Proposed for Interspecific Hybrids of Exacum Species Native to Sri Lanka

HORTSCIENCE 40(5):1580–1583. 2005. breeding programs using this species as the sole germplasm source have yet to produce a com- mercially viable cultivar. Despite garnering New Name ‘Styer Group’ Proposed signifi cant attention for their beautiful fl owers, failure to produce a worthy introduction was for Interspecifi c Hybrids of Exacum attributed to the lack of genetic variation of those nonfl oral traits important for horticultural Species Native to Sri Lanka acceptance (e.g., height, branching habit and garden performance). Therefore, a controlled Andrew Riseman1 breeding program was initiated at The Penn- University of British Columbia Botanical Garden and Centre for Plant Research, sylvania State University using fi ve taxa of Sri 6804 SW Marine Drive, Vancouver, BC, V6T 1Z4 Lankan Exacum to combine desirable traits into genotypes worthy of commercialization. V.A. Sumanasinghe Morphology of the primary hybrids was Department of Agricultural Biology, Faculty of Agriculture, University of used to confi rm hybrid status through the appearance of unique trait combinations not Peradeniya, Peradeniya, Sri Lanka present in the parental taxa. In addition, the Douglas Justice2 current population exhibits acceptable fertility levels (e.g., both pollen and ovule viability are University of British Columbia Botanical Garden and Centre for Plant Research, typically at levels suitable for production of 6804 SW Marine Drive, Vancouver, BC, V6T 1Z4 large progeny populations; however, overall 3 fertility of individual genotypes varies) and Richard Craig important production characteristics (i.e., Department of Horticulture, The Pennsylvania State University, University reliable vegetative reproduction, high density Park, PA 16802 bench spacing). Based on these observations, we propose a new name for this group of hy- Abstract. We propose the name Exacum Styer Group for an interspecifi c population brids derived from Exacum taxa native to Sri derived from several Sri Lankan Exacum taxa. Confi rmation of hybrid status was deter- Lanka. The cultivar-group name ‘Styer’ is in mined by the appearance of either unique trait combinations or intermediate forms of tribute to J. Franklin Styer (Botany, University traits originally represented by individual native taxa. Through 12 sexual generations, of Pennsylvania), who was recognized by the proposed cultivar-group continues to exhibit these unique traits and now forms a Swarthmore College as one of the “...most cohesive fertile population. creative and broadly competent horticultur- The genus Exacum L. (Gentianaceae Juss.) contains about 65 species, most of which are an- nual or perennial herbs (Klackenberg, 1985). Of E. trinervium the recognized species, only E. affi ne L., native to Socotra, has been successfully domesticated and introduced to commercial fl oriculture. However, several species and taxa native to Sri Lanka have been identifi ed as potential new horticulture crops based on the presence of horticulturally desirable traits (Riseman and Craig, 1995). These traits include prolifi c fl owering during the summer; abundant, large fl owers (4 to 7 cm in diameter) that range in E. trinervium E. macranthum color from light blue to dark violet with con- ssp. ritigalensis trasting bright yellow anthers; diverse growth habits from erect to decumbent; attractive, glossy green foliage; and effi cient production attributes (e.g., easily propagated via vegetative cuttings). Unfortunately, no single species or taxon possesses suffi cient desirable traits for either direct introduction or domestication. For example, E. macranthum Arn., known primar- ily for its large, dark blue-violet fl owers, was E. pedunculatum E. pallidum identifi ed over 30 years ago as a potential new horticultural crop (S. Suda, personal commu- nication). Unfortunately, several independent

Received for publication 11 Dec. 2004. Accepted for publication 20 Mar. 2005. The authors would like to thank the H.M. Eddie Ornamental Plant Foundation = production of many primary interspecific hybrids (>50) and The Pennsylvania State University, Department of Horticulture, for their fi nancial support. We would also like to thank Iain Taylor and Sean Graham for = production of some primary interspecific hybrids (> 10<50) their critical review of the manuscript. 1Assistant professor of plant breeding. Author of correspondence; e-mail: ariseman@interchange. = production of few primary interspecific hybrids (<10) ubc.ca. 2Associate director and curator of collections. Fig. 1. Crossing diagram representing production of viable hybrids from among Sri Lankan Exacum taxa. 3J. Franklin Styer professor emeritus of horticultural Arrow heads point to the taxon used as the seed parent in the cross. Double-headed arrows indicate botany. successful reciprocal crosses.

