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North American horticulture cultivates an astonishing diversity of ornamental species, from nearly every floristic region, but its landscapes are dominated by temperate species drawn from eastern Asia. The East Asiatic floristic region is one of the most diverse in the world with a high level of endemism across taxonomic ranks and ancient relicts of a once widespread flora. From this, a large number of ornamental genera and species have been introduced, from either a long history of cultivation in Asia or directly from the wild, where they have since become fixtures in European and American gardens. The success of Asian germplasm in American horticulture is attributable, in part, to a shared evolutionary history, climate matching, and pre-adaptability. Continuing access to these genetic resources is now governed by national legislation and influenced by an evolving international regime of access and benefit-sharing influenced by the Convention on Biological Diversity. Furthermore, updated plant quarantine regulations have added additional requirements for the importation of foreign plant genetic resources. The newly created category within the USDA Animal and Plant Health Inspection Service (APHIS) Q37 regulations known as “not authorized pending pest risk analysis” (NAPPRA) restricts the import of plants for planting that may harbor pests or become pests that are not already established within the United States. To this end, scientists involved in the collection or use of Asian plant genetic resources are affected by recent changes in international and national laws, regulations, and access and benefit-sharing regimes.
A diverse collection of germplasm representing 24 taxa from Catalpa sect. Catalpa Paclt and sect. Macrocatalpa Grisebach, Chilopsis D. Don, and ×Chitalpa Elias & Wisura were screened for susceptibility to powdery mildew (PM), Erysiphe elevata (Burr.) U. Braun & S. Takam, and Catalpa sphinx larvae (CSL), Ceratomia catalpae (Boisduval), feeding. The PM screening was conducted in 2004–05, with plants grown in a lathhouse (50% shade) in 2004, and a gravel pad (100% full sun) in 2005. The PM causal organism was identified as Erysiphe elevata both years. Disease incidence and severity were recorded at 2-week intervals for 6 weeks and used to calculate area under the disease progress curves (AUDPC) for each taxon for each year. North American Catalpa in sect. Catalpa, Chilopsis, and ×Chitalpa taxa were all moderate to highly susceptible to PM. Chinese Catalpa in sect. Catalpa and West Indian species in sect. Macrocatalpa were resistant to PM. Hybrids between North American and Chinese Catalpa in sect. Catalpa varied in susceptibility, indicating inheritance of partial resistance to PM. A no-choice feeding study conducted with CSL in 2005 found no differences in survival or growth of larvae reared on taxa from Chilopsis, ×Chitalpa, or either section of Catalpa. Future breeding of ×Chitalpa can utilize two different sources of PM resistance, but a source for resistance to CSL was not identified.
Inheritance of two mutant foliage types (purple and mottled variegated) was investigated for diploid, triploid, and tetraploid tutsan (Hypericum androsaemum). Segregation ratios were determined for diploid crosses in reciprocal dihybrid F1 and F2, BC1P1, and BC1P2 families. F2 tetraploids were derived from autotetraploid F1s. Triploid segregation ratios were determined from crosses between autotetraploid F1s and diploid F1s. Diploid di-hybrid crosses fit the expected 9:3:3:1 ratio for a single, simple recessive gene for both traits, with no evidence of linkage between each trait. Data from backcross and triploid crosses generally supported this model. In tetraploid crosses we observed twice as many variegated phenotypes as predicted which was not explained by random chromosome or chromatid assortment. Inheritance of purple foliage did not deviate from random chromosome assortment at the tetraploid level.
