Integrated Physiological and Multi-Omics Analysis Reveals Selenium-Mediated Drought Tolerance in Brassica napus Seedlings

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.