Genetic and physiological analysis of biennialism in Hyoscyamus niger

A genetic and physiological study of biennialism in the diploid selfer Hyoscyamus niger (black henbane), an obligate long‐day plant, is described. Three annual and two biennial accessions that were homozygous for their respective growth habits were selected. The early‐flowering trait of two annual accessions was dominant over the late‐flowering trait of the third annual accession. The late‐flowering annual accession, but not the early‐flowering ones, responded to vernalization. Two biennial accessions remained vegetative after more than 1 year in soil and thus had an obligate vernalization requirement. Crosses between annual and biennial accessions showed that biennialism was conferred through a single dominant gene. However, plants containing only one copy of this dominant gene were transformed from biennials into very late‐flowering winter‐annual plants that responded more rapidly to vernalization than biennials. Taken together, these results indicated that there were allelic differences in photoperiod‐specific flowering time genes and that biennialism was a dose‐dependent trait with incomplete dominance. Models for flowering time regulation in henbane involving photoperiod‐, vernalization‐, and most likely gibberellin‐specific pathways are discussed.