Cold treatment modifies the photoperiodic flowering response of Lobelia x speciosa

Lobelia x speciosa Sweet 'Compliment Scarlet' was grown under a range of ph otoperiods and low temperature treatments to determine their effects on flo wering. In the first experiment, plants were held at 5 degrees C for 0 or 1 5 weeks, then grown at 20 degrees C under the following photoperiods: 10, 1 2, 14, 16, or 24 h of continual light or 9 h with a 4 h night interruption (NI). Non-cooled 'Compliment Scarlet' flowered as a qualitative long-day pl ant (LDP) with a minimum flowering photoperiod of 14 h. Following cold, flo wering was quantitative with respect to photoperiod, until approximate to 1 4.2 h, when the calculated rate of progress toward flowering reached a plat eau. In cooled plants, node number below the inflorescence decreased from 2 7 to 16 as the photoperiod increased from 10 to 24 h. Cooled plants develop ed 61-149% more flowers and were 217% taller than non-cooled ones under the same photoperiod. To determine the cold duration required for flowering un der short days (SD), plants were held at 0, 3, 6, 9, 12, or 15 weeks at 5 d egrees C then grown at 20 degrees C under SD (9 h photoperiod) or long days (9 h photoperiod with a 4 h NI). Under SD, few plants flowered after less than or equal to 6 weeks of cold. As cold treatment increased from 9 to 15 weeks, flowering percentage increased, time to flower decreased from 93 to 64 days, and node count decreased from 24 to 13. Cold treatment did not aff ect flowering percentage or time under NI, but plants always had more flowe rs and were taller than reproductive ones under 9 h day lengths. Thus,'Comp liment Scarlet', is a qualitative LDP, but an extended cold treatment can p artially substitute for the long day (LD) photoperiodic requirement. (C) 19 99 Elsevier Science B.V. All rights reserved.