Sunflower (Helianthus annuus L.) seed dormancy and its regulation by ethylene.

Freshly harvested sunflower (Heliarthus annuus L.) seeds germinated poorly, with a temperature optimum close to 25 °C. The inability to germinate at high temperatures (25 to 40°C) resulted principally from a seed-coat imposed dormancy, whereas poor germination at temperatures below 25 °C was mainly due to an embryo dormancy. Dry storage greatly improved germination at temperatures ranging from 5 to 40°C, by eliminating both the seed-coat imposed and the embryo dormancy. A too high temperature (45°C) always completely inhibited germination and rapidly induced a secondary dormancy which was broken by redrying and dry storage of the seeds. The depth of embryo dormancy varied during the development of the seeds. It was low in very immature embryos, but increased and then decreased during desiccation of the seeds on the mother plants. Ethylene and its immediate precursor (1-aminocyclopropane-l-car- boxylic acid) strongly stimulated germination of primary dormant seeds. On the contrary, inhibitors of e...