Introduction

Hibiscus is a genus of flowering plants in the Malvaceae family that are naturally growing in tropical, subtropical and warm-temperate regions throughout the world. Hibiscus genus contains 250-300 species having diversified plant structure and habit such as trees, shrubs, and herbs with full range of flower colors (Akpan 2007; Beers and Howie 1992). Hibiscus is the most variable genus in the family in relation to its vegetative, floral, and canopy expressions. Among different species of Hibiscus, H. rosa-sinensis is generally planted as flowering shrub throughout the tropics, including China, India, Pakistan, Phlippines, and South-Indian Islands (Akpan 2007). Only a few species such as H. syriacus, H. sinosyriacus, H. moscheutos, H. paramutabilis, and H. militaris are naturally distributed in the temperate areas including China, Korea (Bates 1965; Park et al. 2005; Van Laere et al. 2007). Hibiscus syriacus L. (rose of Sharon) is a hardy deciduous ornamental shrub that tolerates low temperature and prolonged dry conditions (Lawton 2004). This species can be grown in diverse climatic and soil condition such as, dry rocks, farmlands, coastal areas, along road-side and in garden as flowering ornamental plant. The part of this species name “syriacus” indicates that the origin of this hibiscus species seems to be Syria, but the exact origin is not yet confirmed. Traditionally, it was cultivated in China as a garden plant and then it got popularity as an ornamental plant in Korea and considered as the national flower of Korea (Kim et al. 2009; Van de Laar 1997).

This species exhibits diversity and variations in the plant structure and other morphological characteristics such as, flower color, form and shape, leaf color and appearance etc. Although they flower only in spring, and appears a stiff and ungainly plant in fall after pruning, but still planted as ornamental shrub due to its diversified adaptability to soil and climatic conditions with least maintenance practices. Generally, this species bears solitary flowers in the axils having color range from white, pink, red to purple and blue, sometimes with a large crimson splash near the petal base. Flower size varies from single, semi double to double in different cultivars and flowering time lasts from the July to October (Yu and Yeam 1972; Bean 1973; Yu et al. 1974; Kim and Lee 1991).

Identification and germplasm characterization is an essential prerequisite for the its utilization in any breeding program and also for conservation of plant genetic resources. Morphological characterization proved to be one of the important approach for identification of species and cultivars by categorizing on the basis of growth habit, plant shape, leaf and floral morphology and other growth characteristics of the plants. The inordinate diversity of species in Hibiscus genus were distinguished by many systematists and a comparative limited acquaintance with botanical and genetic studies of Hibiscus, making considerable hitches to conduct interspecific crossing in this genus (Shim 1994). Nevertheless, the taxonomic description, interpretation and interrelationships of this species are complex and challenging (Bates 1965; Wang et al. 1995) and there is a need of time to conduct morphological analysis of various Hibiscus commercial cultivars.

Nowadays, consumers are more interested in flowering plants for their gardens and landscape (Van Laere et al. 2007) and this change in customer preference proves a major driving force for new cultivar development. Hybridization is a major source of new cultivar development and genetic variation in breeding of ornamentals. Plant breeders focused on this species for assortment of new varieties with variable plant size and shape with compact structure, long-lasting flowers of different colors and shape for multiple use of this species in landscape. The process of hybridization facilitates to accumulate different traits from various gene-pools in a single cultivar. Therefore, hybridization may have the potential to enhance quantitative traits in existing cultivars (Van Tuyl and Lim 2003). Also, the agro-morphological characterization of Hibiscus germplasm is fundamental prerequisite to initiate plant breeding programs. The objective of present study was to evaluate the genetic diversity within H. syriacus using morphological characteristics. The collected data may be useful for future Hibiscus breeding programs.

Material and Methods

Plant materials

In year 2011-2014, 107 Hibiscus varieties (Local and exotic) were collected from various nurseries in Korea (Table 1). The collected varieties were planted in field conditions at Kyungpook National University, Daegu (35.8914° N, 128.6106° E) for their morphological evaluation. The recommended cultural practices in hibiscus cultivation were carried out. The phsio-chemical analysis showed soil pH was around 6.2, organic matter was 0.66%, and available N, P, K were 0.2, 0.3 and 10.2 mg • L^–1, respectively.

Morphological analysis

Phenotypic characterization of collected varieties was carried out according to Korean National Seed Characteristics guidelines (http://www.seed.go.kr). All plants were about three years old at the time of data collection. Data were collected from five samples and then average was computed.

Plant

Plant height was measured by using measuring tape from ground level to the top. Data were collected from five plants of each cultivar and then average was computed. Plant shape was recorded for each cultivar by observing visually and comparing it with as mentioned in Korean National Seed Characteristics guidelines (http://www.seed.go.kr).

Flowers

Flowers were categorized on the basis of number of petals and its arrangement. Flowers were grouped into single, semi double and double based on number of petals and its arrangements both for local and exotic cultivars (Table 2, Fig. 1). The flower color of studied Hibiscus cultivars was recorded using The Royal Horticultural Society color chart. H. syriacus cultivars were also classified based on the angle of petal formation. Hibiscus flowers were grouped into three types based on petal’s angle, that were bowl, intermediate, and flat (Fig. 2). In bowl-type petals are standing erect over 45o from the single-core. In intermediate-type, petals are upright over a single point, but outermost layer is of flat shape. Whereas, in flat-type flowers all petals are straight and flat from the single-core.

