Common lilac (Syringa vulgaris) is a clonally propagated woody shrub that has been the subject of intense breeding for centuries due to its fragrant, colorful spring blooms (Fiala and Vrugtman, 2008). The majority of species originated in Asia, with only S. vulgaris and S. josikaea originating in Europe (Fiala, 1988). The center of diversity of common lilac lies in the Balkans, and native populations can be found throughout southeastern Europe (Fiala, 1988). The impact of lilacs on the U.S. nursery industry is substantial, with nearly $20 million in revenue generated from more than 1.8 million plants sold in 2014 (USDA, 2016). Still, improvements remain possible, such as improved disease resistance, reblooming, and combining other ornamental traits of significance.
A major limiting factor involved in woody plant breeding is the length of time between successive generations. Juvenility in woody plants is a natural process that prevents flowering in seedlings. Woody plants in nature produce vegetative growth for years under competition before diverting resources to fruit and seed production (van Nocker and Gardiner, 2014). Lilacs typically begin sporadic flowering 3 years after germination, with consistent flowering after 4 years. Breeders have several techniques available for circumventing this mechanism that mostly focus on the cultural conditions after germination. One method is to provide optimal growing conditions to promote vigorous, vegetative growth. Apple breeders growing seedlings under optimal conditions have reduced juvenility to 10 months compared with 5 years for field-grown seedlings (Aldwinckle, 1975). Plant growth regulators (PGRs) can promote early flowering, although this approach has proven highly variable and has not been widely adopted for reducing juvenility (Zimmerman et al., 1985). For large trees with longer juvenility periods, combinations of PGR applications, root restriction, and girdling have been proven effective for reducing juvenility (Philipson, 1996; Snowball et al., 1994). When seedlings reach maturity, forcing is a cultural method that can be used to trigger vegetative and floral development in lilacs using high temperatures from 37 °C in November or 15 °C in March (Jędrzejuk et al., 2016b). However, high temperatures required for forcing often degrades pollen and ovules, making this technique problematic for breeding (Jędrzejuk et al., 2016b). Recent research has also proved that low-temperature forcing reduces oxidative stress in lilac flowers, which may be useful for lilac breeders (Jędrzejuk et al., 2016a).
Expediting seed germination is another option for shortening the generation time of woody plants. Efforts to overcome lengthy periods of seed dormancy have proven effective, including “green” seed germination, embryo culture, bioactive gibberellic acid treatments, and nitric oxide treatments (Bethke et al., 2007; Bridgen, 1994; Shen et al., 2011; West et al., 2014; van Nocker and Gardiner, 2014). The simplest is early or “green” seed collection, which has been effective for seed germination in several woody plant taxa, including Tilia americana (Dirr and Heuser, 2006) and Syringa reticulata (West et al., 2014). Seed development and the depth of physiological dormancy vary in lilacs, partly because of genetic variations and environmental conditions such as temperature postpollination (Junttila, 1973b). In tree lilac, S. reticulata, seeds were determined to be fully mature and capable of germinating as the green capsule color began to fade (West et al., 2014). Germination was optimized in this study by collecting capsules at 1-week intervals just as the green color began to fade in early fall. Germination diminished precipitously as moisture content was lost (West et al., 2014).
In a previous study of cross-compatibility among lilac cultivars (Lattier and Contreras, 2017), germinated seedlings from cold-stratified seed were observed to have a quiescent phase during their first year when vegetative growth was limited, but the seedling developed a large root system (Fig. 1A). Seedlings produced a large flush of vegetative growth in their second year (J. Lattier, personal observation). In 2014, a preliminary trial of immature seed germination was conducted. Predehisced green, yellow–green, and yellow capsules were collected in summer from a random assortment of lilac parents. Seeds were direct-sown in containers (Fig. 1B) and in petri dishes with moistened filter paper (Fig. 1C). Only seeds excised from yellow–green and yellow capsules germinated. Direct-sown seeds grew to their quiescent phase in a glasshouse before being moved to an unheated polyhouse for winter dormancy. In spring, these seedlings produced a large flush of vegetative growth and quickly achieved the same size as stratified seedlings from the previous year (J. Lattier, personal observation). These preliminary results provided a proof-of-concept that germinating immature seed may be a means of reducing generation time for lilac breeders.
This study aimed to 1) determine the optimum germination treatment to overcome the quiescent phase before the first year of growth, and 2) determine if germination treatments improved growth and reduced juvenility in common lilac.
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