An in vitro bioassay for the evaluation of cold treatment on flower bud dormancy in Camellia

Camellia japonica L. is an evergreen shrub whose cultivars are of great ornamental value. Initiation and early differentiation of flower buds starts from late spring on while flower bud development and visible bud enlargement sequel until autumn. In many temperate woody ornamentals, dormancy is installed after flower bud differentiation. The exposure of floral buds to cold temperatures (between 2-7°C) will enhance dormancy release and initiate normal growth and anthesis during the next spring. To quantify the cold requirement five low temperature regimes, consisting of darkness and a constant temperature of 7°C applied for 0, 2, 4, 6 and 8 weeks, were tested on budded plants of the cultivar ‘Nuccio’s Pearl’. As indicator of dormancy release, an inexpensive and easygoing test was conducted by forcing excised flower buds in an in vitro bioassay. Simultaneously a possible hormonal basis for this endodormancy was assessed by measuring the concentration of abscisic acid in floral buds. Our results indicated that the bioassay is a suitable indicator of the moment of dormancy release as it was able to highlight a reduced amount of dormancy when buds stayed longer at cold. Abscisic acid content in floral buds decreased after 6 and 8 weeks of cold, with the latter promoting the most precocious dormancy breaking registered in vitro. Hence, the bioassay appears as a cost-effective tool that could be of interest for breeders, to characterize the chilling requirements of flowers of parental plants or their offspring.