Introduction

Hibiscus belongs to the Malvaceae family and is a polymorphic genus consisting of 200-250 species of trees, shrubs, and herbs with wide distribution in tropical and subtropical regions (Bailey, 1950; Bates, 1965; Beers and Howie, 1992). A few species, including Hibiscus syriacus L., Hibiscus sinosyriacus Bailey and Hibiscus paramutabilis Bailey, are naturally found in temperate regions of the world. According to Bates (1965) these three species are native to China and similar pattern of natural distribution is an indication of similar tolerance to environmental factors. H. syriacus (althea or rose of Sharon), a deciduous shrub with more or less distinct trilobite leaves, is the most highly used species in

breeding programs. The color of the solitary flowers in the axils on the young wood varies from white, light pink or red to purple and blue and with a large crimson splash at the base of the petals. Single, semi-double, and double blooms are observed (Yu and Yeam, 1987; Kim and Lee, 1991) and flowering occurs from the end of July till the first part of October (Bailey, 1950; Bean, 1973).

H. syriacus is the most popular species, and about 100 different cultivars of varying flower color and shape are commonly cultivated in Europe, with more genotypes being present on different collections (Van De Laar, 1997). Furthermore, large collections of Hibiscus plants exist in Korea; given that Hibiscus is the national flower (Yu et al., 1976; Yu and Yeam, 1987; Van de Laar, 1997; Bae et al., 2015). The diversity of Hibiscus species cultivated in Korea has been enhanced by cross breeding of cultivars and numerous cycles of interspecific cross breeding (Kim, 2016; Tachibana, 1958). Hibiscus hybrid ‘Daewangchun’, ‘Daeil’, ‘Lohengrin’, ‘Yeonam’, ‘Joomong’ and ‘Jina’, and H. syriacus ‘Honggarosu’ were selected as street trees because of their vigorous shoot growth and big tree size. In contrast, H. syriacus ‘Tamla’, ‘Melrose’, ‘Bidan’, ‘Hi Lea’, ‘Byeollee’, ‘Byunghwa’, ‘Mibaek’, ‘Hanyang’, ‘Chungam’, ‘Lil Kim Violet’, ‘Jongmoo’ and ‘Eunhasu’, Hibiscus hybrids ‘Saehanseo’ and ‘Yousoon’ have smaller size making them suitable for pot planting (Kim, 2016; Shim and Ha, 2010). Hibiscus cultivars also differ in their flower characteristics. ‘Daewangchun’, ‘Daemang’, and ‘Jina’ have large flowers, while H. syriacus ‘Ggoma’, ‘Mibaek’, ‘Andong’, ‘Lil Kim’, and ‘Eunhasu’ have flowers of smaller size. The flowers of ‘Hunjang’ and ‘Hi Lea Red’ cultivars were additionally decorated with long red eyes. The recently developed cultivars, namely ‘Ggoma’, ‘Mibaek’, ‘Andong’, ‘Lil Kim’, and ‘Eunhasu’, have small flowers making them suitable to be used as woody landscape plants (Kim, 2016; Shim and Ha, 2010). These breeding programs of Hibiscus cultivars focused on the selection of large flowers with unique and a desirable flower characteristics such as size, color intensity and pattern.

Our Hibiscus breeding program was initiated in 2008 aiming to develop new Hibiscus cultivars with a uniform growth, upright and compact branches, and unique flowers with long red eyes through crossings.

Materials and Methods

Plant Materials

The breeding program, which produced Hibiscus ‘Mikyung’, spanned the period of 2008-2014. The objective of the breeding program was to develop new Hibiscus cultivars with a compact growth habit and dense branches via specific hybridization. The new Hibiscus cultivar was developed via cross-pollination of H. syriacus ‘Andong’, used as seed parent, with H. syriacus ‘Namwon’, used as pollen parent. ‘Andong II’ was used as the control plant. A total of 70 flowers were pollinated with 50% of them resulting in capsules, which contained 6.0 seeds per capsule on average. Among the germinated seedlings, 57 were planted in the greenhouse and bloomed in 2011.

