Induction photopériodique de l'organogenèse et de la floraison in vitro chez le Cichorium intybus: aspects histologiques

In root explants of a long-day plant, Cichorium intybus L., grown in vitro, inflorescences were produced under short-day conditions (9 h) if long days were applied on days 10 to 18 of culture. Long days consisted of either 16 h fluorescent light (5.6 W∙m−2) or daily cycles of 9 h white fluorescent plus 15 h red (660 nm, 0.3 W∙m−2) or blue (440 nm, 0.5 W∙m−2) light. The structural changes in the meristems of buds regenerated under these conditions were studied. The influence of photoperiod appeared to be critical between the 10th and 16th days, during the progressive transition from vegetative to prefloral stage, which occurred more rapidly under photoperiodic conditions with red or blue light. In noninductive short days, intermediate meristems could be observed tardily on day 24, but no flower formation took place after transfer to long days. Increasing the quantity of light under short-day conditions up to a level comparable to long-day conditions resulted in a rise in mitotic activity, mainly in the peripheral zone of the meristem, but the vegetative zonation was retained. These data therefore indicate that the duration of light really commits the meristem to the subsequent formation of flower primordia; in contrast, the quantity of light controls the bud formation during the first part of development. Key words: Cichorium, tissue cultures, flowering, meristems, organogenesis, photoperiodism.