Introduction

Phalaenopsis hybrids, or the moth orchid, which include Doritaenopsis production is now international trend for ornamental flower market. In 2007, the U. S. Department of Agriculture reported that 75% or more orchids among 15.4 million potted orchids sold are potted flowering Phalaenopsis, Doritaenopsis, and their related hybrids (Lopez et al. 2007). With increasing market demand, large scale production is carried out in Netherlands, Germany, China, Taiwan, The United States, and Japan (CBI 2014). Production quality including flower number, color, and longevity became more important in recent years in Phalaenopsis cultivation to meet consumer’s needs compared to year 2000. It is widely expected that sales will increase as production quality increases. Demand for high quality Phalaenopsis should continue well into the future as new scopes are increased.

Phalaenopsis have light-intensity and photoperiodic requirements. Wang (1995) reported that Phalaenopsis must be exposed to light above a certain level to respond to cool air to activate the reproductive bud. Night interruption (NI) can be a strategy to apply long-day (LD) condition with different light intensity for plants. NI has been used for accelerating flowering and/or improving flower of LD herbaceous or orchid plants including Dianthus (Park et al. 2013), Cyclamen (Oh et al. 2008), Coreopsis (Runkle et al. 1998), and Cymbidium (An et al. 2012, Kim et al. 2011). When NI applied to orchid, light intensity and timing of NI application could be determined time to flowering or flower quality. For example, in Cymbidium, growth was promoted by 16 weeks NI with 3 or 120 μmol • m^–1 • s^–1 during their vegetative growth stage, and flowering could be ultimately promoted (Kim et al. 2011). Photoperiodic response of Doritaenopsis is different according to cultivars (Blanchard and Runkle 2006). Flower bud initiation occurs after the reproductive spike has reached a certain length under the required environmental conditions.

Photoperiodic control during reproductive growth stage can be an important factor to increase flower quality since Doritaenopsis has long reproductive growth period, approximately 5-6 month after flower spike initiation. A major factor in determining production cost is flower quality. Increased number of inflorescences and flowers, inflo- rescence length increases production costs. Controlling of crop time also will help to ensure a profitable crop. To our knowledge, little or no study has been conducted on the NI effect in reproductive growth stage on flower quality in orchid plants. The present study aimed at examining the flower quality of four different Doritaenopsis species subjected to NI after their flower spike initiation.

Materials and Methods

Plant and growth conditions

Tissue culture propagated clones of 15-month-old Doritaenopsis ‘Fuller’s Sunset’, ‘Green Beer’, ‘Little Gem Stripes’, and ‘Queen Beer’ were transplanted into 12 cm pots filled with 100% sphagnum moss (Chilean sphagnum moss, Lonquen LTD., Pueto Montt, Chile). All plants were grown in a commercial greenhouse at Gobong Orchid Farm, Gobong, Korea. In all cultivars, the plants had 6-7 fully developed leaves and 1 or 2 inflorescences with 1 ± 0.25 cm length. The mean leaf width/leaf length in ‘Fuller’s Sunset’, ‘Green Beer’, ‘Little Gem Stripes’, and ‘Queen Beer’ were 18.0/7.0, 15.6/ 7.2, 21.4/9.3, and 18.8/7.5 cm, respectively. The plants were irrigated every week with a water-soluble fertilizer (EC 1.0 dS • m^–1; Hyponex professional 20N-20P-20K, Hyponex Japan Corp., LTD., Osaka, Japan). Average day/night temperatures inside the greenhouse were 19 ± 5/13 ± 2°C during the experiment from November, 2011 to April, 2012.

NI treatment

The plants were grown under 10 h natural light (control) or natural light plus NI using high-pressure sodium lamps (SKL- 01; GEO, Hwasung, Republic of Korea) from 22 : 00 to 02 : 00 HR with low light intensity (LNI) at 3-7 μmol • m^–1 • s^–1 or high light intensity (HNI) at 120 μmol • m^–1 • s^–1. The NI was employed for 21 weeks (November, 2011 to April 2012).

Data collection and analysis

The widest point of the leaf and longest leaf measured from base of the above ground were used to assess plant volume width and height, respectively. Dry weight of leaves, inflorescences, and roots were determined after drying in an oven at 80°C for 1 week after the NI treatments. Weeks to visible bud (VB) was measured from the start of the treatments, when the bud was 0.5 cm in length. The time at which the first floret was fully open was regarded as weeks to flowering. The number of VBs and flowers, flower diameter, flower height, and the inflorescence length above the medium were measured in each plant once every three weeks. Photographs were taken at 20, 21, 17, and 21 weeks after the treatment, respectively, in ‘Fuller’s Sunset’, ‘Green Beer’, ‘Little Gem Stripes’, and ‘Queen Beer’ when the plants were blooming for commercial market.

