Genetic architecture and combining ability of Indian mustard (Brassica juncea L.) under climate-induced late sown stress: Insights for climate-resilient hybrid breeding

Delayed sowing in rice-mustard cropping systems frequently pushes Indian mustard (Brassica juncea L.) planting beyond optimal windows, exposing the crop to late sown condition associated with terminal heat stress and causing substantial yield reductions. To address the critical gap in late-sown adaptation research, this study evaluated the quantitative genetic basis, combining ability and trait interrelationships of 7 genetically diverse parents and 21 F1 hybrids developed via a half-diallel mating design. The genotypes were evaluated for 12 agro-morphological traits under very late-sown conditions (December) in Varanasi, Uttar Pradesh. Results revealed significant genetic variability for all traits. Exceptionally high broad-sense heritability and genetic advance as percent of mean (GAM) were observed for seed yield per plant (YPP) (85.77 % and 52.55 %, respectively), test weight (TW) (93.23 % heritability) and the number of siliquae per plant (NSP) (73.39 % heritability and 42.07 % genetic advance). Hybrids generally outperformed parental lines, exhibiting earlier maturity, superior branching and higher yield. Parent P4 exhibited significant positive general combining ability effects for seed YPP and silique number. Additionally, the cross P5 × P6 exhibited the highest specific combining ability for yield and hybrid H17 demonstrated the maximum better-parent heterosis (50.0 %) for seed yield. Path coefficient analysis confirmed that the NSP exerted the strongest direct positive effect (1.045) on seed yield. Furthermore, principal component analysis (PCA) showed that the first four components explained 81.5 % of the total phenotypic variation. These findings demonstrate exploitable genetic variability among mustard genotypes under late-sown stress conditions. Selecting for the NSP and utilising superior heterotic combinations will be highly effective for developing climate-resilient mustard hybrids adapted to delayed sowing environments. Ultimately, deploying these targeted breeding strategies offers a practical pathway to secure crop productivity and stabilise smallholder farm income across late sown rice-mustard cropping systems.