Irrigation scheduling and foliar bio-regulators mitigate heat and moisture stress in three Brassica species

Rising terminal heat and shrinking irrigation water threaten rapeseed-mustard productivity across the Indo-Gangetic plains, yet comparative evidence on how irrigation scheduling and foliar bio-regulators jointly buffer these stresses across different Brassica species remains limited. A two-year (2023–24 and 2024–25) split-plot field experiment was conducted at Punjab Agricultural University, Ludhiana evaluating two irrigation schedules (Irrigation 35 DAS versus irrigation 35 DAS and 70 DAS) as main plots and eight foliar bio-regulator treatments (thiourea, potassium nitrate, salicylic acid, trehalose, brassinosteroid, jasmonic acid, water spray and control) as sub-plots, replicated across three phylogenetically distinct Brassica species: oilseed rape ( Brassica napus ), Indian mustard ( Brassica juncea ) and Ethiopian mustard ( Brassica carinata ). A morphology-based vulnerability index identified flowering as the physiologically critical stage for stress-response sampling. Irrigation 35 DAS and 70 DAS consistently improved growth (plant height, dry matter accumulation , photosynthetically active radiation interception, SPAD values), membrane stability, relative water content and canopy temperature relative to irrigation 35 DAS, while raising seed yield by 10–34% depending on species. Among bio-regulators, foliar salicylic acid (100 ppm) was consistently the top performer, followed closely by brassinosteroid (20 ppm) and jasmonic acid (10 ppm) it raised proline and soluble-sugar accumulation while lowering peroxidase/superoxide dismutase activity relative to the control, indicating pre-emptive osmotic protection that reduced the need for a reactive, inducible antioxidant response. Principal component analysis separated the three species along physiological-tolerance axes with Brassica carinata showing an inherently muted yield response to irrigation frequency consistent with greater innate drought resilience. These findings indicate that salicylic acid-based priming combined with irrigation 35 DAS and 70 DAS is a promising, potentially transferable strategy for climate-resilient Brassica production, and that pre-flowering physiological vulnerability screening is a candidate tool for phenotyping abiotic stress responses in oilseed crops, pending validation across additional locations, seasons and genotypes.