Introduction
Caryophyllaceae is one of the major families in dicots, including some horticulturally important genera, such as Dianthus and Gypsophila. Many commercially popular species belong to this family, and these plants account for a major portion of flower consumption as cut flowers or pot plants (Chandra and Rawat 2015;Fukai et al. 1991). Eremogone juncea (M. Bieb.) Fenzl, a Korean native plant, is also included in this family and has been considered a new ornamental plant for use as a cut flower, pot plant, or garden plant. This plant is a perennial herbaceous crop and is distributed throughout Korea, China, Mongolia, and the Far East of Russia. It grows wild in the mountainous regions of central and northern Korea and has the strong cold hardiness; thus, it has the advantage of being able to overwinter anywhere in the Korean Peninsula (KBIS 2022). White flowers of this plant bloom from June through mid-August. Although it is a dicot plant, its long and thin radical leaves make it possible to produce sedge-like leaves, and the abundant white florets in summer highlight its potential value as an ornamental plant.
Plant hormones as plant growth regulators (PGRs) can be used to improve the flower quality and yield of herbaceous ornamentals. GAs are the most common plant hormones used for horticultural purposes, and the hormonal effects include the promotion of elongation growth, dormancy breaking, seed germination, and the induction of flowering (Hatamzadeh et al. 2010). GAs are mainly used to promote flowering and stem elongation of cut flowers and to produce more flowering shoots (Rademacher 2015). Cytokinin (CK) is also an important hormone known to stimulate cell division, delay leaf senescence, and induce floral transition (Bernier 2013). Some studies have reported that CK regulates floral genes and the flowering process in Arabidopsis (Bartrina et al. 2011;Bernier 2013;D’ Aloia et al. 2011). BAP, which is one of the synthetic CKs, has been commonly used to improve flower yield along with other plant growth regulators (Gaj 2004).
Some studies have introduced E. juncea as a new ornamental plant for commercial cultivation. Kang et al. (2022) observed the flowering responses to environmental factors such as photoperiods and chilling temperature and conducted a study on cold storage for year-round cultivation of potted plants. For the purpose of using E. juncea as a ground cover plant in the garden, the soil moisture content was investigated to suggest appropriate irrigation conditions in the open field (Gil et al. 2020). Regarding the vase life of E. juncea, research was conducted on how the temperature and the type of holding solutions affected the vase life of cut flowers (Kwon et al. 2020).
To our knowledge, there has been no study on hormone application to E. juncea; thus, we examined the exogenous hormonal effects on the horticultural characteristics of this species. We focused on the value of E. juncea as a cut flower, especially as a filler flower. In some previous studies, it has been reported that BAP and GA promoted floral development and increased flower yield per plant in some varieties of Gypsophila paniculata (Davies et al. 1996;Doi et al. 1989;Li et al. 2019). BAP application could increase the number of flowers and yield of saffron (Javid et al. 2022). Therefore, the objective of this study was to investigate the growth and flowering characteristics of E. juncea after exogenous BAP and GA applications to improve its ornamental value as a cut flower.
Materials and Methods
Plant Materials and Growth Conditions
Four-year-old plants of Eremogone juncea that received natural low temperatures during the winter in an open field at Seoul National University Farm (Suwon, Korea; 37°27'N, 126°99'E) were used in this experiment. On March 16, 2021, plants that started sprouting were transplanted into 12 cm top diameter plastic pots filled with commercial peat-perlite medium (Baroker; Seoul Bio Co., Ltd., Eumseong, Korea) and transferred to environment-controlled growth chambers (HB-301MP, Hanbaek Scientific Co., Bucheon, Korea).
The environmental conditions for the growth chambers were as follows: temperature at 22 ℃, 60% relative humidity, 200 μmol·m-2·s-1 light intensity with 250 W metal halide lamps (Han Young Electrics Co., Gwangju, Korea), and 12/12 h of photoperiod (day/night). The plants were watered with tap water as necessary and fertigated once a week with a water-soluble fertilizer (EC 0.8 dS·m-1; HYPONeX professional 20N-20P-20K; HYPONeX Japan Co., Ltd., Osaka, Japan).
