Nyssa biflora Walt. (swamp tupelo) is indigenous to swamps and bottomlands of the southeastern United States. The species distribution comprises the coastal plain from Delaware to south Florida and east Texas, and extends north to southern Illinois. In contrast, N. sylvatica Marsh. (black gum) is indigenous to much of the eastern United States and occurs from Maine to southern Ontario, to central Missouri, and to eastern Texas and southern Florida. Although swamp tupelo is not marketed for horticultural use, black gum is promoted in the industry as a specimen tree with a strong central leader that supports distinctively stratified scaffold limbs and glossy, colorful foliage. Considered among the most beautiful trees native to North America, cultivars of black gum provide outstanding autumnal foliar color and desirable canopy architecture (Dirr, 2009). Taxonomic confusion may help to explain why swamp tupelo is not a nursery crop. Black gum and swamp tupelo can be challenging to differentiate (Outcalt, 1990), and swamp tupelo has been regarded as a variant of black gum [N. sylvatica var. biflora (Walt.) Sarg.]. Embryological studies suggest that swamp tupelo is more closely related to Nyssa aquatica L. (water tupelo) than to black gum (Tandon and Herr, 1971), and Burckhalter (1992) distinguished swamp tupelo from black gum as one of five North American species of Nyssa.
Naturally occurring in soils usually, but not necessarily constantly, waterlogged (Applequist, 1956; Penfound, 1952), swamp tupelo may thrive where fluctuations in soil moisture and aeration are common. Unlike swamp tupelo, wild plants of black gum are not restricted to wetlands, but instead occur on a soil moisture gradient of dry uplands to alluvial stream bottoms and occasionally on edges of swamps (McGee, 1990). Plants that occupy sites in nature that are flooded seasonally or continuously not only resist stresses that afflict other species in inundated soils, but may also tolerate or resist stress from water deficit (Li and Zhong, 2006). The popular tree bald cypress (Taxodium distichum L.) is one of the numerous species important for horticulture that illustrate the tendency for flood-resistant trees to resist drought also. Within the genus Nyssa, differences in their occurrence in nature underscore the possibility that the highly flood-tolerant swamp tupelo may respond more favorably than black gum to a wide range of soil water contents. Aesthetic features of swamp tupelo include glossy green summer foliage and scarlet-red autumnal color.
Its beauty and potential resistance to extremes in soil moisture justify exploring the potential for using swamp tupelo in horticulture. Propagation protocols are needed by horticultural scientists to generate plants for evaluation and by commercial growers. We focused on propagation of swamp tupelo from seed because of success with other members of the genus. All species of Nyssa form seeds within fleshy drupes. Dirr and Heuser (2006) noted that seeds of black gum can germinate with pulp of drupes intact, although it is unknown whether pulp enhances or impedes seed germination of Nyssa spp. Fagan et al. (1981) demonstrated that seeds of lily turf [Liriope muscari (Decne.) L.H. Bailey] with their mealy covering intact had a lower germination rate than seeds with coverings removed. Norton (1980) found similar effects of removing the aril from seeds of southern magnolia (Magnolia grandiflora L.). Kobayashi et al. (2010) suggested that accumulation of abscisic acid (ABA) in the flesh of fruit of watermelon [Citrullus lanatus (Thunb.) Mansf.] may prevent viviparous germination. Seed germination of spicebush (Lindera benzoin L.) was nearly 100% when fruits were manually cleaned or regurgitated by American robins (Turdus migratorious L.), but was <1% when pulp remained intact (Cipollini and Levey, 1997). We therefore quantified the seed germination of Nyssa spp. by using both intact drupes and seeds cleaned of pulp. Cold stratification is required to overcome dormancy of seeds of many species from temperate climates. Although stratification effects may vary with seed provenance, seeds of black gum stratified at 4 °C for 3 months germinated well (Dirr and Heuser, 2006). On the basis of its distribution in regions with relatively mild winters, we hypothesized that seeds of swamp tupelo are less resistant to germination than seeds of black gum, thus requiring shorter durations of stratification to release embryo dormancy.
Our first objective was to determine effects of pulp removal and duration of cold stratification on the speed, synchrony, and total germination of seeds of swamp tupelo. Because treatments were based on those reported to promote seed germination of black gum, seeds of both species were studied. After results suggested that the pulp of drupes of swamp tupelo contains inhibitors that impede seed germination, our second objective became to test that possibility with seeds of swamp tupelo and three other species.
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