Yardlong bean being a leguminous vegetable crop is likely to arose from the domestication of cultivated Cowpea in Asia. It is a diploid species with a chromosome number of 2n = 2x = 22. According to
Xia et al., (2019), a K-mer distribution analysis estimated the genome size of the Yardlong bean to be approximately 590 Mb. Gene prediction was carried out using a combination of RNA-Seq-based, homologous and de novo approaches, resulting in the identification 42,609 protein-coding genes. Furthermore, their findings indicated that the quality of the Yardlong bean genome assembly is superior compared to that of the common cowpea. In India, the crop occupies an area of about 23,012 hectares, producing around 1,33,587 tonnes of green pods with an average productivity of 5,800 kg per hectare
(Leelavathi et al., 2021).
It is widely grown as vegetable across China and South east Asian region. This crop can survive in warm, humid conditions but is sensitive to low temperatures. Yardlong bean and cowpea exhibit distinct phenotypic differences that have arisen due to divergent selection during their evolutionary process. Cleistogamous flower structure makes this crop highly self-pollinated. The flowers of the Yardlong bean appear in a range of colors, including white, cream, yellow and purple. Its seeds vary in texture from smooth to wrinkled and range in color from red to black, with a distinct hilum encircled by a dark band (Fig 1). It is vigorous, annual climber that can reach a height of three to four meters. It bears exceptionally long, slender and tender pods that vary in color (Fig 2), ranging from white and light green to dark green, brownish red, or purple (
Kamala et al., 2014). Its productivity and pod quality often decline during the monsoon season, primarily because of heavy rainfall and the prevalence of pests and diseases (
Varghese and Celine, 2015). The plant typically exhibits a creeping habit with an indeterminate growth pattern; however, some varieties tend to be more compact and bushier. Its leaves are trifoliate, consisting of three leaflets arranged alternately along the stem and they show considerable variation in size and shape. The crop develops a strong taproot system and bears flowers in racemes along the stems. These racemes are borne on peduncles measuring about 10 to 30 cm in length, each typically carrying two to three pods. The distinctively long peduncles are a characteristic feature of the Yardlong bean, facilitating easy hand harvesting compared to other leguminous crops. From a nutritional standpoint, the immature pods are an excellent source of nourishment, containing 3.5-5% crude protein and appreciable amounts of vitamins A and C, iron, calcium, phosphorus and dietary fiber, making them a valuable vegetable for human consumption
(Singh et al., 2001). These attributes make yardlong beans a highly nutritious
(Hazra et al., 2007) and valuable vegetable crop. However, in India, only a limited number of commercial Yardlong bean cultivars with high yield potential are available. Moreover, suitable breeding approaches for the genetic improvement of this nutrient-rich vegetable remain insufficient. Therefore, the objective of this article is to compile existing information on Yardlong bean improvement, identify diverse parental lines for use in hybridization programs and understand the inheritance patterns and genetic factors influencing yield and its contributing traits. Assessment of genetic diversity, variability parameters and inheritance pattern is important part of crop improvement as these biometrical tools provide insight into available genetic variation and gene action governing the inheritance pattern of characters from one generation to another generation and also in the selection of genotypes for further breeding programme, it also provides idea to plant breeder about need of creation of variation through hybridization, mutation, genetic engineering and introgression of exotic genotypes.
Review methodology
Relevant literature published between 1987-2026 was searched using different academic search tools like Google scholar, Scopus, PubMed, web of science and research gate. Representative Boolean strings combined these axes. One string took the form of the resource and technology axes combined as follows: “yardlong bean” AND “genetic diversity”, “yardlong bean” AND “genetic, improvement”, “yardlong bean” AND “breeding”, “yardlong bean” AND “germplasm”, “yardlong bean” AND “yield”, “yardlong bean” AND “quality”. Research paper was selected based on their relevance to the objectives of review and reporting information on genetic diversity, variability, heritability, heterosis breeding, combining ability and gene action in yardlong bean. Studie were selected when they addressed aspects relevant to the objectives of the review, including origin, domestication, genetic diversity, germplasm characterization, breeding, yield, quality, stress tolerance, molecular genetics, genomics, or crop improvemen, peer-reviewed scientific journal, conference proceeding, thesis or authoritative institutional publication, where appropriate. The publication contained sufficient methodological or scientific information to evaluate its findings. Both recent studies and important earlier publications were considered so that current developments could be discussed while retaining historically important information Research papers excluded when they were unrelated to yardlong bean or the specific objectives of the review; contained insufficient scientific or methodological information.
