Liriopogons (most recently Ruscaceae s.l. Hutch, formerly assigned to Convallariaceae Horan., Asparagaceae Juss., Haemodoraceae Arnot, Ophiopogonaceae Kunth, and Liliaceae Juss.) (Kim et al., 2010) comprise a class of valuable evergreen groundcovers (Skinner, 1971). Liriopogons are native to China, India, Japan, Korea, the Philippines, and Vietnam with Liriope consisting of approximately eight species (Chen and Tamura, 2000a) and Ophiopogon consisting of ≈65 species (Chen and Tamura, 2000b). Popularity of liriopogons is attributable, in part, to their adaptability (Li et al., 2011) and versatility in the landscape, easily filling the roles of groundcovers, foundation plants, edging and massing plants, and understory plants (Fantz, 1993).
The complex taxonomy of liriopogons has been developing since the initial designation of Convallaria japonica by Thunberg (1780). The following centuries resulted in many genera designations (Anemarrhena Bunge, Chloopsis Blume, Convallaria L., Flueggea Rich., Liriope, Mondo Adans., Ophiopogon, Polygonastrum Moench, and Slateria Desv.) and common names (aztec grass, bordergrass, lilyturf, liriope, mondo grass, monkeygrass, and snakesbeard) (Fantz, 1993; Nesom, 2010). Nevertheless, liriopogons’ attractiveness, resistance to pests and diseases, hardiness, and utility in the landscape have made them important nursery crops. Wholesale values of liriopogons in North Carolina are estimated to be over $41 million for 2009 (Trueblood, 2009).
Much confusion surrounding liriopogons lies in morphological similarities between the two genera. Both Liriope and Ophiopogon are acaulescent, evergreen herbs that set summer/fall racemes of small pink to purple or white flowers. Floral whorls are found in multiples of three (dichasia to compound dichasia to small cymes) (Fantz, 2008a). The perianth has six indistinguishable sepals and petals and six stamens. Fruits of liriopogons are blue/black and berry-like or a three-celled capsule (Fantz, 2008a).
Anatomical studies by Cutler (1992) and Rudall (2000), as well as a molecular marker investigation by Mcharo et al. (2003), concluded that similarities between Liriope and Ophiopogon were too great to warrant separation into two genera. However, morphological studies by Bailey (1929), Conran and Tamura (1998), Hume (1961), and Skinner (1971), molecular phylogenetic studies by Kim et al. (2010), and a molecular marker study by Li et al. (2011) provided evidence supporting separation of Liriope and Ophiopogon.
A recent overview of Liriope and Ophiopogon cultivated in the United States by Nesom (2010) found floral characteristics the best method of distinguishing between Liriope and Ophiopogon, supporting Fantz (2008a). Flowers belonging to Liriope are erect with corollas cupulate to rotate and free anthers with apical poricidal openings and long filaments. In contrast, flowers of Ophiopogon are nodding with corollas campanulate and connate anthers in a column, which narrow apically, dehisce longitudinally, and have subsessile filaments (Fantz, 2008a; Nesom, 2010).
In addition to the historically complex taxonomy of liriopogons, nursery practices including sexual propagation of cultivars, plant substitution, mislabeling of cultivars, and seedling invasion of stock plants have resulted in cultivar degradation within the nursery industry (Fantz, 1994). Fantz (1994) investigated 22 named species and 88 labeled cultivars of Liriope and Ophiopogon collected from nurseries and found 17% of germplasm misidentified to genus and 36% misidentified to species.
A variety of ornamental features such as flower color, inflorescence height, inflorescence branching and fasciation, fruit color, foliar variegation, and medicinal qualities such as steroidal glycosides in tubers (Cheng, et al., 2006; Wang et al., 2012; Yu et al., 1996) indicate a high potential for breeding and improvement of liriopogons. A recent study by Zhou et al. (2009) also demonstrated that hybridization between tetraploid L. spicata and diploid Ophiopogon may be occurring naturally in the wild, suggesting new possibilities for breeding between genera in liriopogons. However, breeding systems and cytogenetics of liriopogons are complex. Previous karyological studies have demonstrated the basic chromosome number for liriopogons to be x = 18 (rarely x = 17) with high levels of polyploidy in many species (Table 1). Also, Fukai et al. (2008) investigated ploidy level and relative genome size through flow cytometry of six species (plus cultivars) of liriopogons (Table 1). Oinuma (1946) reported polyploid forms of liriopogons exhibited increased vigor and grew over a wider geographic distribution than diploid forms. In addition to various ploidy levels, many studies have reported liriopogons to be uniquely tolerant of high amounts of aneuploidy (abnormal number of chromosomes) and cytochimerism (different chromosome numbers among cells in the same plant) (Table 1). Therefore, evaluating the cytogenetics of individual clones and cultivars is critical to developing a breeding strategy for liriopogons.
Previous cytological and cytometric analyses of liriopogons.
As a result of the wide range of ornamental traits found in liriopogons and evidence of interspecific and intergeneric hybridization, there is considerable potential for breeding and improvement of liriopogons. However, these efforts are constrained by confusion over proper taxonomy, lack of information on ploidy levels, and lack of information on cytogenetics of individual clones and cultivars. Objectives of this study were to 1) validate the identification and nomenclature; and 2) determine genome sizes and ploidy levels for an extensive reference collection of liriopogons.
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