Young plants of Gardenia (Gardenia jasminoides) were maintained in growth chambers where photoperiods of 8h, 12h or 16h were applied for 4 weeks. Short photoperiods promote flower bud initiation in Gardenia plants; a higher percentage of visible flower buds were counted after 4 weeks of 8 and 12 hour days followed by 4 weeks of 16 hour days. Long 16 hour days for the second period enhanced flower bud development leading to earlier flowering in plants that had received 8 hour days in the initial 4 weeks. In order to define the effect of the night temperature on flower bud set, plants were placed in growth chambers under 8 hour photoperiods and 24°C day temperature while the night temperatures were kept at 12°C, 18°C or 24°C. After 4 weeks the night temperature was changed to 18°C for the next 4 weeks for all plant groups. This was repeated with 12°C and 24°C as the night temperatures. All night temperatures, which were tested, promoted flower bud set and also all six combination temperatures showed a positive effect on flower bud development. In addition it was noticed that plants with a night temperature of 24°C for both initiation and development stage showed an earlier flowering. E.P. Makridou, A.S. Economou.

ISHS Acta Horticulturae www.acthort.org