Propagation protocols determined for two Nyssa species
The genus Nyssa L. includes several woody species with traits valued by horticulturists, but only black gum (Nyssa sylvatica Marsh.) is prevalent in the nursery trade. Considered among the most beautiful trees native to North America, cultivars of black gum provide outstanding foliage color in autumn. The authors of a new study (HortScience, June 2016) say that another Nyssa species, swamp tupelo (Nyssa biflora Walt.), which is indigenous to swamps and bottomlands of the southeastern United States, might also be a marketable shade tree. "Its beauty and potential resistance to extremes in soil moisture justify exploring the potential for using swamp tupelo in horticulture," they explained.
Frank Balestri and William R. Graves from the Department of Horticulture at Iowa State University said that information about propagation protocols is needed by horticultural scientists to generate plants for evaluation and potential use by commercial growers. The scientists focused their research on propagation of swamp tupelo from seed because there have been successes with other members of the genus. "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," Graves and Balestri said.
The study determined the 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," the authors said. Balestri and Graves compared germination of cleaned seeds (surrounding pulp of fruits removed) of swamp tupelo and black gum that were stratified at 5 °C for 0, 14, 28, 42, 56, 70, 84, and 112 days. Seeds of swamp tupelo within intact drupes were also stratified. Analyses showed that, across all durations of stratification, 79% of cleaned seeds of swamp tupelo germinated, whereas 11% of seeds within drupes germinated. Germination value of cleaned seeds of swamp tupelo increased from 1.26 to 3.23 as duration of stratification increased.
Although cleaned seeds of black gum responded similarly, the benefit of stratification was more pronounced, and the mean germination percentage was lower than for swamp tupelo (66% vs. 79%).
"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," the authors said. In the second experiment, irrigation with low and high concentrations of an extract of fruit pulp of swamp tupelo reduced germination of seeds of basil, spinach, zinnia, and swamp tupelo by 25% to 63% (low concentration) and 40% to 70% (high concentration).
Balestri and Graves said the experiments yielded significant findings for horticulture. "We have shown that cold stratification enhances speed and uniformity of germination of cleaned seeds of swamp tupelo, a species that may have untapped potential for the nursery industry. Second, our hypothesis that seeds of swamp tupelo are less resistant to germination than seeds of black gum was supported. Lastly, we have shown that extracts of the fruit pulp of swamp tupelo inhibit germination of seeds of swamp tupelo and other species," they said.
The researchers said that swamp tupelo can be propagated reliably from seeds removed from fruits, which would allow for selection from a wide array of genotypes for desired ornamental or physiological traits.