Flower differentiation of azalea depends on genotype and not on the use of plant growth regulators

Flowering is a complex process which starts with the induction and development of the flower buds. For azalea (Rhododendron simsii hybrids), flower induction was hastened by the application of chlormequat and took place within 11 days after treatment. Subsequent flower bud differentiation was not altered by the application rate of the plant growth regulators (PGR) chlormequat and paclobutrazol, nor by temperature or light sum. There were however, large genotypic variations in flower bud differentiation rate. For all cultivars a linear phase until flower primordia were fully differentiated and the style started to enlarge (flower bud stage 7), was followed by a slower final development (to stage 8). The linear phase was fastest for the semi-early-flowering cultivars ('Mont Blanc', 'M. Marie' and 'Otto'), requiring only 46 or 48 days to reach flower bud stage 7 after the first PGR treatment. Two late-flowering cultivars ('Thesla' and 'Sachsenstern') had the slowest differentiation, requiring 64 days to reach stage 7. The early-flowering cultivars ('H. Vogel' sports) and two late-flowering cultivars ('Mw. G. Kint' and 'Tamira') required 54 and 52 days, respectively, after the first PGR treatment to reach stage 7. To reach flower bud stage 8, a similar trend in velocity was seen, the semi-early-flowering cultivars requiring the least amount of days (17-18 days), the late-flowering cultivars 'Thesla' and 'Sachsenstern' requiring the highest amount of days (24 days) and the early-flowering cultivars and the late-flowering cultivars 'Mw. G. Kint' and 'Tamira' requiring an intermediate number of days (20-22 days).