From Forest Nursery Notes, Summer 2011

Styrax americanus Lam. (American snowbell) is a deciduous shrub or small tree seldom produced in nurseries. This species is distributed in patchy populations found mainly from Florida to southern Illinois, although a small, disjunct population exists in northern Illinois. The winter-hardiness and loss of hardiness during a period of increased temperature (deacclimation) of plants from this disjunct population may differ from those of S. americanus elsewhere. We examined cold-hardiness and deacclimation of stems of plants from the disjunct population, from southern Illinois, and from Florida. Segments of stems removed from plants grown outdoors in Ames, IA, were exposed to low-temperature ramping, and the temperature at which stems showed 50% damage (LT50) was determined by using the tissue-discoloration method. To assess deacclimation, stem segments were collected from cold-acclimated plants during winter in a minimally heated greenhouse and exposed to controlled warm temperatures for various time intervals followed by low-temperature ramping. Plants from Illinois were =15 °C more cold-hardy than plants from Florida in Feb. 2008. Plants from the disjunct population in northern Illinois showed less stem tip injury than did plants from southern Illinois. Deacclimation patterns were similar between plants from both Illinois populations. Plants sampled in Apr. 2009 from Florida deacclimated more rapidly than corresponding samples from Illinois, and the chilling required to overcome endodormancy increased with increasing latitude of plant origin. This research suggests that germplasm from the Illinois populations should be used in regions where the poorer hardiness and deacclimation resistance of most S. americanus would not permit survival. The Styrax L. genus contains w! 30 species distributed in Asia, the Mediterranean, South America, and North America (Fritsch, 1999). Two species from Asia, Styrax japonicus Sieb. & Zucc. (Japanese snowbell) and Styrax obassia Sieb. & Zucc. (fragrant snowbell), are the most frequently planted members of the genus in landscapes in the United States. Styrax americanus Lam. (American snowbell) is rarely cultivated (Dirr, 1978) but forms attractive shrubs or small trees up to 4 m tall that occur in nature at moist sites alongside streams, near swamp margins, and in floodplains (Gonsoulin, 1974). The species blooms in late spring, producing dainty inflorescences of white fragrant flowers with reflexed petals. In fall, hard drupes adorn the branches, and striped bark adds ornamental interest. Received for publication 21 June 2010. Accepted for publication 11 Oct. 2010. Journal paper of the Iowa Agriculture and Home Economics Experiment Station, Ames, IA, and supported by the Hatch Act and State of Iowa funds. We acknowledge technical assistance from Ed Moran, James Schrader, Arlen Patrick, and Peter Lawlor. Allan Trapp provided helpful guidance with statistical procedures. 'To whom reprint requests should be addressed; e-mail graves@iastate.edu. Styrax americanus occurs throughout much of the southeastern United States and is distributed continuously as far north as southeastern Virginia and southern Illinois. We are particularly interested in the northern limit of the distribution, which is a disjunct population located in the Momence Wetlands of northern Illinois, ~500 km north of the closest populations to the south (Phillippe et al., 2003). Numerous factors, including its resistance to environmental stressors, must be examined before the potential for S. americanus as a species for the nursery and landscape industries is understood. Low temperatures restrict where many woody species can be used, and little is known about the cold tolerance of S. americanus. Therefore, we investigated midwinter-hardiness and deacclimation patterns of populations of S. americanus from northern and southern locations within its natural distribution. Cold acclimation is the accrual of coldhardiness in response to lengthening nights and declining temperatures. Deacclimation is the loss of cold-hardiness in response to environmental and phenological changes (Kalberer et al., 2006). Increasing temperatures, rather than lengthening photoperiods, principally govern the deacclimation process (Beck et al., 2004). The capacity of a woody species to resist deacclimation when exposed to unseasonably warm temperatures in late winter and early spring is an important trait to consider when specifying taxa for placement in horticultural landscapes (Taulavuori et al., 2004; Wolf and Cook, 1992). Our objectives were to determine if plants from the northernmost disjunct population of S. americanus are more cold-tolerant than plants from other populations and to compare plants from different provenances for their resistance to deacclimation and their chilling requirement. Our results provide needed information about variation in tolerance to low temperature among provenances of this North American species. Materials and Methods Plant material. Styrax americanus were grown singly in plastic pots (18 cm tall, 16.5 cm wide, 3.8 L) in a mixture of 50% processed pine bark, 35% sphagnum peat, and 15% perlite (by volume) (Fafard® 51; Fafard, Inc., Agawam, MA). Plants were from three sources in the wild (Table 1). Both rooted stem cuttings and plants grown from seed represented the northern, disjunct population in Illinois (lat. 41°9' N; long. 87°33' W). Seedlings from a wetland in southern Illinois (lat. 37°22'20" N; long. 88°40'44" W) and seedlings grown by Superior Tree Nursery (Lee, FL) from seeds collected near Lee, FL (lat. 30°41' N; long. 83°30' W), were used. All plants were 2 years old and sexually mature when their midwinter-hardiness was assessed during the winter of 2007-2008. Tissues for midwinterhardiness determinations were taken from a field plot established on the campus of Iowa State University in Ames, IA (USDA coldhardiness zone 5a) (lat. 42°06' N; long. 93°65' W) in 2007. The plot was composed of 30 plants, 10 from each provenance, arranged in a completely random design. Plants were spaced 1 m apart and were irrigated with tap water weekly. Organic mulch was used on the plot to minimize weed pressure. Air temperature was recorded by two i-buttons (Maxim Integrated Products, Sunnyvale, CA) attached to the shoot of two plants «66 cm above the ground. Tissue for deacclimation experiments came from plants arranged in a completely random design in a greenhouse in Ames, IA. There were 113 seedlings total, 18 from the habitat in northern Illinois, 40 from southern Illinois, and 55 from Florida. Plants were exposed to natural photoperiod. The greenhouse was vented but not otherwise cooled and was minimally heated to prevent temperatures less than 2 °C during winter. Mean air temperatures, logged with i-buttons located «35 cm above the greenhouse bench, for Jan. and Mar. 2008 were 5.8 and 10.5 °C, respectively; means in Jan., Feb., Mar., and Apr. 2009 were 7.5, 10.7, 10.5, and 14.8 °C, respectively. Deacclimation treatment and seasonal/ field cold-hardiness sampling. Deacclimation trials took place in Jan. and Mar. 2008 and from Jan. to Apr. 2009. Stems formed in the previous year were randomly chosen from all plants in the greenhouse. Shoots «8 cm in length were collected, after which the terminal 4 cm of the shoot was removed. Stems HoRiSciENCE VOL. 45(12) DECEMBER 201