Factors Influencing Simultaneous Floral and Vegetative Budbreak In Rhododendron Subg. Hymenanthes

Use of broad-leaved, evergreen rhododendrons (Rhododendron subg. Hymenanthes) in northern landscapes is primarily limited by their cold hardiness. The University of Helsinki introduced several cultivars derived from R. brachycarpum ssp. tigerstedtii with flower buds cold-hardy to -39°C.  When these cultivars are grown at latitudes south of Helsinki, Finland (60°N) such as in St. Paul, MN (45°N), initiation of vegetative growth in the spring often occurs simultaneously with flowering resulting in ‘bypass shoots’ which can obscure flowers, reducing their ornamental value. We initiated experiments comparing bypass shoot development between a Finnish hybrid ‘Haaga’ and ‘Catawbiense Album’, a cultivar that does not typically exhibit simultaneous vegetative and reproductive budbreak when grown in Minnesota.  In one experiment, cultivars were placed in one of four growth chambers with temperature and photoperiod conditions representative of those found in St. Paul or Helsinki during spring budbreak.  Bypassing was quantified using the number of days between floral and vegetative budbreak.  Both cultivars developed bypass shoots in all four environments, although bypassing was greater in St. Paul photoperiod treatment chambers.  Cultivars differed with vegetative budbreak occurring nearly 13 days before flowering for ‘Haaga’ compared to two days for ‘Catawbiense Album’. We analyzed whether auxin may be involved in bypass shoot development.  Indole-3-acetic acid (IAA) and napthalaneacetic acid (NAA) were applied to decapitated shoots at the beginning of forcing to determine if auxin could suppress the growth of lateral vegetative buds.  Both auxins suppressed bypass shoot development, although synthetic NAA was more effective.  Tissue samples from floral organs as well as upper shoot segments were collected and analyzed for endogenous free IAA levels using gas chromatography-mass spectrometry.  Samples were collected at three growth phases:  at Day 0 of being placed in a warm greenhouse for forcing, at the first sign of floral budbreak, and when all flowers in an inflorescence were open.  The amount of free IAA was highest at the beginning of forcing and declined throughout flowering.  IAA was found in both stamens and carpels and was greater in ‘Catawbiense Album’ than ‘Haaga’ at all growth phases and tissue types. A diverse collection of USDA Zone 5-hardy rhododendrons was evaluated at Holden Arboretum (Kirtland, OH) for bypass shoot development over two years.  This germplasm evaluation revealed the trait is heavily influenced by environment; more bypassing occurred after colder winters when flower buds were partially or fully aborted.