TEMPERATURE EFFECTS ON SEED-GERMINATION OF EAST-ASIAN AND TERTIARY RELICT SPECIES OF DIOSCOREA (DIOSCOREACEAE)

To obtain information relevant to the evolution of dormancy, germinati on responses to temperature of intact seeds and naked zygotic embryos were compared among taxonomically closely related species of Dioscorea in the section Stenophora. We examined five species from the northern half of the East Asian distribution area and four species considered to be Tertiary relict species from the Appalachians, the Caucasus, and the Balkans. Although features of germination of the intact seeds dif fered from species to species, the naked embryos of all species germin ated over a similar wide range of temperatures without any marked diff erences among species. This unitary physiological feature of the embry os suggests the possibility that the temperature responses of the embr yos have not changed since the Tertiary period. In the East Asian spec ies, as the distribution area shifts to the north the seeds gradually lose their dormant features, and consequently the germination behavior of the seeds gradually changes to resemble that of their embryos. The seed of the northernmost species has no dormant features at all, and temperature responses of the seed are the same as those of the embryo. Full germination of the intact seeds of East Asian and Tertiary relic t species required prior chilling treatment. Unlike the East Asian spe cies, however, the relict species germinated to some extent at higher temperatures over a narrow range without prior chilling. However, the resultant germlings died or elongated poorly. Thus, the germination pr ocess of relict species may have become less sensitive to high-tempera ture inhibition after their isolation from Asian species.