The experimental design was a completely randomized block with twelve replications of one plant each. Statistical analyses were performed using the SAS system for Windows V8 (SAS Inst. Inc., Cary, NC, USA). Differences among the treatment groups were assessed by Tukey’s honestly significant difference test at P < 0.05. Regression and graph module analysis were performed using Sigma Plot software (Systat Software Inc., Chicago, IL, USA).

Results and Discussion

Volume height was higher in the plants grown under LNI and HNI conditions than that of control in Doritaenopsis ‘Fuller’s Sunset’, ‘Green Beer’, and ‘Little Gem Stripes’ (Table 1). Photoperiod affects stem growth by modulating the rate of internode extension or the number of nodes in plants (Mattson and Erwin 2005; Thomas and Vince-Prue 1997). Similarly with this results, plants height and node number increased as photoperiod increased from 9 to 16 h in Dianthus, Zinnia, Pelargonium, and Tecoma (Torres and Lopez 2011; Park et al. 2013). In this study, no significant difference among NI treatments was observed for the dry weight of leaves, inflorescences, and roots (Table 1). When the light intensity was 2 μmol • m^–1 • s^–1 of day-extension lighting, no significant difference in biomass accumulation in Tecoma stans (Torres and Lopez 2011). Similarly in Dahlia, no significant difference in total plant fresh and dry weight was observed between LD and short day (SD) plugs, indicating that growth differences were a result of photoperiod alterations in carbohydrate partitioning (Legnari and Miller 1998). In the present study, NI applied in their reproductive growth stage not in vegetative stage, therefore NI lighting had a limited effect on the total carbon gain (Kjaer and Ottosen 2011).

Inflorescence length was significantly longer in the plants grown under LNI and HNI conditions than that under control in all cultivars except for ‘Little Gem Stripes’ (Fig. 1). There was no difference on inflorescence length of the plants grown under between LNI and HNI conditions. The number of inflorescences and florets were greater for the plants grown under HNI compared to those grown under LNI condition (Table 2). The number of florets was significantly (P < 0.01) greater in the plants grown under HNI condition in all cultivars. LD by NI at high light intensity can be prolonged duration of photosynthesis (Kim et al. 2013), thus plant development progress more under LNI or HNI conditions than under control condition. CAM plants including Doritaenopsis produce sugars as photosynthates. Sugar content in the leaf was lower than that in the flower stalk and flower, revealing a translocation gradient of total sugar form the leaf to the flower stalk (Endo and Ikushima 1997). There was no significant different in flower diameter and flower height among the NI treatments in all cultivars, except for flower height of the plant in D. ‘Fuller’s Sunset’ (Table 2). Doritaenopsis is well-known as thermosensitivity flowering plants, thus photoperiodic response has not been studied well. Some reports showed that photoperiodic response of Phalaenopsis is different according to cultivars (Blanchard and Runkle 2006).

During the floral inductive phase, plants can be induced to flower when they become sufficiently matures (Lopez et al. 2007). Plants did not flower until the plants reaches mature stage with minimum leaf number of pseudobulb size in orchid (Kim et al. 2011). In the present study, NI applied to the plants at their reproductive growth stage when the inflorescence has already been initiated, which is the stage the plants were already matured for flowering. For the plants grown under HNI condition, number of VBs and flowers increased more than those grown under control or LNI conditions (Table 2). Longdays provided by a 4-h NI reduced flowering time by 1 week compared with control in D. ‘Fuller’s Sunset’ (Table 2). Similar results that promoted flowering under NI compared with SD was shown in many herbaceous plants including Cyclamen and Dahlia (Legnari and Miller 2000; Oh et al. 2008). In orchid, duration from inflorescence initiation to flowering in Doritaenopsis, it generally takes more than 16 weeks, which is 4 times longer than herbaceous plants, but no study have been described the NI effect in the reproductive growth stage of orchid. When NI applied to the plants at their vegetative growth stage, flowering promoted by 1 year compared to the control which takes 3 years from mericlonal stage to flowering (Kim et al. 2011). Plants responded light treatment or light intensity more effectively when they were in young growth stage than mature growth stage (Jeong and Park 2012). Figure 2 shows effect of NI treatments on flowering quality. For those grown under HNI of the all cultivars, the number of VBs and flowers increased more than those in LNI and control. Selling price of Phalaenopsis is very much dependent on plants quality (An et al. 2013). For example, in Korea, potted flowering Doritaenopsis ‘Mantefon’ with 2 inflorescences and many number of florets are usually sold at a price 2 or 3 times more than the price of the plants with only 1 inflorescence.

In conclusion, effect of NI of Doritaenopsis cultivation was manifested not only in the initial and vegetative stages but also in the final reproductive stage. Flowering quality can be increased by NI as evidence by increased the number of flowers and inflorescence length. Economical method of NI for Doritaenopsis would be to expose the plant to exposed NI at night in their reproductive stage.