Introduction

Cymbidium is a popular floriculture plant in many countries. In Korea, it has a long flowering period of two to three months (from late October to early April) making it one of the most representative winter flowers (Kim et al. 2015). Cymbidium production area was 33 ha and sales amount 6.3 billion won in 2019 (MIFAFF 2018) and 51 pot plant cultivars were bred (Park et al. 2017) in Korea. Although Cymbidium was mainly produced in pot plants, some farms are switching their sales patterns to cut flowers as exports have declined. There has been also an increasing demand worldwide for Cymbidium cut flowers (CBI 2013). However, Cymbidium cut flowers were produced in the same cultivation method like pot plants in Korea.

It is necessary to establish an appropriate production system to produce and export high-quality Cymbidium cut flowers. Cymbidium pot plants are shipped out at the third year after vegetative growth of 2-3 years, but cut flowers are harvested at the third year and shipped out for several years from the same plant (An et al. 2016). It is important to maintain continuous high-quality during the cut flower production. Market quality of the cut flowers is determined by spike production, spike length, flower numbers. It is also necessary to identify the appropriate age to continuously produce of high-quality Cymbidium cut flowers (Bill and Bruce 2012). Most of the studies of Cymbidium are focused on pot plant production in Korea, therefore research is also needed for the production and export of cut flowers. Several studies were reported about cultivation and postharvest management of Cymbidium cut flowers (An et al. 2015;An et al. 2016;Kim et al. 2017;Lee et al. 2012).

According to Kim et al. (2011), minimum number of leaves and pseudobulb diameter were required for transition to the reproductive stage in Cymbidium. The increases of starch in leaves during vegetative growth and soluble sugars in pseudobulbs and roots during reproductive growth were significant for increasing plant growth and promoting flowering in C. ‘Red Fire’ (Kim et al. 2013). Therefore, appropriate nutrition would play an important role in Cymbidium considering the relationship of carbohydrate changes of leaf and pseudobulb and flowering. Nutrient supply of manipulated nutrient solution will be more effective for high quality spike production but there are not many studies about appropriate nutrient solution conditions. Sung et al. (1998) compared the effects of organic fertilizer and nutrient solution on Cymbidium young plants, and recommended nutrient supply with rockwool or peatmoss mixed media. Nurient solution of EC of 1.5 dS·m^-1 showed the highest spike production and stem length than lower EC treatments (Won et al. 1998). De Kreij and Van den Berg (1990) reported that higher EC (1.4 dS·m^-1) resulted in increased shoot production and spikes in C. Red Beauty ‘Del Rey’. More researches on various EC levels higher than 1.5 dS·m^-1 of nutrient solution supply are needed for improved spike production of Cymbidium.

The objectives of this study were to establish an appropriate production system to produce and export high-quality Cymbidium cut flowers and determine the appropriate electrical conductivity (EC) treatments of nutrient solution which gives the highest spike production and quality.

Materials and Methods

Plant materials and growth conditions

Three-year-old plants of Cymbidium ‘Lovely Smile’ plants, bred by the National Institute of Horticultural and Herbal Science (NIHHS), were transported to the environmentally controlled Information and Communication Technology (ICT) smart greenhouse at Seoul Women’s University from a commercial greenhouse in Gongju (25 May 2020). Plants were grown in a 15 cm × 21 cm (diameter × height) plastic pot filled with bark (Seungjin bark, Seungjin fertilizer Co., Korea). The maximum and minimum temperatures inside the greenhouse were 34.3/17.5℃ in summer (from June to September) and 26.5/7.1℃ in winter (from October to January), which were monitored by data logger (SH-VT250, Soha-Tech Co. Ltd., Seoul, Korea). The maximum and minimum humidity were 94/28% in summer and 96/23% in winter. The maximum photosynthetic photon flux range was 52.0 - 838.5 μmol·m^-2·s^-1 in summer and 30.0 - 399.5 μmol·m^-2·s^-1 in winter.

