Isoenzyme Verification of American-Chinese Hybrids of Liquidambar and Liriodendron

Control-pollinated interspecific hybrids between Liquidambar styraciflua and L. formosana and between Liriodendron tulipifera and L. Chinese were verified by comparison of isoperoxidase banding patterns developed by starch gel electrophoresis of cambial extracts. The relationships between some of the trees of eastern Asia and eastern United States (Li, 1952) are especially evident in Liriodendron, in which the only two species, the American Liriodendron tulipifera L. and Liriodendron Chinese Helmsley, occur in these regions. There are only four species in Liquidambar: Liquidambar styraciflua L. occurs in eastern United States, with disjunct populations in Mexico and Central America, and Liquidambar acalycina and Liquidambarformosana Hance are native to China. A fourth species is found in Turkey. These two genera were among the first to be investigated after the initiation of a project on cytogenetics, breeding, and improvement of landscape trees at the U.S. National Arboretum in 1967. Santamour (1972a) reported successful crossing among three Liquidambar species, using leaf morphology to verify interspecific hybridity. Because verification of young hybrids would be more difficult in Liriodendron, in which leaf shape of the two species is very similar, Santamour used the biochemical evaluation of leaf flavonoids (Santamour, 1972b) to establish the true hybridity of seedlings from crosses of L. Chinese made in 1970. This was thought to be the first controlled hybridization between these species. However, similar research in China was unknown in the West. According to the Nanjing Technological College of Forest Products (1980), they and the Jiangsu Institute of Botany had successfully crossed the two tuliptrees first in 1963, and then several times between 1963 and 1980. They also reported that the hybrids were superior in growth to L. Chinese at 12 years of age. In addition, Huang and Chen (1979) verified both natural and artificial interspecific hybrids by analyses of isoenzyme banding patterns from dormant bud extracts. They found that by acrylamide gel electrophoresis of peroxidase isoenzymes they could distinguish the hybrids that contained the isoenzyme bands of both parents as well as two new bands. Inasmuch as our joint research efforts at the U.S. National Arboretum involved considerable work in isoenzyme electrophoresis, we decided to investigate the potential of using this technique to verify the Liquidambar and Liriodendron hybrids produced at the U.S. National Arboretum and the Liriodendron hybrids produced in China. MATERIALS AND METHODS All of the parent trees and the progenies derived from controlled crosses at the National Arboretum were available for study. The first material received from China included dormant twigs from one tree each of the parent species and one putative hybrid. A second shipment contained dormant twigs from three putative hybrids. Although most isoenzyme studies in plants have utilized leaf tissue, Santamour and Demuth (1980) found that cambial tissue was equally as effective for isoperoxidases. Furthermore, studies of cambial isoenzymes can be made at any time of the year and dormant twigs can be shipped halfway around the world with no adverse effects. The methods used for starch gel electrophoresis and staining of cambial peroxidase isoenzymes were the same as those reported by Santamour (1982) and have been successful in a wide range of woody genera (Santamour & Demuth,