Approaches of genetic improvement in Yardlong bean
In India, Yardlong bean remains an underutilized vegetable, primarily grown in the peninsular regions such as Kerala, Tamil Nadu, coastal Andhra Pradesh and Odisha
(Sherly et al., 2025). India is believed to be the centre of cowpea diversity because of two important “cultigroups”
viz., Biflora and Sesquipedalis which have evolved from Unguiculata (
Ng and Marechal, 1985). The evolution of vegetable cowpea (yardlong bean) from cowpea in the Asian region was also reported by
Kongjaimun et al., (2012b). The domestication genetics of the Yardlong bean hold considerable scientific importance, as this crop followed an evolutionary pathway distinct from that of cowpea, which is primarily grown for seed production. It has been further reported that the yard-long bean has evolved in Asian region from cowpea following intense selection by farmers for long pods that are consumed as a vegetable
(Tripathi et al., 2019). Yardlong beans are mainly cultivated in kharif season when availability of other vegetable crops is less
(Sultana et al., 2020). In India, few commercial Yardlong bean cultivars with high yield potential are available and approaches for genetic improvement of Yardlong beans are stilled less explored by the breeder. Therefore, the objective of this article is to compile existing information on genetic variability and divergence which will be useful for selection of parental material for hybridization programme. Understanding the inheritance pattern of yield and yield attributing characters is essential for developing superior variety. Appropriate breeding procedure can be predicted for improvement of the traits based on gene action involved in its expression. Yield being a complex and highly variable traits influenced by numerous yield attributing factors
(Ullah et al., 2011). Therefore, the relationships among yield and its component traits are fundamental to achieving yield improvement. In conventional breeding programs, understanding the extent of genetic variability and selecting appropriate cultivars or breeding lines becomes an essential step toward successful commercial production of this vegetable.
Genetic diversity and genetic variability
Genetic diversity and degree of genetic variation within population is fundamental concept behind crop improvement efforts. D-square statistic, proposed by Indian statistician Prasanta Chandra Mahalanobis (1928), it is effective in measuring genetic divergence and develop strategies for embedding germplasm diversity in crop improvement programme. Knowledge of variability and genetic diversity is pre-requisite for designing any crop improvement programme. The clustering pattern reflects the amount of genetic divergence in the germplasm, while magnitude of intra-cluster distance, gives an idea about magnitude of genetic diversity among genotypes of same group. Intercluster distance must be taken into account to explore genetic diversity through hybridization
(Valarmathi et al., 2007). The relative contribution of individual characters towards expression of genetic diversity is also important as cluster mean performance also help in the selection of parents for hybridization programme
(Haripriya et al., 2024). Yardlong bean genotypes are highly variable, which offer significant potential for increasing pod yield through designing appropriate improvement approaches. Among yield attributing characters pod length showed maximum contribution towards genetic diversity followed by test weight, days to maturity and pod yield per plant and due to maximum intercluster distance, it was depicted that hybridization between IC-630381, IC-630379, IC-630383 and IC-626143, will provide superior recombination with desirable traits in segregating generations
(Haripriya et al., 2024). Sathish et al., (2023) divided 40 genotypes in nine cluster by Tocher’s method and found maximum divergence between cluster for yield, pod length, number of pods per plant, number of primary branches per plant and pod width, importance of number of pods per plant, number of pods per cluster and vegetable pod yield per plant in diversity analysis also reported by
Huque et al., 2012. Eighty-nine genotypes were classified into twelve clusters by
Lal et al., (2017b) they depicted maximum contribution of Plant height towards divergence (29.37%) followed by number of peduncles per plant (26.30%), number of pods per plant (18.39%) and pod weight (16.42%) among vegetable cowpea genotypes. The integration of molecular markers along with morphological markers, enables a more encyclopedic assessment of genetic diversity within breeding materials. RAPD analysis depicted substantial polymorphism (79.42%) and also predicted that some hybrids with similar pod performance were genetically distinct while other show both molecular and phenotypic similarity, demonstrating that phenotypic uniformity should not be interpreted as evidence of genetic uniformity
(Truong et al., 2026). However, RAPD is a dominant marker system with limited reproducibility; therefore, validation using co-dominant markers, such as SSR or SNP, is recommended.
