Variation among Kalanchoe species in their flowering responses to photoperiod and short-day cycle number

Our objectives were to identify the critical daylength and number of short-day (SD) cycles necessary for flowering in Kalanchoe glaucescens, K. manginii, and K. uniflora. In Experiment I, plants were grown for 20 weeks under 9,10,11, 12,13,14, or 15 h photoperiods at 300 μmol m^-2 s ^-1 for 8 h 55 min (9 h photoperiod), or 9 h and extended with day-extension (3 μmol m^-2 s^-1) lighting (10 - 15 h photoperiods). All species flowered when grown under photoperiods ranging from 9 -12 h. The percentage of flowering plants decreased for all species as the photoperiod increased from 12 h to 14 h. No flowering occurred on plants grown under a 15 h photoperiod. Node numbers below the terminal inflorescence increased from 18 nodes to 28 nodes on K. glaucescens, from 12 nodes to 14 nodes on K. manginii, and from 12 nodes to 16 nodes on K. uniflora as the photoperiod increased from 12 h to 14 h, from 10 h to 12 h, and from 12 h to 13 h, respectively. Total flower numbers on K. uniflora decreased from 45 flowers to 13 flowers as the photoperiod increased from 9 h to 13 h. In Experiment II, plants were exposed to 0,1,2,3,4,5,6,7, or 8 weeks of SD (8 h photoperiod) before being placed under night-interruption lighting (2 μmol m^-2 s^-1; between 22.00 - 0.200 h). One-hundred percent of K. glaucescens, K. manginii, and K. uniflora plants flowered when they received more than 1, 3, or 6 weeks of SD, respectively. The node number below the terminal inflorescence in each species was not affected by SD cycle-number. Total flower numbers per plant, and days to first open flower, were unaffected as the number of SD cycles exceeded the number required to induce flowering for all species.