Exogenous BAP and GA Treatments
Two types of hormones, BAP (6-benzylaminopurine; Duchefa Biochemie, Haarlem, The Netherlands) and GA (gibberellic acid3; Duchefa Biochemie, Haarlem, The Netherlands), were used to investigate the effect of exogenous hormone application on the growth and flowering characteristics. Each hormone was dissolved in a 4 mL solution of 100% ethanol and 0.1 N NaOH and then diluted with distilled water with 0.1% Tween-20 (P1379, Sigma–Aldrich Korea Ltd., Yongin, Korea). There were seven treatment groups: control (no hormones), 200 mg·L-1 BAP, 400 mg·L-1 BAP, 800 mg·L-1 BAP, 200 mg·L-1 GA, 400 mg·L-1 GA, and 800 mg·L-1 GA. Nine plants per treatment were used, and 5 mL of diluted hormone solution per plant was sprayed twice at one-week intervals. After transplanting, the first treatment was performed 6 days after transplanting (March 22, 2021) and a total of 10 mL per plant was sprayed. The second treatment was carried out 44 days after transplanting (April 29, 2021), when floral buds were induced, and a total of 10 mL per plant was sprayed.
Data Collection and Statistical Analysis
Days to flowering were counted from March 16, 2021, after the plants were transplanted. For floret diameter, six florets per plant and a total of 54 florets per treatment were measured. At 15 weeks after transplanting, the number of inflorescences and the length of the inflorescences and leaves were measured. Photos were taken 13 weeks after transplanting. Statistical analysis was performed using ANOVA (analysis of variance) in SAS (Version 9.4; SAS Institute Inc., Cary, NC, USA). A comparison of parameters among the treatment groups was performed by Duncan’s multiple range test (p ≤ 0.05). All graphs were drawn using SigmaPlot (version 10.0; Systat Software, Inc., Chicago, IL, USA).
Results and Discussion
BAP and GA are known to stimulate floral initiation in some plant species. In Gypsophila paniculata, GA treatment decreased the time from planting to flowering (Hwang et al. 2003) and could effectively stimulate early flowering (Li et al. 2019). BAP application promoted floral initiation of Gypsophila paniculata (Doi et al. 1989) and stimulated the flowering of Pharbitis nil (Galoch et al. 1996). In this study, the percent flowering was 100% in all treatment groups, and Eremogone juncea treated with BAP and GA did not show a significant difference in days to flowering (Fig. 1). Likewise, there were no significant differences in the number of days to bolting and floral bud initiation (data not shown). The promotion of flowering initiation has been commonly observed in long-day plants (Galoch et al. 1996;Hwang et al. 2003). GA has an accelerating effect on photoperiodic floral initiation (Garner and Armitage 1996). CKs also showed a promoting effect on flowering under subthreshold photoperiod conditions (Galoch et al. 1996). Therefore, an early flowering response to BAP and GA treatments could be observed in some long-day plants (Davies et al. 1996;Doi et al. 1989;Li et al. 2019). Kang et al. (2022) reported that, E. juncea showed no photoperiodic responses. Thus, it could be assumed that these hormone treatments did not have a promoting effect on floral induction of E. juncea in this study.
Although the hormone treatments did not induce significant changes in the number of inflorescences, exogenous BAP and GA influenced the flower development of E. juncea. When treated with BAP, the floret diameter decreased compared with that of the control (Figs. 2 and 3). The inflorescence length was also shortened compared to the control when treated with BAP, and it showed a tendency to be shorter as the concentration increased (Figs. 2 and 4). In general, it is well known that CK promotes cell division and increases the number of cells to make floral organs larger (Rademacher 2015). Nishijima et al. (2006) showed that when three types of CK, including BAP, were applied, the number of cells increased, and the flower size of Petunia hybrida increased accordingly. In Arabidopsis, mutations in the genes for two CK degrading enzymes resulted in larger floral organs because of the increased number of cells (Bartrina et al. 2011).