Cultivation Method

The new Hibiscus plant was discovered and selected by the inventor in August 2011 as a flowering plant within the progeny of the stated cross-pollination in a controlled outdoor nursery environment in the Research Institute of Rose of Sharon and Tiger Lily, Cheonan, Korea. It was selected for testing and propagation for the size and color of flowers, flower longevity, number of seed pods produced, strength of upright stems and the landscape value of the plant. Asexual reproduction of the new Hibiscus ‘WR-99’ by hard wood, semi hardwood and softwood cuttings was used in a controlled environment since spring of 2011 to determine the stability of the unique features of the new Hibiscus and to reproduce true-to-type progenies (Fig. 1).

Fig. 1. Pedigree diagram of a newly-bred F₁ hybrid ‘Mikyung’.

Investigation of Characteristics

After 4-years (2011-2014) of evaluation of plant characteristics such as growth, leaf shape, leaf size, flower characteristics and flowering output, the new cultivar was named ‘Mikyung’. The characteristics were evaluated based on the manual for agricultural investigation and according to the guidelines defining uniqueness, uniformity and stability of Hibiscus plants (UPOV, 2005).

Results and Discussion

Plants of ‘Mikyung’ had uniform growth with compact, upright and stiff branches and abundant production of white (RHS NN150C) (Royal Horticultural Society Flower Council of Holland, 2001) flowers of moderate sizes (10.6 cm diameter on the average) decorated with red (RHS 60A) eye spots. One-year-old grafted plants were 43 cm in height and 27 cm in crown width while they grew up to 91.7 cm in height and 52.5 cm in crown width after three years. Plant height of this cultivar was taller than the seed parent and the control plant, while it was about half the size of pollen parent plant ‘Namwon’ (Table 1). ‘Mikyung’ was more vigorous as compared to H. syriacus ‘Andong’ and ‘Andong II’. H. syriacus ‘Andong II’, commonly sold under the trademark name of ‘Lil Kim’, is a miniature cultivar which was awarded an U. S. Plant Patent PP#19,547 on 2 December, 2008.

Fig. 2 shows the overall appearance of the new Hibiscus cultivar with the color as true as it was reasonably possible to obtain in colored reproductions of this type. The ‘Mikyung’ cultivar was very fertile with abundant seed pods produced after insect pollination.

Table 1. Growth characteristics of a new cultivar ‘Mikyung’ produced by an intraspecific hybridization

zMean ± standard deviation (n=5).

Fig. 2. Flower of the new Hibiscus cultivar ‘Mikyung’ with a semi-dwarf habit and dainty white flowers.

Table 2. Leaf characteristics of the new Hibiscus cultivar ‘Mikyung’ produced by an intraspecific hybridization

zMean ± standard error (n=10).

Table 3. Flower characteristics of a new cultivar ‘Mikyung’ produced by an intraspecific hybridization

zUPOV TG/ (I-a, Single flower and not overlap; I-b, single flower and slightly overlap; and I-c, single flower and overlap). yUPOV TG/ (I-type, spatulate; II-type, Fan; and III-type, Spoon).

Leaves were 6.3 cm long and 3.7 cm wide of medium green color. They were alternate and had an oval shape with acute leaf apex and base. Mature plants produced leaves with leaf shoulder diameter of 4.6 cm and petiole diameter of 0.8 cm. Leaf margins were irregularly dentate with medium incisions and weak undulation (Table 2). Among the Hibiscus cultivars, ‘Joomong’ produces the largest leaves, 13.1 cm in length, followed by H. syriacus ‘Moonwon’, ‘Minerva’, ‘Samchulli’, ‘Reddish Ball’, and Hibiscus hybrid ‘Daeil’, which produce leaves of 10.0 cm. Cultivars with smaller leaf sizes (> 5.0 cm) were Hibiscus hybrid ‘Arirang’, Hibiscus hybrid ‘Daemang’, and H. syriacus ‘Andong’ (Kim, 2016).