Luther Burbank (1849–1926) was a prolific ornamental plant breeder, who worked with 91 genera of ornamentals, from Abutilon to Zinnia, and released nearly 1000 cultivars to the industry. His innovative work included both herbaceous and woody plant materials as well as ornamental vegetables such as corn, tomatoes, and spineless cacti. His most popular ornamental release, the shasta daisy hybrids—first released in 1901, is still on the global market. This article focuses on Luther Burbank’s breeding techniques with ornamental plants and how both the germplasms that he developed and his methodologies used permeate modern flower breeding. Genera with the highest number of cultivars bred and released by Burbank include Amaryllis, Hippeastrum, and Crinum followed by Lilium, Hemerocallis, Watsonia, Papaver, Gladiolus, Dahlia, and Rosa. With Lilium, he pioneered breeding the North American native lily species, particularly those from the Pacific coastal region, producing the eponymous Lilium ×burbankii. Burbank’s breeding enterprise was designed to be self-sustaining based on profits from selling the entire product line of a new cultivar or crop only to wholesale firms, who then held exclusives for propagation and selling, although financial hardships necessitated selling retail occasionally. Entire lots of selected seedlings were sold to the highest bidder with Burbank setting the price in his annual catalogs such as the Burbank Hybrid Lilies lot for U.S. $250,000 or some of the “very handsome, hardy ones” for U.S. $250 to U.S. $10,000 each. Other flower cultivars also commanded high prices such as seedling Giant Amaryllis that sold for U.S. $1.55/bulb in 1909. Cacti were another area of emphasis (he released more than 63 cultivars) from the spineless fruiting and forage types (Opuntia ficus-indica, O. tuna, O. vulgaris) to flowering ornamentals such as O. basilaris, Cereus chilensis, and Echinopsis mulleri. Interest in cacti during 1909–15 rivaled the Dutch Tulip mania with exorbitant fees for a single “slab” of a cultivar, speculative investments, controversy with noted cacti specialists (particularly David Griffiths), and lawsuits by The Burbank Company. Although most cultivars have been lost, Burbank’s reputation as the Father of American Ornamental Breeding remains admirable from critics and devotees alike.
Flowering cherries belong to the genus Prunus, consisting primarily of species native to Asia. Despite the popularity of ornamental cherry trees in the landscape, most ornamental Prunus planted in the United States are derived from a limited genetic base of Japanese flowering cherry taxa. A diverse collection of ornamental Prunus germplasm is maintained at the U.S. National Arboretum as part of an ongoing flowering cherry improvement program, but the genetic backgrounds of many trees are unclear. We characterized this germplasm using five simple sequence repeat (SSR) primer pairs, including one chloroplast primer pair. These primers generated 140 unique alleles that were used to assess genetic relationships among species, hybrids, and cultivars in this collection. We found that these markers followed expected Mendelian inheritance from parents to progeny in controlled hybridizations. In general, species clustered according to published taxonomic groupings, including a distinct separation of the ornamental cherries (Prunus subgenus Cerasus section Pseudocerasus) from other subgenera. Individual accessions of several taxa did not cluster with other samples of the species, indicating possible misidentification or interspecific combinations. The resulting information will be useful in guiding decisions on breeding methodology and germplasm preservation.
×Chitalpa tashkentensis Elias & Wisura is a sterile intergeneric hybrid [Catalpa bignonioides Walt. × Chilopsis linearis (Cav.) Sweet]. To restore fertility in ×Chitalpa the following were evaluated: 1) oryzalin as a polyploidization agent, 2) fertility of induced polyploids, and 3) in vitro culture methods for embryo rescue of interploid crosses. Meristems of ×Chitalpa `Pink Dawn' were submerged in an aqueous solution of 150 μm oryzalin for 0, 6, 12, or 24 hours and ploidy analyzed via flow cytometry. As treatment duration increased, recovery of diploids decreased as mixoploids and shoot mortality increased. Two tetraploid shoots occurred in the 24-hour treatment. Four tetraploids and two cytochimeras were stabilized in total. Tetraploids flowered sparsely; however, cytochimeras flowered