Photoperiod is a major factor in flower development of the opium poppy (Papaver somniferum L. 'album DC') which is a long-day plant. Predicting time to flower in field-grown opium poppy requires knowledge of which stages of growth are sensitive to photoperiod and how the rate of flower development is influenced by photoperiod. The objective of this work was to determine when poppy plants first become sensitive to photoperiod and how long photoperiod continues to influence the time to first flower under consistent temperature conditions. Plants were grown in artificially-lit growth chambers with either a 16-h photoperiod (highly flower inductive) or a 9-h photoperiod (non-inductive). Plants were transferred at 1 to 3-d intervals from a 16- to a 9-h photoperiod and vice versa. All chambers were maintained at a 12-h thermoperiod of 25/20 degrees C. Poppy plants became sensitive to photoperiod 4 d after emergence and required a minimum of four inductive cycles (short dark periods) before the plant flowered. Additional inductive cycles, up to a maximum of nine, hastened flowering. After 13 inductive cycles, flowering time was no longer influenced by photoperiod. These results indicate that the interval between emergence and first flower can be divided into four phases: (1) a photoperiod-insensitive juvenile phase (JP); (2) a photoperiod-sensitive inductive phase (PSP); (3) a photoperiod-sensitive post-inductive phase (PSPP); and (4) a photoperiod-insensitive post-inductive phase (PIPP). The minimum durations of these phases for Papaver somniferum 'album DC' under the conditions of our experiment were determined as 4 d, 4 d, 9 d, and 14 d, respectively. (C) 1997 Annals of Botany Company.