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AAugustBook.indbugustBook.indb 11580580 66/14/05/14/05 112:29:252:29:25 PPMM Table 1. Distinguishing morphological characters of Sri Lankan Exacum species and E. Styer Group. Flower color Calyx Petal apex Stem shape Leaf shape RHS code lobe arrangement Anther Species (ht at anthesis) (L × W) (petal number) length (L × W) length E. macranthum Cylindrical Ovate to oval-suborbicular Dark violet RHS 89B 8–9 mm Acuminate overlapping 6–13 mm (29–55 cm) (4.5–9.7 × 1.8–4.3 cm) (5-merous) 14–32 × 8–25 mm E. pallidum Cylindrical with 2 opposite lines Ovate Pale blue RHS 91C 9–10 mm Acute not overlapping 2–4 mm (17–48 cm) (3.0–7.5 × 1.6–2.3 cm) (5-merous) 10–11 × 6–8 mm E. pedunculatum Quadrangular Elliptic to ovate Medium blue RHS 90D 3–4 mm Acute, refl exed overlapping 4 mm (8–22 cm) (1–2 × 0.5–0.6 cm) (4-merous) 6–8 × 4 mm E. trinervium Quadrangular Lanceolate to Medium to light blue 7–10 mm Acute not overlapping 6–13 mm with wings narrowly elliptic RHS 92B 6–16 × 5–15 mm (39–73 cm) (5.6–11.8 × 1.8–4.7 cm) (5-merous) E. trinervium Quadrangular Lanceolate to ovate Medium to dark blue 10–24 mm Acute overlapping 6–13 mm ssp. ritigalensis with wings or narrowly elliptic RHS 93B-D 6–25 × 6–20 mm (20–63 cm) (6.5–14.8 × 1.3–4.3 cm) (5-merous) E. Styer Group Cylindrical to Lanceolate to ovate Pale blue to dark violet 3–24 mm Rhomboidial to broadly 8–18 mm quadrangular, with or narrowly elliptic RHS 89B-C through 93A-C obovate, overlapping or not and without wings (5.5–11 × 1.8–3.7 cm) (5-merous) 20–28 × 16–21 mm or lines (12–36 cm) ists in North America.” Styer helped write the prior research and further refi ning descrip- hybrids were then incorporated into a pedigree the fi rst edition of the International Code of tions based on his research using cytogenet- breeding program where in each generation, Nomenclature for Cultivated Plants was a ics, isozyme profi les, and crossability (prior superior progeny were identifi ed and used as strong supporter of horticultural education treatments were based only on anatomy and the following generation’s parents. Through and introduced many plant cultivars including morphology). In addition, research on hybrid 12 generations beyond the primary hybrids, Cedrus deodara (D. Don) G. Don ‘Kashmir’, fertility (Riseman, 1990) and edaphic condi- we have selected for fertility (individual geno- Ilex crenata Thunb. ‘Green Island’, I. crenata tions of each taxon’s native habitat (Riseman, types’ pollen viability range between 0% to ‘Birmingham’, and Berberis ×gladwynensis E. 1997), support Sumanasinghe’s conclusions on 96%; data not shown), greenhouse production Anders.‘William Penn’. taxon identity and rank. traits, and combinations of horticultural traits The following taxa were collected from that are indicative of hybrid status. Origin their native habitats in 1983 and 1984 and used in controlled hybridizations: E. pedunculatum Description The taxonomic treatment of Sri Lankan L., E. macranthum Arn., E. pallidum Trimen., Exacum has been revised three times during E. trinervium (L.) Druce, and E. trinervium ssp. The distinguishing traits among the original the past 20 years (Cramer, 1981; Klackenberg, ritigalensis (Willis) Cramer. All taxa are con- taxa used in the breeding program and the 1985; Sumanasinghe, 1986). The primary dif- sidered cross-pollinated in nature. However, distinguishing traits of the Styer Group are ference among these treatments is centered despite this and the production of inter-taxa summarized in Table 1. Color descriptions are on taxon rank, not taxon recognition (except hybrids from hand-pollinations, natural hybrids based on the RHS Color Chart (Royal Horticul- for Klackenberg’s treatment of E. trinervium among these taxa are suspected to exist only ture Society, 1998). Through repeated cycles (L.) Druce ssp. ritigalensis (Willis) Cramer between E. pallidum and E. trinervium (Klack- of seed production and selection, the proposed which he collapsed into E. trinervium). We enberg, 1983). Primary interspecifi c hybrids cultivar-group includes all distinguishing have chosen to use the treatment of Sumanas- were produced in 1984 and 1985 with varying traits identifi ed in the original populations. inghe for several reasons. As the most recent, success, depending on the parental combination However, as a result of the initial interspecifi c Sumanasinghe had the advantage of reviewing (Sumanasinghe, 1986) (Fig. 1). These primary hybridizations and subsequent breeding, these

