Japanese tree lilac (Syringa reticulata) is a popular ornamental tree in the northern United States because of its flowering characteristics and its outstanding winterhardiness [U.S. Department of Agriculture hardiness zone 3–7 (USDA, 2012)] (Fig. 1A). Japanese tree lilac has a mature height of 8 m with a width approximately two-thirds of the mature height. Dirr (2009) reported that japanese tree lilac is possibly the most trouble-free lilac and is an excellent specimen and street tree. It bears large clusters of creamy white flowers in early summer and produces seed panicles, which have a slight ornamental appeal. Japanese tree lilac cultivars grown from seed are not true to parent and require clonal propagation for commercial production. Cultivars of japanese tree lilac are propagated either by stem cuttings, tissue culture, or grafting (Dirr and Heuser, 2006).
Grafting is generally performed on seedling rootstocks of japanese tree lilac. Commercial nurseries produce seedling rootstocks for grafting, which require stratification (moist-chilling or moist-warm conditions) to overcome physiological or embryo dormancy. Seed dormancy is variable in japanese tree lilac with a recommendation of 40- to 60-d cool (1–5 °C) stratification (Hartmann et al., 2011). Shugert (1973) reported that fall planting of japanese tree lilac seed in Nebraska yielded 60% to 65% germination rates without stratification. Unfortunately, there is no information reported on timing of seed collection or any other criteria for collection time to maximize germination rates without stratification (Shugert, 1973). Commercial nurseries in the northern United States sow stored dry seed in field beds in mid-July to produce a cotyledon or first true leaf seedling for overwintering with germination rates of ≈75% (N. Maren, personal communication). The following growing season, the seedlings are covered and uncovered to protect from early spring frosts. This frost prevention covering is often placed directly over the seedlings for insulation. The covering process often results in crooking of the seedling, making them unsalable for grafting use. It would be advantageous to rootstock seedling production if seed stratification can be eliminated to produce a sizable seedling that would not get seedling crooking from frost protection practices. This would allow for seedling production to be moved from field beds to greenhouses for quicker production of high-quality rootstock plants suitable for grafting.
There are three primary stages of seed development: stage I—embryo differentiation, stage II—cell expansion, and stage III—maturation drying (Hartmann et al., 2011). Hartmann et al. (2011) state that at the end of stage II the seed has reached physiological maturity and can be removed from the fruit without lowering germination rates. This process allows seed to be germinated without stratification. Japanese tree lilac seed capsules change color from green to yellow and finally brown before dehiscing. Japanese tree lilac seed is considered to be mature once seed capsule color change commences.
Research objective of this study was to determine if “green” seed could be used without the need of stratification for japanese tree lilac seedling production. Seed capsule fresh weight and seed moisture content were evaluated to determine if these factors could be used as predictors of germination percentages.
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