Flowering in celeriac ( Apium graveolens L. var. rapaceum (Mill.) DC.): effects of photoperiod

Abstract

The role of photoperiod in time of flowering in celeriac ( Apium graveolens L. var. rapaceum (Mill.) DC.) was analysed for three different phases, 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°C with a photoperiod of 8 or 16 h, to 20°C. The degree of thermoinduction at the time of the transfer was determined by later counting the final number of leaves. The degree of thermoinduction 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 until thermoinduction was completed after 39 (8 h) and 34 (16 h) days. Rate of initiation of leaf primordia on the apex decreased with increasing 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 followed 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 strongly retarded, and initiation of the final leaf was delayed. If the photoperiod was varied after initiation of the main umbel on the apex, time of flowering of the main umbel at a photoperiod of 24 h was approximately 8 days earlier than at a photoperiod of 10 h. The main inflorescence stalk developed only at a 24 h photoperiod; at 10 h the main umbel was directly implanted on the tuber.