Temperature and heterogeneity of emergence time in oilseed rape

SUMMARY Emergence of oilseed rape cultivars with known differences in germination profile was examined by controlled sowings in the field at mean temperatures between 2 and 10°C. Early percentiles, up to the 50th in two cultivars (Rocket and Martina) and up to the 20th in a third (Comet), responded to temperature similarly at all sowings, in that 1/time to emergence of a percentile (emergence rate) increased exponentially with temperature above an intercept of‐1°C. For these percentiles, emergence occurred at a similar thermally weighted time over the whole range of temperature, indicating the peak of the first flush of seedlings is predictable. Percentiles higher than those cited above displayed increasing non‐germination (probably secondary dormancy) as temperature declined towards winter. The maximum non‐germination ranged between 50% and 80% among the cultivars. In sowings during and just after winter, the emergence rate of these higher percentiles was lower than at a comparable temperature before winter. A portion of non‐germinating seeds of the two most heterogeneous cultivars overwintered and emerged synchronously in April from a range of pre‐ and post‐winter sowings, probably stimulated by increasing diurnal temperature amplitude from 7 to 15K. At low temperature, therefore, cultivars exhibited great heterogeneity in the emergence profile, those with more heterogeneity being more likely to give rise to temporally separate weed or feral populations. The findings are discussed in the context of weediness and transgene movement in the oilseed rape metapopulation.