Ombrohydrochory and its relationship to seed dispersal and germination strategies in two temperate North American Oenothera species (Onagraceae)

Capsule dehiscence, seed dispersal, and seed dormancy and germination were examined in Oenothera triloba and Oenothera macrocarpa. The hygrochastic capsules of O. triloba, resembling splash cups, opened, and seeds were dispersed during rain. Capsules retained viable seeds for at least 1 yr, and the seeds were nondormant and could germinate at any time of the year as long as temperatures were ca. 15°/6°C or higher. In contrast, the xerochastic capsules of O. macrocarpa, which also mature by July, opened during late autumn and winter with moisture and temperatures ≤5°C. As opposed to O. triloba capsules, O. macrocarpa capsules were fully expanded when dried after rain. Seeds of O. macrocarpa required cold stratification for release from dormancy, which occurred while the capsules were gradually opening and releasing seeds, and germination was confined to late winter and early spring. Precipitation plays an important role in capsule opening of both species, which coincides with their seed dormancy and germination strategies. Ombrohydrochory is a rare seed dispersal strategy in temperate regions and is usually found among plants of arid regions. Although ombrohydrochory includes hygrochasy, we suggest that xerochasy as influenced by rain should be recognized as an indirect mechanism of this dispersal syndrome as well.