RH 1975: An Indian mustard (Brassica juncea) variety suitable for timely-sown conditions

Introduction

Indian mustard (Brassica juncea (L.) Czern. & Coss.) is a major oilseed crop of the Indian subcontinent, largely because of its high oil content, wide environmental adaptability, and reliable performance under moisture stress ((Chauhan et al., 2020) [4] ). Compared to other species in the Brassicaceae family, B. juncea exhibits greater yield stability and production reliability under Indian agro-ecological conditions. This makes it a key crop for achieving self-sufficiency in edible oils (Singh et al., 2021) [10] . North Indian farmers prefer cultivating it in every soil type under both irrigated and rainfed conditions. The well-drained nature of these soils facilitates root growth and nutrient uptake, thereby contributing to higher crop productivity and yield potential. A major challenge in plant breeding is to develop high-yielding varieties that fully exploit the genetic potential of a crop while maintaining stable performance across diverse agroecological environments. For a variety to gain widespread acceptance across diverse regions, high seed yield must be complemented by desirable agronomic traits such as appropriate flowering and maturity duration, manageable plant height, bold seed size and high oil content (Yadav et al., 2018) [11] . Evaluation of these traits in segregating populations enables breeders to identify stable, adaptable, and high-yielding genotypes. The pedigree method offers clear advantages over other conventional breeding methods (Allard 1999) [2] . As a hybridization-based approach, it allows freedom for the selection of diverse parents and subsequently generation and selection of superior transgressive segregants. Selection begins in the F₂ population and continues until the lines reach genetic uniformity (homozygosity). Ultimately, the performance of transgressive progenies in the pedigree program largely depends on the selection of parents and the extent of variability generated through hybridization of parents.

Materials and Methods

Two genetically diverse lines, RH 1140 (bold seed and long siliquae) and RH 1378 (a good combiner parent with basal branches and long main shoot) were crossed to generate transgressive segregants. The resulting segregating population was handled through the pedigree method from the 𝐹 2 to 𝐹 5 generations, with selection of desirable plants in each generation. Selection was focused on plant types characterised by early-to-medium flowering, early-maturity, medium-to-high plant biomass, manageable plant-height, more branching, high siliqua density, long main shoot, more seeds per siliqua, bold seed size, and high oil-content. Up to the 𝐹 5 generation, selected individual plants were harvested and threshed separately. Final selection was based on desirable seed characteristics and progeny performance, whereby lines exhibiting smaller seed size and lower oil content were systematically eliminated. By the 𝐹 5 generation, most lines had achieved desirable homozygosity, and heritable variation diminished within these lines; at this stage, these promising individual lines were harvested in bulk for further evaluation in subsequent yield trials. The systematic breeding pathway from the initial cross to advanced Multi-environment Coordinated Trials is mapped out below (Figure 1). For the evaluation of these promising homozygous bulks, small-scale trials under irrigated, timely-sown conditions against standard checks were constituted. Promising entries were further promoted to large-scale and final yield trials.

To be officially released and notified at the national level, promising entries were contributed to the All India Coordinated Research Project on Rapeseed-Mustard (AICRP-R&M) trial (Anonymous, 2023) [1] . These trials were conducted annually across diverse agro-ecological zones. This designated entry, RH 1975, was contributed to the AICRP-R&M trial during Rabi 2020-21 for rigorous multi-environment evaluation. In these trials, comprehensive data were generated to evaluate the overall potential of a variety, particularly focusing on:  Agronomic performance: Seed yield and crucial yield-related traits.  Pathology & entomology: Host-pathogen interactions and insect-pest resistance.

To accurately evaluate pest and disease resistance, standardised scoring scales are used for comparison with well-known susceptible checks (Bakhetia et al., 1989; Conn et al., 1990) [3,5] .

Results and Discussion

Biotic stress performance (Pathogens and insect pests)

Biotic stress directly threatens seed yields, making genetic resistance/tolerance essential to reduce chemical use and protect the environment. Screenings under both natural stress pressure and artificial inoculations confirmed that RH 1975 is less prone to major foliar diseases and insect pests than standard checks.

Alternaria blight

Alternaria blight, caused by the necrotrophic fungus Alternaria brassicae, reduces seed yields by 32% to 57% in typical years (Meena et al., 2010) [7] .

White rust

White rust, caused by the biotrophic oomycete Albugo candida, severely damages crops by its capacity to inflict damage to all aerial parts and, in the later stage, severe malformation of the inflorescence known as staghead formations (Saharan et al., 2016) [8] . Replicated artificial inoculations across multiple seasons showed that RH 1975 recorded white rust severity to a mean PDI of 31.2% (ranging from 16.0% to 42.0%). RH 1975 demonstrated less susceptibly compared to the susceptible check, Rohini, which recorded an average disease severity of 36.4% (Table 5). [12] . Since absolute immunity rarely exists in B. juncea germplasm, breeding efforts focus on partial, polygenic resistance. Under combined natural and sick-plot screenings, RH 1975 reported a mean incidence of 35.3%, showing an approximate 47% reduction in disease severity compared to the heavily infected susceptible check NRCYS-5-2 (66.9%, Table 6).

Mustard aphid

The mustard aphid (Lipaphis erysimi) remains a major threat, reducing seed yield, oil yield, and oil quality by sucking sap and promoting the growth of black sooty mould (Singh et al., 2018) [9] . Evaluation for the Aphid Infestation Index (AII) from 2020-21 to 2022-23 shows that RH 1975 maintained a lower mean AII of 1.9 compared to the national check, Kranti (2.2). This confirms it can withstand heavy insect pressure across different testing environments (Table 7).

References

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