Germination Responses of Proso Millet (Panicum miliaceum L.) to Different 2,4-Epibrassinolide Priming Concentrations Under Drought Stress

Chemical regulators play an important role in alleviating abiotic stress during seed germination and early seedling establishment, providing a low-cost and effective solution for crop production under stress conditions. The seed germination stage is critical for crop establishment under drought stress. Proso millet (Panicum miliaceum L.) is relatively drought-tolerant during later growth stages but remains sensitive during seed germination and early seedling establishment. In this study, two P. miliaceum varieties (‘Yumi 2’ and ‘Yumi 5’) were used as materials. Seeds were soaked with 2,4-epibrassinolide (EBR) at concentrations of 0, 0.01, 0.1, 0.5, and 1 μmol·L−1, and then germinated under polyethylene glycol (PEG-6000) solutions at 0%, 15%, and 20% (w/v) to simulate drought stress. Drought stress progressively reduced seed germination potential, germination percentage, germination index, and vigor index, while increasing mean germination time and decreasing germination synchrony. EBR soaking, particularly at 0.01 μmol·L−1, consistently alleviated the above inhibitory effects. Under 20% PEG stress, 0.01 μmol·L−1 EBR increased the germination potential of ‘Yumi 5’ to 82% and significantly improved the germination percentage of both varieties. Compared with the control, low concentrations of EBR (0.01 and 0.1 μmol·L−1) increased the germination synchrony of ‘Yumi 2’ by up to 89.5% and 93.6%. Under 20% PEG stress, the germination synchrony of ‘Yumi 2’ treated with 0.01 μmol·L−1 EBR was 54.7% higher than that of the control. EBR treatment increased α-amylase activity, with the highest activity observed at 0.01 μmol·L−1 under non-stress conditions, and it maintained relatively high activity under drought stress, especially in ‘Yumi 5’. The two varieties exhibited genotype-dependent response differences: ‘Yumi 5’ responded to a wider range of EBR concentrations under drought stress, whereas ‘Yumi 2’ showed a selective response. Soaking seeds with 0.01 μmol·L−1 EBR is a promising pre-sowing treatment strategy for improving seedling establishment of P. miliaceum under drought-prone conditions. In summary, this study provides a low-cost and easily applicable hormone soaking method that can be directly used to enhance the early drought tolerance of cereal crops. Furthermore, this strategy may offer practical guidance for improving seedling techniques in medicinal plants facing similar drought stress during propagation, thereby supporting sustainable agricultural production in water-deficient regions.