Pidigam et al. (2019) emphasized the importance of Random amplified polymorphic DNA marker, it is informative, scorable and reproducible and classified 20 genotypes into five major clusters. Importance of Inter-retrotransposon amplified polymorphism (IRAP) and retrotransposon microsatellite amplified polymorphism markers (REMAP) with scoring percentage of 52% was reported by
Widyawan et al., (2021). Ogunkanmi et al. (2007) investigated genetic diversity among fifty accessions of yardlongbean using SSR markers and he concluded that accessions from India showed more genetic diversity than those from other countries, thereby providing further evidence in support of the suggestions that India may be the centre of diversity for the crop and SSR marker are useful in accessing genetic diversity in Yardlong bean.
Anwer et al. (2019) revealed that RAPD markers can be an efficient, fast and, inexpensive way for differentiation of yardlong bean, F1 progenies and their parents, as well as to evaluate their genetic relationships. Genetic diversity in yardlong bean was evaluated by using different DNA markers AFLP
(Coulibaly et al., 2002), RAPD
(Alam et al., 2013), SSR
(Kongjaimun et al., 2012a), SNP
(Xu et al., 2011), ISSR and SCoT
(Igwe et al., 2017). A study by
Rambabu et al. (2016) observed substantial variability in Yardlong bean collection and revealed that growth habit varies from pole type to bush type, while flower colour varied from light purple, dark purple, pink and white, pod colour also varied from light green, dark green, purple colour with green tip to green colour with purple tip and genotype IC-582859, NSJ-362, IC-582850 and IC-582872 were found to be superior with pod yield and yield attributing characters. Higher Pod length observed in Githika (51 cm) followed by Vellayani Jyothika (45 cm) and Manjari (44 cm)
(Sangeetha et al., 2022) as pod length is one of the important yield attributing traits, Githika recorded significantly maximum yield (259 q/ha) followed by Vellayani Jyothika (215 q/ha) and Manjari (199 q/ha).
Genetic variability within Yardlong bean cultivars will indicate the need for the introgression of exotic germplasm into breeding programs
(Fang et al., 2007). In Yardlong bean high GCV, PCV along with high heritability observed for green pod yield per plant, green pod width, green pod length
(Devan et al., 2021; Merin and Sarda, 2022). Phenotypic coefficient of variation (PCV) is indicator of total variations which includes genetic as well as environmental effect while genetic coefficient of variation (GCV) gives a basis to access and compare the range of genetic variability for quantitative traits. High PCV and GCV reported for pod yield per plant (45.07, 44.15) followed by number of pods per plant (34.85, 34.77), pod girth (21.11, 20.96), number of seeds per pod (20.33, 20.20) and plant height (20.84, 20.26) respectively by
Haripriya et al. (2024), higher GCV and PCV for these traits also reported by
Shivakumar et al. (2014). Estimates of heritability gives idea about characters which are least sensitive to environmental factors and strength of these characters for inheritance to next generation and phenotypic selection through these characters are effective. Highest estimates of heritability observed for number of pods per plant (99.49%), number of seeds per pod (98.76%), plant height (91.55%). The sole estimation of heritability doesn’t give idea about progeny progress in the subsequent generation, but still it should be interpreted in connection with the genetic advance. High value of genetic advance as percent of mean was obtained for pod yield per plant (89.11%) followed by number of pods per plant (71.44%), pod girth (42.89%), number of seeds per pod (41.36%), plant height (40.58%) and pod length (37.60%). The high heritability and genetic advance observed for these traits suggest that their phenotypic variation is relatively less influenced by environmental effects and that additive gene action may have a substantial role in their inheritance. Therefore, these traits can be effectively improved through selection in breeding programmes. So simple selection may be suggested for improvement through these characters
(Haripriya et al., 2024; Vidya et al., 2002; Resmi et al., 2019). High heritability along with moderate genetic advance as per cent of mean suggest the predominance of both additive and non-additive gene action.
