Genetic profile and horticultural evaluation of Styrax americanus (American snowbell)

Styrax americanus Lam. (American snowbell) is an ornamental tree or shrub seldom used in the nursery industry. The species is common in the southeastern United States and is distributed continuously as far north as southeastern Virginia and southern Illinois. We studied a small, threatened population in northern Illinois (latitude = 41°10N), about 500 km disjunct from populations to the south. Little is known about the genetic diversity and landscape potential of this species. The first objective was to evaluate the genetic structure of the northern disjunct population and compare those individuals genetically to populations from the northern and southern extremes of the continuous range in southern Illinois and Florida, respectively. Percentage of polymorphic loci of the disjunct population was 64%, greater than that of the population in Florida. We propose that the diversity of the disjunct population is a consequence of its history as a component of the Grand Marsh that existed in the region until about 100 years ago. The second objective was to evaluate variation in floral morphology among plants from populations that span the distribution of S. americanus grown in a common environment. Results suggest that plants from Illinois have larger flowers than plants from Florida, new knowledge about phenotypic variation of traits with both horticultural and ecological implications. The third objective was to evaluate the tolerance of S. americanus to water deficit and partial flooding, and to determine how soil-water status affects cold tolerance during cold acclimation. Across hydration treatments, there were no provenance differences in CO2 exchange rates between the northern Illinois and Florida populations. Results suggest that plants exposed to prolonged water deficit exhibit less cold tolerance than well hydrated plants. This study provides initial evidence of a dynamic interplay of moisture supply and cold hardiness. Finally, the fourth objective was to characterize this species' capacity to resist deacclimation. Analyses suggest that plants from Florida deacclimated more rapidly in April than plants from Illinois. Selection of germplasm from the Illinois populations appears advantageous for horticultural applications in regions with harsh winters due to the greater mid-winter hardiness, and particularly the capacity of the northernmost population to resist deacclimation.