198 Daylength and Temperature Affect Rate of Flowering in Primula obconica

Abstract

Primula obconica (`Libre Light Salmon') was grown from transplant at 16 or 20 °C in combination with 8- or 16-h daylength. Irradiance was adjusted to 10 mol/'m2 per day. Germination took place at 20 °C and seedlings were transplanted into 10-cm (800-mL) containers. Growing conditions until treatment 58 days from seeding were 20 °C and 16-h daylength. Rate of flower bud formation was independent of photoperiod at 16 °C requiring 88-93 days from seeding. A similar rate was observed at 20 °C and 16-h daylength. However, short days (SD) delayed flower bud appearance with 10-15 d at 20 °C. Open flowers were first recorded 110 days from seeding at 20 °C and long days (LD). At 16 °C, 12 more days were required for flower development at LD and 22 days at SD. Open flowers did not develop by termination 145 days from seeding at SD and 20 °C. Treatments within each temperature were included where plants were moved weekly to the alternate photoperiod and allowed to flower. There was no significant difference in flower bud appearance among plants exposed to 16 °C or LD at 20 °C. Flower bud formation progressively slowed at increasing duration beyond 4 weeks of SD and 20 °C. Any duration of LD at 20 °C resulted in similar flowering time while more than 1 week at SD slowed or prevented flower appearance. At 16 °C, more than 6 weeks of exposure to SD delayed, while LD hastened flowering.