Evaluation of Osmotic Stress Tolerance in Brassica napus, B. juncea, and B. carinata under PEG-6000-Induced Osmotic Stress at Germination and Early Seedling Stage

Climate change-driven water deficit severely constrains seedling establishment and productivity of oilseed Brassica crops. This study evaluated the effects of polyethylene glycol (PEG-6000)-induced osmotic stress (0, –0.20, –0.40, –0.60, –0.80, –1.00, and –1.20 MPa) on germination and early seedling growth of 12 genotypes belonging to Brassica juncea (L.) Czern., Brassica Napus L., and Brassica carinata A. Braun. A completely randomized design with three replications was used. Germination percentage (GP), speed of germination (SOG), mean germination time (MGT), shoot and root length, biomass parameters, vigour indices (VI-I and VI-II), antioxidant enzyme activities (CAT, SOD, POD), and oxidative stress markers (H2O2, MDA, proline, MPI) were assessed. Osmotic stress caused progressive, dose-dependent declines in all growth parameters, while antioxidant enzymes and osmoprotectants were induced. At –1.20 MPa, mean GP fell to 9.8% of the control value, and VI-I and VI-II declined by 97 and 99%, respectively. A critical stress threshold was identified between –0.80 and –1.00 MPa, beyond which most genotypes failed completely. Three genotypes: B. napus GSC-7, B. juncea Giriraj, and B. carinata PC-6, maintained measurable germination (38–40%), seedling growth, biomass, elevated antioxidant enzyme activities, and superior proline accumulation at –1.20 MPa. Principal component analysis (PC1 = 53.8%, PC2 = 18.2%) and hierarchical clustering confirmed the segregation of genotypes into tolerant and sensitive clusters. Network centrality analysis identified germination percentage, biomass traits, and catalase activity as hub traits driving stress-tolerance networks. These genotypes represent valuable genetic resources for drought-tolerance breeding in oilseed Brassica crops.