Development of a molecular test to ensure good flowering of Viburnum Opulus Var. Roseum

During wintertime Viburnum opulus var. roseum (snowball) shrubs are forced in warm greenhouses to harvest early cut flowers. Early forcing is occasionally unsuccessful. This is probably due to a lack of hours with low temperatures that is needed to break bud dormancy. To gain more insight about the effect of temperature on breaking dormancy and early forcing results, shrubs were transferred into cold storage in week 41, 43 and 45 (2009) for 4, 6 or 8 weeks at a temperature of 2, 5 or 8°C. After storage, the shrubs were forced at a day-/night temperature of 28/23°C. Shrubs stored for at least 8 weeks from week 41, 6 weeks from week 43 and 4 weeks from week 45 gave good forcing results, regardless of storage temperature. Since growing conditions outside vary every year and differ between nurseries, it is difficult to determine when forcing can be started. To prevent poor forcing results, we aimed to develop a molecular diagnostic assay to determine the moment at which winter dormancy of snowball flower buds is sufficiently broken. In order to identify genes involved in bud breaking dormancy, an experiment was performed on a commercial nursery from October 2010 to January 2011. Flower buds of snowball shrubs outside were sampled weekly and in December and January batches of shrubs were placed every week in a greenhouse to observe forcing results. Genes involved in bud breaking dormancy were identified by performing next generation Illumina RNA sequencing on bud samples with poor and good forcing results. A set of candidate genes was validated during a second trial at two different commercial nurseries in 2011-2012.