Wallerstein, I., Department of Ornamental Horticulture, Agricultural Research Organization, The Volcani Center, P.O. Box 6, Bet Dagan, 50250, Israel
Hackett, W.P., Department of Horticultural Science, Landscape Architecture, University of Minnesota, St Paul, MN, United States

At 16°C Hedera helix L. is an obligatory short-long day (SLD) plant as regards inflorescence initiation (12-h photoperiod for short days (SD). 16-h photoperiod for long days (LD)). The presence of a developing terminal inflorescence (for at least 24 days after initiation) can substitute for SD and allow inflorescence initiation on a pruning- induced branch of a flowering shoot under LD only. The temperature at which exposure to LD occurs is critical, such that the inflorescence initiation response obtained is saturating at 16°C, only very marginal at 21°C, and absent at 24"C. A partial evocation (a prefloral stage) tends to occur under environmental conditions in which inflorescence initiation and development do not occur. At 16°C this is expressed as a production of bracts (scales), a change in apical mcristem morphology, and an activation of the axillary meristems near the apical one. At the end of this process, the partially evoked meristem can either reduce in size or entirely cease its activity. In both cases, the apical bud will undergo a quiescent period. The results suggest that floral initiation in Hedera helix is composed of at least two major evocation steps with different requirements. The first step apparently is not specific to a particular environment and results in a prefloral stage. The second step involves inflorescence initiation and has an obligatory requirement for sequential environmental conditions consisting of an inductive process under SD followed by the expression of this induction which can occur only under LD at a specific temperature. © 1989 Taylor & Francis Group, LLC.