Bolting induction in Oenothera erythrosepala Borbás in relation to rosette size, vernalization, and photoperiod.

The relationships between rosette size, vernalization, and photoperiod in bolting induction of Oenothera erythrosepala (a facultative biennial) were examined. A natural population of O. erythrosepala in a sand dune system at Azigaura, Japan, behaves as a monocarpic perennial, while a population treated with fertilizer showed the life cycle of a winter annual. In both natural and fertilized populations, bolting was restricted to size classes with rosette diameter greater than 9 cm, regardless of the chronological age and the amount of food reserves accumulated in the tap root. Pot culture experiments showed that the species requires both vernalization and long-day photoperiod for bolting induction, and has a critical leaf area for receiving photoperiodic stimuli. The amount of food reserves in the plant per se does not contribute to the size-dependent flowering of the species.