Abstract
Each of the five species of Zelkova tested could be distinguished by the qualitative or quantitative distribution of flavonol glycosides. Thus, the verification of interspecific hybrids might be achieved by flavonoid analyses. Such analyses indicted that Z. X verschaffeltii is actually a rare hybrid between Z. carpinifolia and Z. serrata.
The genus Zelkova belongs to the elm family (Ulmaceae) and consists of 5 to 7 species in western and eastern Asia and on the island of Crete.
In recent years, the Japanese keaki (Z. serrata (Thunberg) Makino) has been widely planted as a landscape tree in the United States, largely as a supposed “replacement” for the American elm (Ulmus americana). Two vegetatively propagated cultivars, ‘Village Green’ and ‘Parkview’, have been introduced into the American nursery trade and seedling material has even been marketed under the confusing name “Sheerlund elm” (Amer. Forest 81(2: 15, 1975).
Zelkova carpinifolia (Pallas) K. Koch is native to the Transcaucasus and neighboring Iran. It has been widely used in Great Britain, but is rare in cultivation in the United States. The Cretan zelkova is Z. abelicea (Lamarck) Boissier (=Z. cretica Spach) and although reputed to be a small tree, some specimens may reach a height of 49 feet (8).
There are two Chinese species, Z. sinica Schneider and Z. schneideriana Handel-Mazzetti. Both are rare in cultivation in the United States and although Z. sinica has been recommended for greater use (5, 7), it is inferior in most respects to Z. carpinifolia and Z. serrata, because of its irregular branching habit. On the other hand, Z. schneideriana may be worthy of further trial. As far as we know, the only mature tree of this species in the United States is located at the U.S. Plant Introduction Station in Glenn Dale, Maryland. It was originally regarded as a Z. serrata, but may be easily distinguished from that species by the soft pubescence on the underside of the leaf. This particular specimen is remarkable for its russetred autumn leaf color which generally lasts from September 1 to early November.
There is one more species of dubious origin and identity: Z. verschaffeltii (Dippel) Nicholson. Although it has been in cultivation at the Royal Botanic Gardens, Kew, England since 1886 (2), Dippel was the first to classify this small tree as a zelkova in 1892. He considered it to be a variety of the Japanese species. Henry (6) suggested that the tree was a hybrid between Z. crenata (Z. carpinifolia)and Z. cretica (Z abelicea). No wild specimens of Z. vershaffeltii have been observed.
Trees of Z. carpinifolia have been killed by Dutch elm disease in Iran (1), but no published reports on the disease susceptibility of the other species have been found. We inoculated 3 trees of diverse origins of Z. serrata with mixtures of aggressive strains of the Dutch elm disease fungus in 1976 and 1978, and consider the species to be highly resistant.
We were interested in developing flavonoid profiles for the various species so that we could use these chemical criteria to establish hybridity in the various progenies resulting from our planned hybridizations in the genus.
The flavonoid analyses of Giannasi (3) on hydrolyzed dried leaves were of limited value since he did not identify the glycosides present in the individual species.
Materials and Methods
Our chemical analyses were restricted to living trees in the Washington, D.C. area that we intended to use in our hybridization program. Only single trees were available in Z. abelicea, Z. schneideriana, Z. sinica, and Z. X verschaffeltii. Two trees of Z. carpinifolia and several trees of Z. serrata were also utilized.
Fresh leaves collected during the middle of the growing season were extracted by boiling for 1 hour in 70% ethanol, concentrated to about 1 ml. of extract for each gram of leaf tissue. Preliminary chromatograms were prepared by subjecting the raw extracts to 2-dimensional ascending paper chromatography in BAW (n-butanol-glacial acetic acid-water, 4:1:5 (V/V) upper phase) and water. These chromatograms, after being fumed with ammonia and viewed under ultra-violet light, gave an indication of the number and behavior of the compounds likely to be found in the extract. The flavonoids were purified and isolated by repeated banding and elution of 1-dimensional runs in BAW, water, and 15% acetic acid, in whatever sequence was most appropriate for complete separation from other compounds. Purified compounds were chromatographed against certain known compounds in all 3 solvents, and occasionally in TBA (tertiary butanol-glacial acetic acidwater (3:1:1) upper phase).
The pure compounds were hydrolyzed, to remove sugars, by boiling in 2N HCL for 1 hour, and the hydrolyzed product chromatographed in Forestal solvent (acetic acid-HCL-water, 30:3:10) with quercetin control. Sugars were analyzed by ascending paper chromatography in iso-propanol-water (4:1) with glucose control. Ultra-violet absorbtion spectra in 95% ethanol were determined before and after hydrolysis. Spectral shifts with a variety of additives (sodium acetate, boric acid, aluminum chloride) were recorded. The methods of analyses and standards for comparison are included in Harborne (4).
Flavonoid compounds found in the various species are given in Table 1. Each species appeared to have a unique qualitative and quantitative flavonoid distribution pattern that would allow verification of hybridity in young seedling material. Unfortunately, we were never able to attempt these tests. Our controlled pollinations of Z. serrata X Z. carpinifolia and Z. schneideriana × Z. serrata failed to produce a single viable seed.
Distribution of flavonoids in Zelkova species; (+) indicates significant amounts and (tr) indicates trace amounts.
Our analyses did, however, shed some light on the possible origin of Z. X verschaffeltii. Zelkova serrata was unique among good species in producing the flavonol quercetin 7-glucoside. An unidentified UV-absorbing spot (Rf 0.72 BAW, 0.43 water, 0.64 in 15% acetic acid) was only found in Z. carpinifolia. The chromatographic profile of Z. X verschaffeltii was virtually a composite of these 2 species and contained both of the assumed species-specific compounds. Thus the chromatographic evidence would suggest that Z. X verschaffeltii is a hybrid between Z. carpinifolia and Z. serrata.
Z. serrata was first introduced into England in 1861, and Z. verschaffeltii has been cultivated since 1886. (2). It may well have been that some of the Z. serrata trees reached sexual maturity before 1886 and crossed with Z. carpinifolia already in cultivation. The fact that Z. X verschaffeltii is really a clone would indicate that such a cross might be a rare event, and our experience would tend to substantiate this hypothesis.
- © 1983, International Society of Arboriculture. All rights reserved.
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