Fig. 2. Individuals of Exacum Styer Group representing the range of plant habits displayed from 12th generation hybrids.

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AAugustBook.indbugustBook.indb 11581581 66/14/05/14/05 112:29:272:29:27 PPMM Fig. 3. Flowers from individuals of Exacum Styer Group representing the range of fl ower forms and colors displayed from 12th-generation hybrids.

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AAugustBook.indbugustBook.indb 11582582 66/14/05/14/05 112:29:302:29:30 PPMM traits are now expressed in novel combinations wetting the foliage with either water or a dilute displaying novel forms and full sexual com- and as intermediate forms not displayed in the fertilizer solution. patibility. We propose the new name ‘Styer original taxa. Traits critical in the delinea- Pest management. Exacum Styer Group Group’ for the selections from this interspecifi c tion of the original taxa include: stem shape, is susceptible to attack by several plant pests. population, with the following description leaf shape, fl ower color, calyx length, petal Greenhouse whitefl y (Trialeurodes vaporari- (note: cultivated selections designated from shape, and anther length (Table 1). In addi- orum Westwood) can be controlled through interspecifi c crosses with Sri Lankan taxa tion, important horticultural traits contributed applications of several commercial pesticides; Exacum macranthum, E. pallidum, E. pedun- to the cultivar-group by individual species however, Malathion has been shown to have culatum, and E. trinervium): include dark blue-violet, acuminate petals phytotoxic effects on mature buds and open Botanical description. Plants decumbent to (E. macranthum), pale-blue, nonoverlapping fl owers. Fungus gnats (Bradysia spp. Winnertz) erect; stems cylindrical to quadrangular, with petals (E. pallidum), short plant stature and can be controlled through the introduction of wings present or lacking. Leaves glossy green, increased branching (E. pedunculatum), en- commercially available parasitic nematodes lanceolate to ovate or narrowly elliptic; leaf hanced fl ower production and upright growth (Steinernema Travassos) or application of a tips acute to acuminate, leaf bases cuneate to habit (E. trinervium), and large broadly acute Bti (Bacillus thuringiensis Berliner spp. is- rounded. Flowers blue to violet, pale to dark petals (E. trinervium ssp. ritigalensis). The raelensis) based product. In addition, the root hues, fl attened to cup-shaped or rarely with range of phenotypes currently represented in pathogens Pythium Pringsh., Phytophthora petals refl exed, corolla base short tubular, the the proposed cultivar-group is presented for deBary., and Fusarium Link ex Fr. can infect E. petals rhomboidial to broadly obovate, overlap- plant conformation (Fig. 2) and fl ower form Styer Group. These diseases can be controlled ping along their entire length to overlapping (Fig. 3). Voucher specimens representing the though a combination of cultural modifi cation only at their broadest point or not overlapping. Styer Group are maintained at the University (e.g., less frequent irrigations) and medium Anthers bright yellow, 8 to 18 mm long. of British Columbia Herbarium. drenches with commercial fungicides. Due to the interspecifi c origin of these Environmental conditions and fl owering plants, heterozygosity remains high and we Culture time. Exacum Styer Group prefer warm tem- anticipate initial introductions to be vegeta- peratures and high humidity during production. tively propagated. Asexual reproduction. Exacum Styer Group Maintaining aerial temperature between 18 is easily propagated