Factors affecting simultaneous reproductive and vegetative budbreak in Rhododendron subg. Hymenanthes

Abstract

The use of elepidote rhododendrons [Rhododendron subg. Hymenanthes (Blume) K. Koch] in the landscape of the upper Midwest of the United States is primarily limited by their cold-hardiness. Rhododendron ssp. tigerstedtii Nitz. derived cultivars developed in Finland are some of the hardiest rhododendrons available, and have proven to survive in Minnesota’s cold, continental climate. However, when these Finnish cultivars are grown at latitudes south of Finland, such as areas in the northern continental United States, vegetative growth often occurs before or during flowering, effectively obscuring the inflorescence. This process, which reduces the plant’s ornamental value, has been termed ‘vegetative bypassing’. The objectives of this thesis are 1) to identify if environmental factors such as temperature and photoperiod affect vegetative bypassing, 2) to assess the incidence of vegetative bypassing in a diverse germplasm collection of elepidote rhododendrons, and 3) to determine if there is an association between auxin and vegetative bypassing. In all experiments, the Finnish cultivar Haaga was compared to ‘Catawbiense Album’, a cultivar derived from the North American native species Rhododendron catawbiense Michx. that is not known to show vegetative bypassing when grown in the U.S. Cultivars did not vary in bypassing when placed in growth chambers that had temperature and photoperiod conditions representative of St. Paul, MN and Helsinki during budbreak. Substantial bypassing was observed on both cultivars in all photoperiod-temperature treatment combinations, suggesting environmental conditions before forcing into flower may have conditioned plants to show the trait upon placement in growth chambers. Preliminary experiments suggested extended periods of warm temperatures or high soil fertility during bud development may promote vegetative bypassing during budbreak. We also observed that vegetative bypassing was more likely to occur under partially or fully aborted (damaged) flower buds than flower buds that open to full inflorescences. The bypassing trait occurred infrequently in the 40 genotypes evaluated in 2009 and the 55 genotypes evaluated in 2010 at Holden Arboretum (Kirtland, OH). ‘Maud Corning’ was the only cultivar to show bypassing in both 2009 and 2010, but fewer flowering (reproductive) shoots exhibited vegetative bypassing in 2010 (10%) than in 2009 (30%) on that cultivar. In general, less bypassing was observed in 2010 following a milder winter than in 2009, further supporting a correlation between damaged flower buds and an increased incidence of vegetative bypassing. The applications of indole-3-acetic acid (IAA) and naphthalene-acetic acid (NAA) to decapitated shoot tips after flower bud removal limited lateral vegetative growth. The results of our exogenous auxin work support the hypothesis that subtending vegetative buds are inhibited from growing by auxin produced in the flower bud that is transported basipetally. However, quantification of endogenous levels of free IAA revealed higher levels in shoot segments in the region of lateral buds from reproductive shoots with vegetative bypassing compared to shoots not exhibiting the trait for both cultivars. Furthermore, ‘Haaga’ had higher levels of IAA in shoot tissue below the terminal flower bud than ‘Catawbiense Album’ when both showed no bypassing. The results from our auxin work highlight the multi-functionality of this plant hormone. Application of auxin to decapitated shoot tips inhibited lateral budbreak, but the endogenous auxin study found lateral bud growth was associated with higher IAA levels in shoot regions proximal to lateral buds, implicating the role of auxin in vegetative growth in rhododendron. Methods used here may lack the precision to isolate auxin’s role in specific plant growth and development processes, and therefore, multiple analytical and molecular methods are needed.