Electrical conductivity (EC) treatment of nutrient solution

Plants were treated with water soluble Peters fertilizer 20N-20P-20K (Scotts, Inc., Marysville, Ohio). The EC of nutrient solution was changed in three distinct growth stages: vegetative stage (from May to July), flower initiation stage (from August to November), and flower development stage (from November). The EC treatments were 1-0-1 (dS·m^-1, EC101), 1-1-1 (dS·m^-1, EC111), 2-1-2 (dS·m^-1, EC212), 2-2-2 (dS·m^-1, EC222), 3-2-3 (dS·m^-1, EC323), 3-3-3 (dS·m^-1, EC333) and the pH was adjusted to 6.0-6.5 (Table 1). Plants were irrigated with 1 L of fertilizer solution to each pot (three times per week).

Data collection

Each treatment was tested with 10 pots. The leaf length and width, number of pseudobulb per plant, pseudobulb diameter, number of new shoots and spikes were measured every month for 28 weeks after nutrient solution treatment. The longest leaf from the base of the pseudobulb was used to represent the leaf length. The pseudobulb diameter was measured at the widest point of the largest pseudobulb using a digital caliper.

Flower development period (flower bud and flowering stage) were recorded per flower spike. Total number of spikes were investigated per pot and pseudobulb. The stem length, stem thickness, number of flowers, days to flowering, and fresh weight were investigated at harvest of produced flower spikes (10 replicates). Spike length was determined from the base to the top of the flower and stem thickness was measured at the last basipetal node. The day at which the first three floret was fully open was regarded as flowering time and the days were calculated from the nutrient solution treatment to each flowering time. Flower diameter was measured with the average of three flowers per plant, from the margin of the left petal to that of the right petal. Petal color of the surface of three different petals from each plant was measured using a color meter (CR-10 Plus, Minolta Co., Tokyo, Japan) and recorded using the CIE (L*, a*, and b*) from uniform color space, in which the L* scale ranged from no reflection (L* = 0; black) to perfect diffuse reflection (L* = 100; white), the a* scale ranged from negative values for green to positive values for red, and the b* scale ranged from negative values for blue to positive values for yellow (Francis 1980). Flower diameter and petal color were measured of 5 replicates per treatment.

Statistical analysis

Statistical analysis were performed using SAS system for Windows version 9.4 (SAS Inst. Inc., Cary, NC, USA). Differences among treatment groups were assessed using Duncan’s multiple range tests. Graph module analyses were performed using Sigma Plot version 10.0 (Systat Software Inc., San Jose, CA, USA).

Results and Discussion

Plant growth affected by EC changes in nutrient solution of Cymbidium

Leaf length and width, number of pseudobulb showed no significant differences according to the EC treatments at the end of the vegetative stage (Table 2). However, number of leaves decreased and pseudobulb diameter increased in the highest EC treatment (3-3-3). Leaf length increased in EC maintained treatments with no significant differences (EC111, EC222, and EC333). Pseudobulb diameter also increased in EC maintained Treatments.

Pseudobulb diameter increased in the plants treated with EC 1.0 dS･m^-1 (EC101 and EC111) compared to the values of vegetative stage at the reproductive stage 28 weeks after nutrient solution treatment (Table 3). But pseudobulb diameter of the plants treated with EC 2.0 - 3.0 dS･m^-1 remained similar compared to the values of vegetative stage. Arnold and Van den Berg (1982) reported that nitrogen promoted vegetative growth but reduced the number of spikes per shoot. Good vegetative growth is needed but excessive nutrition inhibited reproductive growth in treatments of EC 2.0 - 3.0 dS･m^-1 in this experiment. Leaf length and width, number of pseudobulb, number of spike showed no significant differences according to the EC treatments. Leaf length increased in EC maintained treatments with no significant differences (EC111, EC222, and EC333). Plants began to produce spikes from September. Number of spike increased in EC maintained treatments with no significant differences (EC111, EC222, and EC333).

