Seed dormancy and germination in the summer annual Galeopsis speciosa

This study examined germination and dormancy in Galeopsis speciosa (Lamiaceae), a common summer annual weed in cold-temperate areas. Seeds collected in southern Sweden were subjected to several experiments. The seeds were dormant at maturity. Seeds sown outdoors after collection produced a small number of seedlings, that emerged early in the spring. After long cold stratification or stratification outdoors over two winters, the maximum germination was 40 to 50%; germination occurring over a wide range of temperatures. Warm stratification preceding cold stratification had no effect on germination, but repeated warm and cold periods seemed to promote germination. Gibberellic acid (GA) stimulated germination but full germination was only achieved after more than two months of incubation at the most suitable temperature regime tested. Excised embryos grew and developed into normal seedlings. With these results, the species does not fit into the currently used system for seed dormancy classifications. The response to GA and the growth of excised embryos indicate non-deep or intermediate physiological dormancy, but dormancy alleviation by stratification was not in line with the guiding principles for these classifications. Galeopsis speciosa has a strong dormancy that is sufficiently alleviated during the winter to allow germination of only part of a seed batch each year, hence a stepwise germination pattern occurs over a period of several years.The definitive version is available at www.blackwell-synergy.com: LM KARLSSON, JAL ERICSSON & P MILBERG, Seed dormancy and germination in the summer annual Galeopsis speciosa, 2006, Weed Research, (46), 353-361. http://dx.doi.org/10.1111/j.1365-3180.2006.00518.x. Copyright: Blackwell Publishing www.blackwell-synergy.co