Crop Improvement of Urdbean (Black gram) — Detailed Chapter

Scientific name: Vigna mungo (L.) Hepper | Family: Fabaceae | Chromosome number: 2n = 22

Introduction

Urdbean, commonly called black gram, is an important short-duration pulse crop grown primarily in South and Southeast Asia. It provides an essential source of dietary protein, minerals and vitamins for millions of people. As a legume, it contributes to soil fertility through biological nitrogen fixation and fits well into diverse cropping systems including relay and intercropping systems. Urdbean is predominantly self-pollinated, though low levels of natural outcrossing may occur. The crop is cultivated both for dry grain (dhal/whole pulses) and for green pods/vegetables in some systems, and its straw and haulm have fodder uses.

This chapter synthesizes knowledge on origin and distribution, cultivated and wild species, botanical features, economic importance, breeding objectives and detailed breeding methods—both conventional and modern—used in the genetic improvement and hybrid development of urdbean.

Centre of origin

The primary centre of origin and domestication of Vigna mungo is the Indian subcontinent, particularly the north-western plains and adjoining regions of South Asia. This region harbors the greatest genetic diversity of cultivated urdbean as well as its wild relatives. Historical cultivation patterns, archeobotanical evidence and the concentration of diverse landraces support India as the primary domestication centre.

Distribution of species

Urdbean is widely distributed across the tropical and subtropical regions of Asia. Major production countries include India, Bangladesh, Pakistan, Nepal, Sri Lanka, Myanmar, Thailand, Philippines and Indonesia. The crop has also been introduced to parts of Africa and the Caribbean. Urdbean is commonly grown on rainfed uplands, as a post-rainy or relay crop, and often intercropped with cereals and vegetables.

Extension beyond Asia is limited compared to some other pulses; however, in regions with suitable climates and markets, urdbean can be an important pulse crop for smallholders.

Cultivated species (descriptive)

The cultivated species Vigna mungo exhibits wide phenotypic variation that farmers and breeders exploit. Major cultivated types include:

Short-duration determinate and semi-determinate types: Mature rapidly (60–80 days in some environments), suited to multiple-cropping systems and short rainy seasons.
Indeterminate (viny) types: Climbing or sprawling habit, extended flowering and podding; often higher biomass but may require more space and have asynchronous maturity.
Large-seeded types: Preferred in many markets for dhal and table use; fetch higher prices.
Small-seeded, quick-cooking types: Favoured where cooking time and fuel savings matter.
Dual-purpose types: Selected for grain plus green-manure/forage value — moderate seed yield with higher vegetative biomass.

Seed characteristics vary in colour (black, brown, mottled) and seed coat thickness — traits that influence dehulling efficiency, cooking time and consumer preference. Adaptation to photoperiod and temperature explains large regional differences in maturity and cropping windows.

Wild species

Wild relatives of Vigna mungo (other Vigna spp. and undomesticated V. mungo accessions) are present around the Indian subcontinent. These wild gene pools are valuable for pre-breeding because they carry alleles for pest and disease resistance, abiotic stress tolerance (e.g., drought, heat, salinity) and adaptive traits such as deep rooting or altered phenology. Pre-breeding is essential to convert wild variation into forms that can be crossed readily with elite cultivars without introducing excessive linkage drag.

Botanical description

Habit: Annual herb — erect or climbing depending on genotype and support. Many cultivated lines are semi-erect.

Root: Deep taproot system with lateral roots; forms nodules with Bradyrhizobium/other rhizobia for biological nitrogen fixation.

Stem and nodes: Stems usually branched and may be pubescent; nodes bear leaves and inflorescences.

Leaves: Trifoliate with oblong to ovate leaflets; leaflet size varies by genotype and environment.

Flowers: Papilionaceous pea-type flowers borne in racemes; typical colours range from pale yellow to cream.

Pollination: Predominantly self-pollinated; insect-mediated outcrossing is low but can be exploited if hybrid seed production systems are used.

Pods and seeds: Pods are linear to slightly curved, usually with 4–8 seeds. Seeds range from small to large, with black and brown seed coats being common. Seed composition typically contains 20–25% protein, substantial carbohydrate and variable levels of micronutrients.

