EFFECT OF PHOTOPERIOD ON FLOWER STALK ELONGATION IN CELERIAC (APIUM-GRAVEOLENS L VAR RAPACEUM (MILL) DC)

The effect of photoperiod on elongation of the newer stalk in celeriac (Apium graveolens L. var rapaceum (Mill.) DC.) was studied by subject ing plants, after initiation of the primary umbel on the apex, to phot operiods between 8 and 24 h at a temperature of 16 degrees C. The incr ease in length of the flower stalk with time could be described by a l ogistic curve. The final length of the stalk increased with increasing photoperiod, but did not increase further with photoperiods above 14 h. The stalk elongation rate increased with increasing photoperiod up to 18 h, when a maximum elongation rate of 4.2 cm day(-1) was reached. Plants kept at a photoperiod of 8 or 10 h flowered normally, but with out formation of the flower stalk, if the treatment started before ini tiation of the primary umbel on the apex but after thermoinduction for flowering was completed. Only additional low temperatures, before the start of the photoperiodic treatment, or a longer photoperiod resulte d in elongation of the flower stalk. Time of flowering of the primary umbel was not affected by length of the photoperiod, only time of flow ering of the secondary umbels was delayed in shorter photoperiods. Whe n no stalk elongation was observed, the primary umbel developed direct ly from the bulb and no laterals developed. The results showed that th e requirements for flowering and bolting were different in celeriac. F or stalk elongation a longer period of low temperatures is required th an for flowering. However, the low temperature needed for stalk elonga tion could be replaced by a long photoperiod.