profusely and these were used to study fertility at the tetraploid level. Diploid ×Chitalpa `Pink Dawn' pollen was essentially nonviable, but cytochimera pollen stained and germinated equal to or greater than pollen of C. bignonioides and C. linearis `Bubba'. Cytochimera ×Chitalpa were selfed yielding tetraploid seedlings, crossed with C. bignonioides to yield triploids, but failed in reciprocal crosses with C. linearis `Bubba' and `Burgundy Lace'. To increase recovery of triploids, germination of triploid and tetraploid embryos was investigated, as either intact ovules or excised embryos, on Schenk and Hildebrandt (SH) basal salts supplemented with sucrose at 20, 40, and 80 g·L-1, presence or absence of 2% coconut-water, and gibberellic acid (GA3) at 0, 1, 2, or 4 μm, and harvested weekly beginning 2 weeks after pollination (WAP). Germination of triploids (cytochimera ×Chitalpa × diploid C. bignonioides) and tetraploids (selfed cytochimera ×Chitalpa) were greatest with excised embryos at 7 WAP on SH supplemented with sucrose at 20 g·L-1 and ≥1 μm GA3. Germination of triploids (diploid C. linearis × cytochimera ×Chitalpa) was <5% at 4, 5, or 6 WAP on the same medium as above. Oryzalin effectively induced polyploidy and restored fertility in ×Chitalpa `Pink Dawn'. Successful crosses between hybrid and parental taxa of different ploidy levels, coupled with embryo culture will facilitate a ×Chitalpa breeding program. Chemical names used: 4(dipropylamino)-3,5-dinitrobenzenesulfonamide (oryzalin).
Inheritance of two mutant foliage types, variegated and purple, was investigated for diploid, triploid, and tetraploid tutsan (Hypericum androsaemum). The fertility of progeny was evaluated by pollen viability tests and reciprocal crosses with diploids, triploids, and tetraploids and germinative capacity of seeds from successful crosses. Segregation ratios were determined for diploid crosses in reciprocal di-hybrid F1, F2, BCP1, and BCP2 families and selfed F2s with the parental phenotypes. F2 tetraploids were derived from induced autotetraploid F1s. Triploid segregation ratios were determined for crosses between tetraploid F2s and diploid F1s. Diploid di-hybrid crosses fit the expected 9: 3: 3: 1 ratio for a single, simple recessive gene for both traits, with no evidence of linkage. A novel phenotype representing a combination of parental phenotypes was recovered. Data from backcrosses and selfing support the recessive model. Both traits behaved as expected at the triploid level; however, at the tetraploid level the number of variegated progeny increased, with segregation ratios falling between random chromosome and random chromatid assortment models. We propose the gene symbol var (variegated) and pl (purple leaf) for the variegated and purple genes, respectively. Triploid pollen stained moderately well (41%), but pollen germination was low (6%). Triploid plants were highly infertile, demonstrating extremely low male fertility and no measurable female fertility (no viable seed production). The present research demonstrates the feasibility of breeding simultaneously for ornamental traits and non-invasiveness.
A diverse collection of germplasm representing 24 taxa from Catalpa sect. Catalpa Paclt and sect. Macrocatalpa Grisebach, Chilopsis D. Don, and ×Chitalpa Elias & Wisura were screened for susceptibility to powdery mildew (PM) incited by Erysiphe elevata (Burr.) U. Braun & S. Takam and feeding by catalpa sphinx larvae (CSL), Ceratomia catalpae (Boisduval). PM screening was conducted on plants grown in a lathhouse (50% shade) in 2004 and a gravel pad (100% sun) in 2005. The PM causal organism was identified as E. elevata both years. Disease incidence and severity were recorded at 2-week intervals for 6 weeks and used to calculate area under the disease progress curves (AUDPC) for each year. North American Catalpa in sect. Catalpa, Chilopsis, and ×Chitalpa taxa were all moderately to highly susceptible to PM. Chinese Catalpa in sect. Catalpa and West Indian species in sect. Macrocatalpa were resistant to PM. Hybrids between North American and Chinese Catalpa in sect. Catalpa varied in susceptibility, indicating transmission of partial resistance to PM. No differences in survival or growth were found in a no-choice feeding study with CSL reared on taxa from Chilopsis, ×Chitalpa, or either section of Catalpa. Future breeding of ×Chitalpa can use sources of PM resistance identified in this study, but a source of resistance to CSL was not found.