FLOWERING IN CELERIAC (APIUM-GRAVEOLENS L VAR RAPACEUM (MILL) DC) - EFFECTS PHOTOPERIOD

The role of photoperiod in time of flowering in celeriac (Apium graveo lens L. var. rapaceum (Mill.) DC.) was analysed for three different ph ases, namely thermoinduction, evocation and inflorescence development. The effect of photoperiod during thermoinduction on the relationship between duration of exposure to a low temperature and the progress of thermoinduction was studied by transferring plants at different times from 12-degrees-C with a photoperiod of 8 or 16 h, to 20-degrees-C. Th e degree of thermoinduction at the time of the transfer was determined by later counting the final number of leaves. The degree of thermoind uction at 8 and 16 h did not increase during the first 21 days of the low temperature treatment, but it increased linearly after 21 days unt il thermoinduction was completed after 39 (8 h) and 34 (16 h) days. Ra te of initiation of leaf primordia on the apex decreased with increasi ng photoperiod from 0.65 leaves per day at 8 h to 0.47 leaves per day at 16 h. At a photoperiod of 8 h completion of thermoinduction was fol lowed immediately by initiation of the primordia of the main umbel on the apex. However, if plants remained at 16 h after the completion of the thermoinduction, leaf primordium initiation on the apex was strong ly retarded, and initiation of the final leaf was delayed. If the phot operiod was varied after initiation of the main umbel on the apex, tim e of flowering of the main umbel at a photoperiod of 24 h was approxim ately 8 days earlier than at a photoperiod of 10 h. The main infloresc ence stalk developed only at a 24 h photoperiod; at 10 h the main umbe l was directly implanted on the tuber.