At reproductive onset the marine green alga Acetabularia acetabulum (L.) P.C. Silva redirects growth from vertical elongation of the axis of the plant to lateral expansion of a disk-shaped reproductive structure, the "cap." We used amputation to synchronize cap initiation and to facilitate investigation of the light requirements during amputation-induced cap initiation. Following amputation of a nascent cap, most plants initiate one whorl of vegetative hairs and then a cap. Both hair and cap initiation required photosynthesis, as indicated by studies with 3-(3',4'-dichlorophenyl)-1, 1-dimethylurea, but did not require the nucleus. Amputation-induced hair initiation occurred in red light, but 10 min of blue light given in a background of red light significantly increased hair initiation, supporting previous studies that hair initiation is a blue-light-triggered photomorphogenic event. Amputation-induced cap initiation also occurred in red light, but daily 10-min flashes of blue light given in a background of red light did not significantly enhance cap initiation. We also examined the light requirements of intact plants at each phase of development. In the absence of blue light, juveniles and adults with /=13.7 +/- 4.3 whorls of hairs initiated caps in the absence of blue light, suggesting that there is a point in late adult development beyond which cap initiation does not require blue light. Several plausible interpretations of the role of light and the nucleus in the regulation of reproductive onset are discussed to try to reconcile these data with those in the literature.