throughout the year by to 26 °C day/16 to 20 °C night and relative Literature Cited stem cuttings. However, higher rooting percent- humidity between 70% to 85% throughout Cramer, L. H. 1981. Gentianaceae, p. 55–66. In: ages are observed when cuttings are harvested the production cycle gives good results. Pho- M.D. Dassanayake (ed.). A revised handbook from nonfl owering stock plants. Rooted cut- toperiod control is not required for fl owering; to the fl ora of Ceylon. vol. 3. Amerind Publ. tings are obtained within 4 to 6 weeks from however, fl owering time will vary based on Co. New Delhi. cuttings 2 to 3 nodes in length, basally treated genotype and will decrease with increasing Klackenberg, J. 1983. A reevaluation of the genus with 1% IBA in talc, and placed in cell packs light intensities. The earliest genotypes fl ower Exacum (Gentianaceae) in Ceylon. Nord. J. fi lled with a well-drained substrate under 6 to 8 weeks after transplanting into a fi nishing Bot. 3:355–370. intermittent mist. Rooting percentages range container (e.g., 10 to 15 cm) while the latest Klackenberg, J. 1985. The genus Exacum (Genti- from 50% to 100% depending on genotype and anaceae). Opera botanica, p. 1–144. AiO Print genotypes fl ower between 16 to 20 weeks Ltd. Copenhagen. reproductive status of the stock plant. following transplanting. During naturally Riseman, A. 1990. Examination of the morphol- Growth medium and fertilization. Plants low light months, supplemental irradiance ogy and reproductive biology of interspecifi c perform well in a quick draining peat-based promotes earlier and more robust fl owering hybrids of Exacum. MS thesis. Pa. State Univ., medium. Depending on water quality, media for all genotypes. University Park. pH can be managed through the use of fertil- Riseman, A. 1997. Ecology, physiology, and ge- izer; higher ammonium:nitrate ratio will help Summary netics of zinc nutrition in Sri Lankan Exacum to maintain a low media pH. Excellent results hybrids. PhD thesis. Pa. State Univ., University have been obtained fertigating with acidifying The consistent desire for new and interest- Park. Riseman, A. and R. Craig. 1995. Interspecifi c fertilizers high in ammonical nitrogen, used ing horticultural crops prompted us to evaluate –1 Exacum Hybrids- Novel germplasm for the at a rate of 150 to 200 mg·kg total nitrogen. the commercial potential of Sri Lankan Exacum production of a new fl oricultural crop. XVIIIth The optimal media pH range is 5.0 to 6.0 and taxa, close relatives of E. affi ne (Persian violet). EUCARPIA Symposium Section Ornamentals, that for electrical conductivity (EC) is 1 to Based on initial evaluations, we concluded no Tel Aviv, Israel. 5–9 Mar. 1995. Acta Hort. 2.25 dS·cm–1. single taxon had suffi cient horticultural merit 420:132–134. Irrigation. Plants perform best when grown to begin commercialization. When these taxa Royal Horticultural Society. 1998. Royal Hor- on the ‘dry side’ but without undo wilting. were intercrossed, however, the interspecifi c ticultural Society colour chart. Royal Hort. Typically, excessive irrigation or a severe populations displayed novel forms worthy of Soc., London. drought followed by excessive irrigation leads continued breeding. Through several sexual Sumanasinghe, V.A. 1986. Electrophoretic, cytoge- to increased susceptibility to root pathogens. netic, crossability, and morphological studies of generations, the resulting population has Exacum (Gentianaceae). PhD thesis. Pa. State Also, plant growth is enhanced by periodically distinguished itself from the original taxa by Univ., University Park.

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