Lal et al., (2017b) infers higher magnitude of PCV than GCV for all the characters emphasizing the environmental effect on expression of characters, high GCV, PCV along with high heritability with genetic advance reported for plant height, pod yield per plant, pod weight, number of pods per plant and pod length. Similar result obtained by several workers
(Nath et al., 2009; Huque et al., 2012; Chattopadhyay et al., 2014; Nwofia et al., 2013). High estimates of genotypic coefficient of variation, heritability and genetic advance for the characters,
viz. pod yield per plant, number of peduncles per plants, number of pods per plants, pod weight and pod length indicate that improvement of these characters can be achieved through selection. In Yardlong bean traits such as flower color, pod color, pod size, pod weight, pod texture and fruit characteristics showed maximum variability
(Alam et al., 2013). This variability is an essential resource for breeding, as limited genetic diversity remains a major constraint to their improvement
(Kusmiyati et al., 2021). Several studies have investigated the genetic variability of Yardlong beans and the application of this knowledge in breeding strategies
(Phansak et al., 2005; Chen et al., 2010; Hossain et al., 2013; Huang et al., 2018). Toppo et al., (2018) evaluated twelve Yardlong bean genotypes for growth and yield under Allahabad Agro-Climatic Conditions. Earliness in first pod picking was recorded in three genotypes: Improved AK-57, TS-18 long yard and Richa (65.33 days after sowing). Maximum Pod length was recorded in COPBVAR-6 (51.67 cm), the number of seeds per pod was recorded in Richa (18.67) and pod yield per plant (481.33 g). Six genotypes (VL Lobia-1, VS-3, PVCP-20 Vyjayanthi, Arka Mangala, Lola) were evaluated
(Sarada et al., 2020) in RBD with four replications during 2016-18. They reported that Pod girth was maximum in Arka Mangala, whereas the number of pods per plant was maximum in VS-3. Among the entries evaluated, VS-3 recorded the highest yield (66.2 q/ha), followed by Arka Mangala (54.7 q/ha).
Character association and Cause-effect analysis
Yield is a complex quantitative trait controlled by several genes, so selection through yield is not possible. Correlation and path analysis can increase selection potency for yield improvement and its attributing components. In yardlong bean pod yield per plant, showed strong positive correlation with pod weight, number of peduncle and number of pods per plant
(Lal et al., 2017a; Huque et al., 2012; Chattopadhyay et al., 2014; Madhavi et al., 2014) depicting selection through these traits might be fruitful in further generation.
Sultana et al., (2020) inferred significant positive correlation of number of seed per pod, number of pods per plant with pod yield per plant while significantly negative correlation with days to harvest. Correlation shed light on principal association between dependent and independent variables where fresh pod yield per plant being a dependent variable and other yield attributing traits are considered as independent variables. Path analysis separates correlation matrix into direct and indirect effect.
