Flowering response of an early and late flowering cultivar of azalea (Rhododendron simsii Planch.) to cold treatment

As in other woody ornamentals, azalea flower bud differentiation is followed by a period of dormancy which is broken by cold. In this research two cultivars with different cold requirements (the early flowering cultivar ‘Nordlicht’ and the late flowering cultivar ‘Mw. G. Kint’) were kept at 7°C after flower bud differentiation and monitored weekly. To follow dormancy release, endogenous abscisic acid (ABA) concentrations in the flower buds were measured and opening of buds was assessed by an in vitro bio-assay, testing the sensitivity to exogenous ABA. Flower buds of ‘Nordlicht’ had a steady ABA content for 5 weeks at 7°C, indicating that endogenous ABA concentrations are not directly linked to dormancy breaking. The in vitro bio-assay on the other hand showed that four weeks of 7°C are required for ‘Nordlicht’ to lose sensitivity to exogenous ABA. For ‘Mw. G. Kint’ there was a decrease in ABA content until 7 weeks at 7°C, and also flower buds lost sensitivity to exogenous ABA after 7 weeks. Carbohydrate and starch content in leaves and buds were examined since these are important in the successive forcing period. Starch levels in the leaves decreased with increasing cold period. ‘Nordlicht’ flower buds responded to the cold treatment by formation of raffinose and stachyose, flower buds of ‘Mw. G. Kint’ maintained the initial concentration of those carbohydrates. Finally the actual flowering response was monitored by forcing weekly 10 plants from 7°C in the greenhouse. The results showed that at least 4 weeks are required to break dormancy for ‘Nordlicht’ confirming the results of the in vitro opening of the buds, while at least 6 weeks are required for ‘Mw. G. Kint’.