Different cultivars of H. syriacus were also classified based on petal shape such as broad, intermediate, narrow. The shape of petals was estimated by length and width ratio of petals as described by Song (2004). If length/width ratio greater than 0.91 it was considered as broad. The value between 0.71-0.90 was considered as intermediate type and value less than 0.71 was considered as narrow type (Fig. 3).

Hibiscus flowers were also classified based on eye spot size. According to the size of eye spot 3 groups were identified, that were small, medium, and large. Small-type referred to the eye spot size of not more than 1/5 compared with the petal area. Medium referred to more than 1/5 and less 1/3 compared to the petal area. Whereas, in large type eye spot had more than 1/3 of the total petal area.

Results and Discussion

Morphological analysis of 107 cultivars of H. syriacus was carried out. Out of 107, 69 cultivars were collected locally from various parts of Korea, whereas, 38 were exotic cultivars that were successfully growing in different nurseries in Korea (Table 1).

Plant height of Korean and exotic cultivars were recorded and data revealed that the maximum plant height was recorded in ‘Woodbridge’ (143 cm) which had tall erect shape. ‘Serenade’ and ‘Saemaeul’ had plant height of 141 and 137 cm, respectively and both had tall erect plant shape. ‘Daedeoksahwarip’, ‘Cheoyong’, ‘Pheasant Eye’, and ‘Hwahab’ were grouped into low-heighted cultivars and had plant height ranged 70 to 73 cm (Table 2). Morphological evaluation of various H. syriacus cultivars was also conducted by Kim et al. (2009) to determine its genetic relationships. They concluded that using phenotype information through principal coordinate analysis (PCoA), the well-distributed characteristics of H. syriacus cultivars were very clear in morphological variations, although these are heavily influenced by environmental conditions of the locality (Yu et al. 1979).

Classification with flower morphological data of various H. syriacus cultivars displayed that flowers of various cultivars of H. syriacus were grouped into three types based on the petal arrangements (Table 2). The results showed that among total cultivars studied, 54.8% of Korean cultivars were single, while 5.6% and 4.7% contained semi-double and double flowers, respectively (Table 2). Among exotic cultivars 21.4% born single flowers, whereas, 3.7 and 9.3% had semi-double and double type of flowers respectively. H. syriacus exhibited a divergent floral morphology, which appeared to be fit for selection for hybridization. Maclntyre and Lacroix (1996) also reported diversified flower’s morphology in H. rosa-sinensis and they found that this variation was due to homeosis process during which replacement of any plant part completely or partially occurred.

In present study, the diameters of H. syriacus flower ranged from 4.1 cm in ‘Kakchang’ to 9.4 cm in ‘Pyonghwa’. In a previous study, Van Huylenbroeck et al. (2000) observed flower diameters ranged from 4.5 to 12 cm in ‘Coeruleus’ and ‘Shimsan’ cultivars. In present study, H. syriacus flowers were divided into three types based on the angle of flower formation. The data showed that among the studied Korean cultivars, 52.1% showed the intermediate type of flower formation followed by bowl type (26.1%) and flat type (21.8%). Among exotic taxa also intermediate types of flower formation were observed the maximum among studied cultivars (Table 2). In a previous report, phonological analysis of H. cannabinus cultivars was conducted and variations were observed regarding flower characteristics (Fatanmi et al. 2013). The flower color of studied Hibiscus cultivars is listed in Table 1. The red color was dominating flower color in cultivars studied as the maximum cultivars had red flowers followed by white flowers. The variation in morphological characteristics leads to selection of the best character of choice for further breeding programs.

Data regarding petal overlapping are presented in Table 2. Results showed that among total cultivars studied, 32.5% of Korean cultivars had showed small overlapping of petals whereas, 16.7% exhibited large overlapping while 14.8% displayed valvate appearance (no overlapping). The exotic cultivars showed 14.8, 13.9 and 6.5% of small overlapping, large overlapping and valvate, respectively. Present morphological findings reported that H. syaircus petals can be grouped as narrow, intermediate and broad. 27.8% of the Korean cultivars had broad petals followed by intermediate and narrow type (Table 2). In exotic cultivars the maximum flowers had intermediate type of petals (17.7%) followed by broad type (44.7%).

Eye spot size is also considered one of the important parameter in the identification of flowering plants. Result regarding eye spot size depicted that among total cultivars studied, 49.2 of Korean cultivars possessed small eye spot on flowers while in exotic cultivars only 14.8% cultivars had large eye spot.

Commercial Hibiscus varieties were developed from the interspecific breeding of tropical and subtropical species that had caused the great variability in morphological and agronomical expression (Bruna et al. 2008).

In present study, large variability in morphological characteristics of H. syriacus between Korean and exotic cultivars was observed. Western plant customers/buyers are more conscious about plant shape, flower size, petal arrangements, eye spot color and size in Hibiscus, therefore such cultivars should be selected for breeding which contains these traits for better marketing. The morphological characteristics studied in this study could be useful to perform a more efficient Hibiscus breeding design in future.