Economical importance

Nutrition: Urdbean is a major protein source in vegetarian diets. It also supplies vitamins and minerals and complements cereal-based diets by improving amino-acid balance (higher lysine).
Income: Dhal (split urd) is commercially valuable; farmers obtain cash income from both grain and green-pod markets.
Soil health: As a legume it contributes nitrogen via fixation and improves cropping system sustainability when used in rotation or as green manure.
Livestock feed: Straw and haulm are used as poor- to moderate-quality fodder in mixed farming systems.
Processing and industry: Urdbean is processed into flours, fritters, snacks and other value-added products — driving demand for particular seed qualities (size, colour, dehulling traits).

Breeding objectives

Clear, prioritized breeding objectives guide improvement programs. For urdbean, these include:

Yield and yield stability: Increase genetic yield potential and maintain consistent performance across variable environments.
Duration and phenology: Short and synchronous maturity for multiple cropping and escape from terminal stresses.
Biotic resistance: Durable resistance to major diseases (e.g., Urdbean Yellow Mosaic Virus complex, powdery mildew, rust, Cercospora leaf spot) and insect pests (e.g., bruchids in storage, pod borers).
Abiotic tolerance: Drought and heat tolerance (especially terminal heat stress), and where relevant, salinity tolerance.
Seed and cooking quality: Larger seed size, thin seed coat for easy dehulling, rapid-cooking types, low anti-nutritional factors and improved protein and micronutrient content (iron, zinc).
Adaptation traits: Photoperiod neutrality or predictable photoperiod response; lodging resistance and improved root architecture for water and nutrient uptake.
Post-harvest traits: Seed longevity, storability and resistance to storage pests.

Breeders often use a selection index that combines yield with one or more key traits (e.g., disease resistance or seed size) to ensure that selection gains are balanced with practical farmer needs.

Important breeding methods for crop improvement and hybrid variety development

Improvement of urdbean uses both time-tested conventional methods and modern genomics-assisted approaches. Below is a comprehensive description of methods used along with practical considerations for hybrid development.

Conventional approaches

Germplasm collection and characterization

A broad, well-documented collection of landraces, farmer varieties, released cultivars and wild relatives underpins any breeding program. Characterization includes agro-morphological descriptors, disease and pest screening, quality profiling (seed size, protein, dehulling) and adaptation evaluations under multiple environments.

Selection methods

Given the selfing nature of urdbean, pure-line and pedigree selection are common:

Pedigree method: Cross superior parents (based on combining ability), advance segregating populations (F2 onwards) and select desirable recombinants each generation while recording pedigrees.
Bulk method and single-seed descent (SSD): Useful to advance many lines rapidly to near-homozygosity in early generations before intensive evaluation.
Pure-line selection: Identify superior lines within landraces and fix through selfing and selection.

Backcrossing

Backcross breeding is used to transfer major genes (for disease resistance or quality) into elite backgrounds. Marker-assisted backcrossing (MABC) expedites recovery of the recurrent parent genome and reduces linkage drag.

Mutation breeding

Mutagenesis (gamma irradiation, EMS) generates novel variation for traits such as plant architecture, seed size and maturity. Mutants with desirable traits can be included in crossing programs.

Hybridization and heterosis

Although primarily selfed, urdbean hybrid systems have been developed experimentally. Key components include:

Male-sterility systems: Genetic male sterility (GMS) and cytoplasmic male sterility (CMS) are explored to produce hybrid seed without hand emasculation.
Combining ability tests: Evaluate parental lines for general combining ability (GCA) and specific combining ability (SCA) in factorial crosses to identify parents that produce superior hybrids.
Seed production considerations: Ensure isolation, manage pollinators (if needed), and organize seed-production blocks to maintain hybrid seed purity and reduce costs.

Molecular and modern approaches

Marker-assisted selection (MAS)

MAS uses DNA markers tightly linked to traits of interest (disease resistance, bruchid resistance, quality loci) to select progeny at early stages and increase breeding efficiency. MAS is particularly valuable for traits difficult or expensive to screen phenotypically or those expressed late in the season.

