Subnivean embryo development in the alpine herb Caltha leptosepala (Ranunculaceae)

Caltha leptosepala D.C. (Ranunculaceae) is a perennial herb of alpine wet meadows. Seeds of this species have morphophysiological dormancy; they contain a rudimentary embryo that must undergo further development prior to germination. The embryo itself is also dormant and requires a physiological cue for resumption of development. We ask how C. leptosepala can break embryo dormancy and complete embryo development, germination, and seedling growth under the constraint of the extremely short alpine wet meadow growing season. Seeds were stratified at 2.5 degreesC and collected at regular intervals for germination trials and histological analysis. At dispersal, the embryo occupies approximately 7% of the length of the seed and is dormant. Embryo development resumes within 1 month of stratification. Embryos grow to fill the seed over a 4- to 7-month period. Germination rates are positively correlated with stratification time and reach 50% at 4 months and 70% at 7 months. Because embryo development requires a time period longer than the snow-free period prior to germination in the alpine wet meadow, we conclude that embryo development in C. leptosepala takes place beneath alpine snowbanks. Morphophysiological dormancy in Caltha is not uniquely derived, but the ability of Caltha embryos to develop and germinate at low temperatures may be an adaptation to their environment.