Germination and establishment of pearl millet and sorghum of different seed qualities under controlled high-temperature environments

Larger and/or denser pearl millet [Pennisetum glaucum (L.) R. Br.] and sorghum [Sorghum bicolor (L.) Moench] seed have improved field establishment under temperate conditions. Because soil temperatures are higher in the semi-arid tropics and often are a constraint to successful establishment, effects of seed size and density on germination and establishment under high temperatures (up to 50°C) were evaluated. Effects of seed size and seed density on germination of Senegal Bulk millet and Segaolane sorghum were examined at temperatures of 26-50°C using a thermogradient plate. Seed size and density combinations and timing and duration of 40-45°C stress were evaluated for germination in an incubator. Effects of millet seed size and seed density on establishment were determined at constant temperatures of 35, 43, and 50°C. Sorghum had highest germination at 32-40°C and 32-42°C for medium and large seeds, respectively. Medium-sized millet seed showed higher germination over a wider temperature range than small or large seed. Millet germination rate and plumule emergence had optima at 37-38°C. High-density or large sorghum seed had greater radicle emergence at 24 h with 1 or 2-h high-temperature stress; however, with 4 or 8 h of stress, there was no difference among seed sizes or seed densities. Millet germination at 24 h was lower following 8 h of stress than for 1 or 2 of stress. Millet germination at 48 h was lower for large seed than small seed. High-density millet seeds had higher emergence than low-density seeds at a constant 43°C. At 7 days after planting, seedlings from large and medium-sized high-density seeds were taller than from small or low-density seeds.