FLOWER INDUCTION AND INITIATION IN CELERIAC (APIUM-GRAVEOLENS L VAR RAPACEUM (MILL) DC) - EFFECTS OF TEMPERATURE AND PLANT-AGE

Flower induction in (Apium graveolens L. var. rapaceum (Mill.) DC.) plants was studied by transferring plants from a low (12-degrees-C) to a high (20-degrees-C) temperature and vice versa at regular time intervals after sowing. There were 52 and 40 leaves, respectively, initiated on the apex of the main stem, prior to the initiation of the umbel (final number of leaves), of plants grown continuously from sowing at 20-degrees-C and 12-degrees-C. Temperature during induction determined this final number of leaves. Transfer of plants at different times during the induction process resulted in a final number of leaves between 40 and 52. The degree of induction increased linearly with time of exposure to inductive conditions, after the end of a short juvenile phase lasting until four to five leaves have been initiated on the apex. As soon as the induction was completed the final number of leaves was fixed. Completion of induction and time of initiation of the inflorescence need to be regarded separately, because completion of induction was reached at a much lower number of initiated leaves than the final number of leaves. A temperature-determined final number of leaves and a variable time lapse between completion of induction and initiation of the inflorescence on the apex prevents initiation of the inflorescence in young plants at a low number of leaves.