief fruit characters from the consumer point of view. Fruit shape exhibited tremendous variability among the collections supporting the views of Robinson and Decker-Walters (1999); Behera et al. (2008); Asna (2018) and Asna et al. (2020). Eight descriptor states justifiable to cover all kinds of fruit shape variability viz., globose, discoid, rhomboid, oblong, cylindrical, narrow spindle, broadly spindle and elliptical shapes were observed in the collections (Fig. 7a-h). The fruit ends were either pointed at both ends or only blossom end pointed or both ends blunt (Fig. 8). Pointed fruit ends were one of the distinct features of spindle-shaped fruits possessed by true wild types. Fruit ribs were either continuous or discontinuous (Fig. 9). The variability in tubercles and bumps with respect to their nature, density and ribbing was very interesting. Tubercles were the prominent projections on the fruit surface whereas the bumps were small projections or the illdeveloped tubercles on the fruit surface. Bumps were either raised or faint and not conspicuously raised or, joined at base (Fig. 10). Four states of descriptors were observed for tubercles (Fig. 11a-d). In true wild types, the tubercles were with sharp pointed tip (Marr et al. 2004;Asna et al. 2020), discontinuous and bumps soft, faint, slightly raised, and fruit blossom end with or without a prominent rostrum (whip like elongation). However, tubercles were discontinuous raised ridges with blunt tips in semi-domesticates supporting the views of Marr et al. (2004). As per their observations, ridged tubercles, increase in pericarp thickness and bump height, and decrease in tubercle base ratio were considered indications of domestication events as they were prominently observed in cultivated bitter gourds. This is evident by the presence of continuous ribs in a popular variety of cultivated       Generally, the small fruit bearing wild/semi-domesticated genotypes produce larger number of fruits per plant (even > than 100 fruits per plant), which may be due to the presence of larger number of primary and secondary branches in these wild genotypes (Islam et al. 2014). However, other yield contributing traits like fruit weight, length, width, flesh thickness, and cavity size were found to be higher in the commercial charantia varieties. The seed shape, colour and size are also indicators of the nature of genotype whether wild or cultivated. Seed colour ranged from cream to light brown to dark brown to grey to black coloured. Black seed colour is considered as a marker trait of wild types (Marr et al. 2004). True wild types possess nearly smooth seed surface with less ornamentation resembling cockroach egg casket with feebly sub-tridentate apex (Fig. 12). Seed sides were either dentate or almost smooth (Fig. 13). Seed ornamentation varied from no sculpturing to markedly sculptured seeds (Fig. 14). Broad rectangular, narrowly rectangular and squarish shaped seeds were produced by the genotypes (Fig. 15). Mithipavai genotypes; a semi-domesticate, traditional cultivar from Kaveri Delta of Tamil Nadu, India has almost squarish (length to breadth ratio nearly unity) shape with brown sculptured seeds. Asna et al. (2020) reported that seed characters have an important role in distinguishing wild muricata accessions from the commercially cultivated varieties and the seed characters like surface evenness, ends and sides of seeds and surface ornamentation are unique for each genotype. Seed surface ornamentation and epidermal patterns has been classically and widely used for taxonomic purpose in a variety of plant groups (Sivarajan et al. 1989). Organoleptic qualities are also of paramount importance from the consumers point of view. Less bitter or bitter-less fruits with unique flavour will be more preferred.
Out of the 45 characterization and 35 evaluation descriptors described in this paper, 27 of characterization and 26 of evaluation descriptors are categorized under the minimum descriptor list, which is considered as the minimum number of characteristics needed to describe the variability in the crop, thus enabling the breeders/ germplasm curators to effectively characterize in a cost-effective manner.

Conclusion
M. charantia var. muricata, an under-utilized vegetable was considered for the study owing to our observations on the occurrence of wide variability between collections and, its distinct morphological features from M. charantia var. charantia. In order to describe it under a distinct taxon level, botanical description is also worked out. Variability encountered at seedling, vegetative and reproductive stages of the plants have been included for preliminary characterization and for evaluation purposes, those traits which are important for crop husbandry along with biotic and abiotic stress have also been included. The proposed descriptors and descriptor states will be useful for the characterization of infra-specific morphological variability not only in small bitter gourd but also will serve as Vol.: (0123456789) Fig. 7 a-h Descriptor states of fruit shape a prelude for cultivated bitter gourd improvement, as they are parallel variations of homologous characters, between the cultivated and wild germplasm. Present treatment of the extent of variability in many characters may not be exhaustive as numerous states were difficult to describe in technical terms but easy to depict through illustrations. It is expected that future collections from unexplored diversity rich areas and study of variability may lead to the identification of more diverse types and accordingly descriptor states may be elaborated and revised.