The effect of polyethylene glycol (PEG) osmopriming on plant emergence in laboratory at optimal (25°C) and suboptimal (18 and 14°C) temperatures was assessed in two cultivars of sweet sorghum differing in kernel colour: Brandes, light-coloured, and Roce, dark-coloured. Sterilized sand or non-sterilized soil were used as substrates. PEG osmopriming enhanced seedling emergence in sand. In soil, the light-seeded cultivar produced seedlings only at optimum temperature, whereas the unprimed dark-seeded cultivar performed well as in sand at all temperatures and reduced dramatically its germination capacity at suboptimal temperatures following priming treatment. These results might be related to the presence of tannins in the seed coat. Seed tannin contents of 80 and 590 mg 100 g-1 seed fresh weight (FW) were determined in Brandes and Roce respectively. These amounts consistently reduced after PEG osmopriming, especially in Roce (217.5 mg 100 g-1 seed FW). The decline in tannin content in Roce, because of osmopriming, might explain the poor emergence of this cultivar in soil. These results suggest the opportunity of using high-tannin cultivars of sweet sorghum in the field, which may better growth in this environment when early sowings are required.