However, in this study, plants treated with BAP showed a negative effect of decreasing the floret diameter and inflorescence length compared to the control (Fig. 2). BAP treatment makes the flower smaller and shortens the plant height. When some cultivars of Ranunculus asiaticus were treated with BAP, they showed a decrease in plant height (Kwak et al. 2018). In some cultivars of Schlumbergera truncata, the flower length and width tended to decrease as the concentration of BAP treatment increased (Lee et al. 2020). Although branching was promoted and the number of flower buds increased with increasing concentrations from 50 to 300 mg·L-1 of BAP, exogenous BAP treatment inhibited elongation growth in the stem and flowers of Schlumbergera truncata (Lee et al. 2020). Davies et al. (1996) reported that in Gypsophila paniculata, flower quality decreased since BAP treatment caused the formation of thin and weak pedicel branches, and the intertwining of the pedicels made the flowers unmarketable. In this study, the internode elongation of the flower stalk was also inhibited in the BAP treatments, and more branches of inflorescences were observed compared to the control and GA treatments (Fig. 3). Even if the number of flowers increased as the branching of inflorescences was promoted, it can be considered that BAP treatment is not suitable in E. juncea because its ornamental value decreases as the small floret diameter, short inflorescence length and ununiformed flower stalks, as shown in Figs. 3 and 4.
In contrast, GA treatment resulted in a floret diameter larger than that in the control group, but there was no significant difference in floret diameter between the control and the high-concentration 800 mg·L-1 groups (Figs. 2 and 3). All GA treatments increased the inflorescence length compared to the control, but there was no significant difference among the GA treatment groups (Figs. 2 and 4). Although the highest GA concentration (800 mg·L-1) significantly increased the leaf length compared to the control, it was difficult to detect any remarkable difference visually because the leaf drooped (Figs. 2 and 4). The responses to GA in floret diameter, inflorescence length, and leaf length may be attributed to the general effect of GA in stimulating cell elongation, enlargement, or both. Many studies on floricultural crops have shown the effectiveness of GA in promoting plant height, stalk length, or leaf length (Cocozza Talia and Caputo 1980;Hwang et al. 2003;Kwak et al. 2018;Li et al. 2019;Rebers et al. 1994;Saniewski et al. 1999). GA application was also reported to increase flower size or diameter in Exacum affine and R. asiaticus (Kwak et al. 2018;Neumaier et al. 1987).
E. juncea treated with BAP showed abnormal flowers in which stamens did not develop normally (Fig. 5). The abnormal flowers had a larger gynoecium than the normal flowers observed in the control group. The stamen of the abnormal flowers had a very short filament, and there was a very small anther, and almost no pollen was produced (Fig. 5B). In Arabidopsis, it was reported that exogenous BAP treatment affected stamen development, causing a decrease in filament length and pollen grain production (Marsch-Martínez et al. 2012). Exogenous BAP treatment of Brassica napus also inhibited stamen filament elongation and anther maturation and caused conspicuous overgrowth of tissue in the gynoecia (Zuñiga-Mayo et al. 2018). Since the flowers malformed by BAP treatment do not exhibit the ornamental features of E. juncea, in which purple or yellow anthers are spread on white petals, it would be better to avoid BAP treatment.
In conclusion, exogenous BAP or GA treatment did not appear to have any effect on accelerating flowering induction of E. juncea, while it could be seen that the difference in floret diameter and inflorescence length by BAP or GA treatment considerably affected the ornamental value. BAP treatment could be considered undesirable because it reduced the flower size and caused abnormal flowers. GA treatment could increase the flower size, thereby enhancing the ornamental value. Therefore, GA treatment is recommended for cultivating E. juncea as a cut flower.
