Drought stress severely restricts early growth in Brassica napus, while selenium (Se) can enhance stress tolerance. This study examined how foliar sodium selenite pretreatment affects polyethylene glycol (PEG)-induced drought stress in B. napus seedlings. Seedlings were sprayed with 10, 20, or 40 mg L⁻¹ Na₂SeO₃ for 7 days and then exposed to 10% (w/v) PEG 6000 for 2 days. Drought reduced fresh weight and increased malondialdehyde (MDA), O₂·⁻ production, H₂O₂ accumulation, and antioxidant enzyme activities. The response was nonlinear, and 20 mg L⁻¹ Na₂SeO₃ was most effective. Relative to drought alone, this treatment increased fresh weight from 2.6 to 5.1 g plant⁻¹ and reduced MDA, O₂·⁻, and H₂O₂ by 50.5%, 50.9%, and 44.3%, respectively. Notably, catalase (CAT) and superoxide dismutase (SOD) activities declined markedly from the drought-induced levels, indicating lower oxidative pressure rather than enhanced enzymatic scavenging. Integrated transcriptomic and metabolomic analyses showed associations between selenium pretreatment and pathways related to photosynthesis, carbon fixation, porphyrin metabolism, phenylpropanoid biosynthesis, and selenocompound metabolism. Together, these findings suggest that foliar application of 20 mg L⁻¹ Na₂SeO₃ improves drought tolerance in B. napus seedlings by alleviating oxidative pressure rather than simply increasing antioxidant enzyme activity.

Rapeseed (Brassica napus L.) is an important oil crop, but seedling growth is often restricted by low temperatures in late autumn and winter. Selenium (Se) may modulate plant stress responses, but its dose-dependent effects and associated molecular responses in rapeseed under cold stress remain unclear. Seedlings were exposed to 4 °C for 3 d and received foliar applications of 10, 20, or 40 mg L⁻¹ Na₂SeO₃. Based on physiological responses, the 20 mg L⁻¹ treatment was selected for integrated transcriptomic and metabolomic analyses alongside the normal-temperature control and the cold-stress group without Se. Cold stress reduced whole-seedling fresh weight by approximately 40% and increased O₂⁻ production, H₂O₂ and malondialdehyde (MDA) contents, and superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities. Relative to the cold-stress group without Se, the 20 mg L⁻¹ treatment showed partial restoration of fresh weight and lower oxidative-stress indicators and antioxidant enzyme activities. Integrated analyses highlighted cysteine and methionine metabolism and phenylpropanoid biosynthesis. Treatment-associated changes involved sulfur-related and methyl-cycle genes, a metabolite feature tentatively assigned to 3-sulfopyruvic acid, and a feature putatively annotated as coniferaldehyde. These findings provide a multi-omics basis for understanding rapeseed responses to foliar-applied selenite during short-term cold stress.