Devan et al., (2021) depicted that Pod length (0.769) had the highest significant direct influence on green pod yield per plant followed by days to first harvest (0.728), seeds per pod (0.663), green pod width (0.218), days to first flowering (0.189) and vine length (0.106), similar report was given by
(Lal et al., 2017b; Nath et al., 2009; Chattopadhyay et al., 2014). Direct effect on pod yield depicted by first flowering, days to maturity, number of pods per plant, pod weight and number of seeds per pod. Correlation and path analysis gives idea about the characters on which maximum emphasis should be given for selection in Vegetable cowpea. Studie on correlation by
(Rambabu et al., 2016), revealed traits like plant height, number of primary branches per plant, terminal leaf length, length of harvesting time, length of cluster stalk, number of clusters per plant, numbers of pods per cluster, number of pods per plant, pod length, pod girth, seed number per pod and 100 seed weight showed positive association with pod yield per plant (g). Cause and effect analysis showed that length of cluster stalk (0.8242) and pod girth (0.5550) at genotypic level exhibited high positive direct effect on pod yield per plant. Selection for these associated traits may improve pod output per plant since they are in the right direction. Pod length, pod width and the number of pods per plant positively correlated with pod yield (
Manju 2006;
Jithesh 2009;
Vavilapalli and Celine, 2014;
Rambabu et al., 2016; Bhagavati et al., 2019). Pods per cluster and Plant height had a significant positive connection with each other and pods per cluster had a highly significant positive association with pods per plant and pod production. Pod length had a highly significant positive relationship with pods per plant, seed weight and seed yield
(Pidigam et al., 2019). Pod yield per plant was highly positive significant with pods per plant and pods per cluster
(Ullah et al., 2011; Lovely and Radhadevi, 2006). Positive correlation of pollen viability and fruit set percentage on pod yield per plant reported by
Feba et al., (2022). Correlation merely demonstrates the association between characters while it doesn’t give any idea about cause and effect of that association. So, further Path analysis is done to determined true relationship using correlation matrix for separating variables into direct and indirect effect. Green pods per plant, followed by green pod length, number of seeds per pod and green pod width, had a strong positive direct influence on plant pod yield
(Nanda et al., 2022; Abhimanyu et al., 2025). This inferred that when other variables remain constant, genetic improvement of such traits will result in higher yields (
Manju, 2006;
Lovely, 2005). Increased pod yield can be achieved by increasing the number of pods per cluster and number of pods per plant. As a result, these traits can play a significant role in crop improvement and the better-performing accessions aided the advance into the breeding program. High direct effect of number of pods per plant was earlier reported by
(Jana et al., 1983; Chattopadhyay et al., 1997; Resmi 1998;
Vardhan and Savithramma, 1998). Pod weight is also found one of major contributor to pod yield (
Sobha, 1994;
Chattopadhyay et al., 1997; Resmi, 1998). Number of pods per plant and pod weight should be given due importance in selection programs for yield improvement in yard-long bean since these characters recorded significant genotypic correlation with high direct effect on pod yield.
Gene action
Yield is complex quantitative traits; its expression is governed by several genes. Understanding the genetic makeup of these yield attributing traits is essential for designing any effective breeding programme. Gene action refers to manner in which gene influence the expression of a trait
(Maurya et al., 2026). It can be determined by mating a set of genotypes in all possible combinations by using different mating design like Diallel and line ´ tester to estimate general combining ability (GCA) and specific combining ability (SCA) reflecting additive and non-additive gene actions, while with the help of generation mean analysis, where different generations like F1, F2 and backcross are used to determine additive, dominance and epistasis interaction
via scaling test. Ratio of gca variance and sca variance is good indicator of gene action as if its value is less than one there is predominance of non-additive gene action while when its value is more than one it indicates the involvement of additive gene action in transfer of that trait.
Ratnakumari et al., (2020), conducted a study to get information about the nature and magnitude of gene action for yield and attributing characters, they made crosses in half diallel fashion using Seven yardlong bean genotypes
viz., Babli, Geethika, Bobbili Local, Vizianagaram Local, Lola, Trivendram Local, Bhuvaneswar Local. Magnitude of SCA variance was greater than that of GCA variance, depicting predominance of non-additive gene action for vine length, average leaf area, days to 50% flowering, number of pods per plant, pod length, pod girth, number of seeds per pod and pod yield per plant, Additive gene action observed for traits like primary branches per plant and days to first picking as magnitude of GCA variance was greater than that of SCA variance.