H. syriacus ‘Mikyung’ produced large flowers, 10.6 cm in diameter, compared to that of the seed parent ‘Andong’ (6.2 cm) and ‘Andong II’ (9.0 cm). The corolla composed of five slightly fan-typed petals, which were 5.6 cm long and 4.3 cm wide at the widest point. Petals were white (NN155C) with eye spots being dark red (60A) and approximately 2.9 cm in diameter (Table 3). Kim (2016) assessed 127 Hibiscus cultivars and reported that Hibiscus hybrid ‘Daewangchun’ had the largest flower with 16.0 cm in diameter, while H. syriacus ‘Ggoma’, ‘Mibeak’, ‘Andong’, ‘Andong II’ and ‘Eunhasu’ had the smallest flower with a size of 6.2 cm.

Upon anthesis in early morning the flower petals reflexed, gradually returned to nearly right angles to the central axis and remained in this position throughout the flowering stage (Table 4). Flowering period of ‘Mikyung’ was 34 h, from 4:00 am on 1^st July 2016 till to 2:00 pm on the following day. New cultivar, ‘Mikyung’ started flowering each year in early July and continued for about four months depending on the surrounding environmental conditions, and the plants produced sporadic flowering until frost (Table 5). The stiff, upright main stems of H. syriacus ‘Mikyung’ were derived from the pollen parent plant, H. syriacus ‘Namwon’, whereas its flower color was same as that of seed parent plant, H. syriacus ‘Andong’. The flower size of H. syriacus ‘Mikyung’ was larger than those of its seed parent and control plants. However, its flowers were similar to those of H. syriacus ‘Andong’ and ‘Andong II’ but with more expanded petals. The leaves of ‘Mikyung’ resembled those of H. syriacus ‘Namwon’ regarding the leaf shape. The ‘Mikyung’ cultivar differed from the parent plants primarily in growth habit, flower size and length of red eye spot as its progeny plants were more vigorous than seed parent and the control ‘Andong II’ plants. Unlike larger Hibiscus plants, ‘Mikyung’ is compact flowering shrub and attractable for use in landscapes such as foundations, borders and around decks and patios.

Table 4. Flower size of the new Hibiscus cultivar ‘Mikyung’ produced by an intraspecific hybridization

zMean ± standard error (n=10).

Table 5. Flowering characteristics of the new Hibiscus cultivar ‘Mikyung’ by intraspecific hybridization in 2016

zAll cultivars were cultivated in Cheonan, Chungcheongnam-do province.

The new Hibiscus syriacus ‘Mikyung’ plant grows on well-drained soil, at medium moisture and sunny to partially shaded places, with flowering being superior when exposed to full sun. Although the ‘Mikyung’ cultivar grew better on rich organic soils, it also tolerated soil conditions of low nutrient content and low water availability. It exhibited high tolerance to the summer heat and high humidity conditions. The ‘Mikyung’ can be pruned in shape, and pruning back to 2-3 buds in late winter may produce larger blooms. Benefits are expected from the propagation via stem cutting, which produce dainty flower color as the parental lines. For cultivar protection, ‘Mikyung’ was applied at the Korea Seed and Variety Service (Application no. 2016-105) and the plant variety protection rights registered as grant number 6332 on 27^th October 2016. Additional information on research purposes and the list of nurseries associated with the propagation of ‘Mikyung’ are accessible at the Research Institute of Rose of Sharon and Tiger Lily (RIRS & TL), Cheonan, Korea (e-mail: shimkk@chol.com). The cultivar has already been released by RIRS & TL, Cheonan, Korea and sold as commercial variety at the domestic market. It is requested that appropriate recognition must be made if the breeding line contributes towards research or development of new germplasm, breeding lines or other cultivars.