Quantitative trait loci (QTL) mapping and GWAS

Mapping QTLs for yield components, drought tolerance and disease resistance helps breeders focus on genomic regions with measurable effects. Genome-wide association studies (GWAS) across diverse germplasm panels identify marker–trait associations for complex traits.

Genomic selection (GS)

GS models predict breeding values using genome-wide marker profiles and training populations. This approach is powerful for accelerating genetic gain for complex, polygenic traits such as yield, yield stability and multi-stress tolerance.

Speed breeding and controlled-environment advancement

Manipulating photoperiod and temperature under controlled environments reduces generation time, allowing faster fixation of desirable alleles and more rapid cultivar release.

High-throughput phenotyping (HTP)

Field-based sensors, drone- or pole-mounted imaging, and thermal/NDVI indices are used to phenotype canopy temperature, biomass, early vigor and stress symptoms across many plots. HTP improves selection precision for traits like drought tolerance and disease progression.

Pre-breeding and wild introgression

Wild relatives provide novel alleles for resistance and tolerance. Pre-breeding converts wild accessions into bridge donors with reduced linkage drag using recurrent backcrossing and selection, often aided by markers.

Genome editing and transgenics (prospects)

CRISPR/Cas and transformation techniques hold future promise for targeted improvement of quality (e.g., phytic acid reduction), anti-nutritional factors, or disease-resistance genes. Deployment depends on regulatory frameworks and market acceptance.

Integrated strategies for stress resistance and quality improvement

Durable improvement requires integration of methods rather than reliance on a single approach. Exemplary strategies include:

Pyramiding resistances: Combine major resistance genes with quantitative resistance to reduce the risk of resistance breakdown.
Physiological trait selection: Select for root architecture, early vigor, and canopy temperature depression as indirect traits for drought and heat tolerance.
Marker-aided recurrent selection: Use markers to advance polygenic tolerance while maintaining useful diversity.
Participatory selection: Include farmers in selection for traits such as cooking quality, seed appearance and market preferences to ensure adoption.

Hybrid variety development — practical roadmap

Identify male sterility system: Establish reliable CMS or GMS system with corresponding maintainer and restorer lines where relevant.
Develop and test parental lines: Screen parents for good GCA/SCA, disease resistance and acceptable seed quality.
Hybrid seed production: Design seed-production blocks with adequate isolation and pollinator management. If insect pollinators are required, maintain good nectar sources and pollinator health.
On-farm trials: Conduct multilocation trials to evaluate hybrid performance and stability under farmer-managed conditions.
Economic analysis: Ensure the hybrid seed system is economically viable — higher yield must compensate for increased seed cost and production complexity.

Deployment, seed systems and farmer adoption

Improved cultivars reach farmers through robust seed systems, extension services and private–public partnerships. Important considerations include:

Participatory varietal selection: Farmers evaluate materials in their own fields to ensure local adaptability and preference alignment.
Decentralized seed multiplication: Strengthen community seed producers to ensure availability of quality seed.
Market alignment: Breed for traits that match market and processor demands (seed size, colour, dehulling quality, dhal recovery).
Extension and crop management: Combine improved genetics with agronomy (timely planting, pest management, rhizobial inoculation) to realize yield potential.

Monitoring genetic gain and future directions

Monitoring genetic gain using multilocation performance data and historical checks quantifies breeding program success. Future priorities for urdbean breeding include:

Wider adoption of genomic selection to accelerate gain for complex traits.
Integration of phenomics and genomics to identify robust adaptive traits.
Stronger pre-breeding pipelines to tap wild germplasm for durable resistance and novel adaptation.
Targeted use of genome editing for quality and anti-nutritional trait improvement (subject to policy and public acceptance).
Improved hybrid seed systems if cost-effective and delivering consistent heterosis under farmer conditions.

Key takeaways

Urdbean improvement must balance yield, stability, stress resistance and quality. Combining conventional selection, pre-breeding with wild relatives, marker-assisted tools and modern genomics approaches offers the best route to creating varieties that meet farmer, market and nutritional needs.

Crop Improvement of Urdbean | Agriculture Notes | Crop Improvement-I Notes