Flower spike production affected by EC changes in nutrient solution of Cymbidium

Flower spike production per pot and pseudobulb showed the highest values in the plants treated with EC111 of 3.3 and 1.4 but there was no significant difference according to the treatments (Fig. 1). Nutrient solution of EC111 can be recommended to improve flower spike production of Cymbidium. Flower spike production per pot and pseudobulb increased in EC maintained treatments (EC111, EC222, and EC333). High level during vegetative growth and omitting fertilizer during generative bud emergence gave the highest yield in other studies (Arnold and Van den Berg 1983). But production of spikes in the level of EC 1.0 dS·m^-1 showed increased values in maintained EC treatments in our experiment.

Flower spikes of the plants treated with EC 1.0 dS·m^-1 showed the highest values in stem length, stem thickness, and fresh weight of spikes (Fig. 2). Flower spikes of the plants treated with EC 3.0 dS·m^-1 showed the worst spike quality. Stem length decreased in EC maintained treatments (EC111, EC222, and EC333). According to De Kreij et al. (1990), a higher EC (1.4 dS·m^-1) gave a greater spike production per m², a lower spike production per shoot, a shorter spike and more flowers per m² than lower EC (0.6 and 1.0 dS·m^-1). The plants treated with EC 1.0 dS·m^-1 also showed the highest values in number of flowers and flowered more earlier than other treatments (Fig. 3). Nutrient supply of EC 1.0 dS·m^-1 produced more flower spikes with increased plant characteristics, but higher or maintained EC treatments decreased stem length like the previous study.

Flower development period (flower bud and flowering stage) treated with treatments of EC101, 111 and 212 took place 1 - 2 weeks earlier than the other treatments (Fig. 4). Flower buds developed mainly 22 - 26 weeks after nutrient solution treatment with treatments of EC101, 111, and 212 and that of EC222, 323, and 333 developed 24 - 26 weeks. Flowers developed mainly 30 - 33 weeks after nutrient solution treatment with treatments of EC101, 111, and 212 and that of EC 222, 323, and 333 developed 31 - 33 weeks. Production of total spikes were 18 (EC101), 22 (EC111), 13 (EC212), 13 (EC222), 10 (EC323), 10 (EC333) at the end of the experiment for 10 pots (data not shown). Omitting fertilizer at the stage of generative bud emergence resulted in earlier flowering and harvest compared with continuous fertilization in other studies (Arnold and Van den Berg 1984; Van Os and Van der Wurff 1988a). There was only difference in flower development period according to the EC levels, and fertilization methods of EC treatments showed no distinguished phenomenon in this experiment.

Flower diameter showed the highest values in treatments of EC 1.0 dS･m^-1 and flower color showed higher L* and b* values and lower a* values in treatments with EC 3.0 dS･m^-1 compared to them with EC 1.0 - 2.0 dS･m^-1 (Table 5). Coloration is a important factor determined the quality of cut flowers. Especially high a* values appeared in vivid reddish coloration and total anthocyanin content in Rosa flowers with red or pink petals (Biolley and Jay 1993; Schmitzer et al. 2009). Red coloration increased more under EC 1.0 - 2.0 dS･m^-1 treatments represented by higher a* values. Flower diameter decreased in EC maintained treatments (EC111, EC222, and EC333) and b* values showed an increasing tendency. Higher and maintained EC treatments decreased the flower quality in this experiment.

Nutrient solution of EC 1.0 dS·m^-1 can be recommended to improve plant growth and flowering and EC111 was the most appropriate treatment considered the spike production of Cymbidium. EC maintained treatments increased spike production but spike quality characteristics associated with stem length, flower diameter, flower color were decreased. Most studies of Cymbidium about the EC levels of nutrient solution supply of were performed in the level of 0 - 1.5 dS･m^-1, and 1.5 dS･m^-1 were mainly recommended for improved plant growth and spike production (Van den Berg 1990; Won et al. 1998). Thus we conducted an experiment to determine the effectiveness of higher EC level but there were no increasement on spike production in higher EC levels. Treatments with omitted (April - July) nutrient supply of 2.5 dS·m^-1 were reported for higher spike production in previous study of Van Os and Van der Wurff (1988b). Excessive nutrition inhibited reproductive growth in treatments of EC 2.0 - 3.0 dS･m^-1 in this experiment, and therefore more research will be needed for the relation between EC level and supply period of the nutrient solution in Cymbidium.