(Kumari et al., 2026; Romanus et al., 2008; Badhe et al., 2016; Owusu et al., 2018; Pallavi et al., 2018) in cowpea,
Farag and Afiah (2012) in faba bean,
Das et al., (2014) in Dolichos bean,
Askander and Osman (2018) in Pea,
Latha et al., (2018) in Mung bean.
Bharathi et al., (2019) in Blackgram]. Recombination breeding can be suggested for characters days to 50 per cent flowering, pod weight, pods per cluster, pod yield per plant and length of the harvest period
(Romanus et al., 2008), prevalence of additive gene action for days to maturity and pod length reported by
Owusu et al., (2018), pod weight, number of pods per plant, hundred fresh seed weight, pod width, pod length and seeds per pod had higher SCA variance designing predominance of dominant gene action in the inheritance of yield and most of the characters in Cowpea reported by
Kumari et al., 2018 and
Pampaniya, 2017. Quantitative characters do not follow the Mendelian ratio (
Fehr, 1987). Inheritance pattern in plant growth habit and seed coat colour in crosses between Yardlong bean (
Vigna unguiculata ssp.
sesquipedalis) and Cowpea (
Vigna unguiculata), is assessed through chi square tests of different generations P1, P2. F1, F2 BCP1 and BCP2 and shows segregation ratio of ratio of 3 indeterminate: 1 determinate, while seed coat color segregated in a 3 black: 1 brown ratio. Plant growth habit and seed coat color are controlled by a single dominant gene, with segregation in the F2 generation in accordance with Mendel’s First Law (
Nantawong et al., 2026). Furthermore, the dihybrid cross analysis of both traits revealed segregation in a 9 indetermine black seed coat: 3 indeterminate brown seed coats: 3 determinate black seed coat: 1 de terminate brown seed coat ratio, which is in accordance with the the oretical expectation. This finding demonstrates that the two traits are controlled by genes located on different chromosomes and assort independently, consistent with Mendel’s Second Law. However, In Yardlong bean brown seed coat colour is dominant over white seed coat colour
(Qu et al., 2011). Kusmiyati et al. (2021), assessed segregation ration in F2 population of Yardlong bean. Segregation ratio is assessed in Fn generation to estimate whether the progeny has attained homozygosity or still it will split into different traits. They found that Pod length, Plant height, number of leaves per plant and pod sweetness showed normal distribution in F2 population in yardlong bean were classified as quantitative traits that were controlled by many genes. Selection based on these characters should be done at later generations, as these characters are controlled by many genes having small effect on expression of these characters and also influenced by environment. Pod weight is controlled by epistasis gene action so selection through this character can be done at early stage and it is not inherited directly from parents to offspring.
Lovely and Radhadevi (2025) observed additive x additive, additive x dominance and dominance x dominance gene effects for traits such as pod length, pod yield per plant, number of pods per plant and root weight, indicating the involvement of epistatic gene action. The presence of duplicate and complementary epistasis suggested that hybridization followed by selection in segregating generations could be effective for improving these traits. Recombination breeding was also recommended for enhancing pod yield and pod weight.
Rahman and Saad (2000) studied inheritance of yield and attributing characters utilizing the mean of six population P1, P2, F1, F2, BC1P1 and BC1P2 in four crosses of
Vigna sesquipedalis and shed light on the importance of dominance gene action for number of pods per plant and pod yield per plant also depicted involvement of digenic interaction [additive x additive (i) and dominance x dominance (l)] for these characters which varied among the crosses. Pedigree selection and heterosis breeding can be used to exploit the fixable and non-fixable components of variation respectively in in this crop.
