Modified compact fluorescent lamps improve light‐induced off‐season floral stimulation in dragon fruit farming

Abstract Efficient light‐induced floral stimulation plays a key role in energy conservation and maintaining stable productivity during off‐season periods of dragon fruit plants. In this study, we first reported on results of a survey on dragon fruit farmers regarding use of lamps in performing artificially induced flowering process in Vietnam. It was found that the use of incandescent lamp was prevalent in dragon fruit cultivation practices, resulting in heavy electricity consumption, and that low‐power compact fluorescent light (CFL) bulbs were not extensively utilized, possibly due to low floral induction performance of domestic CFL bulbs. Arguing that emission spectra of currently used lamps were not consistent with adsorption spectra of phytochromes, whose transformation is responsible for flowering process of dragon fruit, we then proposed three improved CFL lamps (power capacity of 20 W) having emission spectra focused on red and far‐red regions. New lamp prototypes were tested in 7 field experiments in three different provinces in Vietnam. One improved CFL bulb (treatment 2) performed relatively well in comparison with the incandescent control lamp (60 W) in six out of seven experiments with regard to some growth indicators (e.g., number of floral stems, number of bubs, number of fruits per plant) and fruit yield. Recent success on commercialization of the improved CFL lamp demonstrates the potential of CFL lamps in floral stimulating irradiation of other crops and plants and in alleviating electricity burden in dragon fruit growing areas.


| INTRODUC TI ON
Vietnam has been one of the major exporters of dragon fruit (pitaya) in the world, possessing about 35 000 ha of total commercial culti- In 2017, export turnover of twelve spearhead exporting fruits from Vietnam contributed around 2.6 USD billion to the national net exports. Of which, dragon fruit accounted for 44.23%, which is 1.15 USD billion and is expected to grow exponentially in the coming years due to growing interest in the fruit in the global market (Nguyen, 2020;Vietnam General Department of Customs, 2017).
Dragon fruit is a photoperiodic plant that optimally blooms in the geographic condition in which daytime is longer than nighttime (Su, 2005). As such, in Vietnam, main cropping season of dragon fruit spans from March to September in the following year and cultivation in the remaining period of the year should be aided with artificial irradiation during nighttime. Off-season floral stimulation of dragon fruit is especially important for farmers because Lunar New Year, which is the most important holiday in Vietnamese culture and has greatly increased demand for fruit products, often falls in the end of the off-season period. This causes the off-season harvest to be the major income source of dragon fruit farmers and necessitates the use of lamps to stimulate off-season flowering. Currently, highpower incandescent lamps with capacity ranging from 75 W to 100 W are widely used to provide irradiation for 4-6 hr per night to dragon fruits in Taiwan (Tran et al., 2015;Yen & Chang, 1997) and Thailand (Saradhuldhat et al., 2009(Saradhuldhat et al., , 2016. Incandescent light bulbs with capacity ranging from 75 W to 100 W are also recommended by Vietnamese Ministry of Agriculture and Rural Development (MARD) to provide around 10 hr of artificial lighting per night (Hoa et al., 2008;Ministry of Agriculture & Rural Development of Vietnam, 2006;Truong, 1999).
To our knowledge, there have been four studies that investigated the effect of irradiation on flowering of dragon fruit plant. The study of Khaimov and Mizrahi (2006) attempted the manipulation of flowering pattern of pitaya (Hylocereus undatus) and yellow pitaya (Selenicereus megalanthus) via a number of interventions including photoperiodic lighting, shading, flower thinning, and application of growth regulators. It was concluded that even 9 hr of lighting extension after sunset induced no effect on flowering, possibly due to the subtropical climate in the study area (Israel). This explanation was further confirmed by Jiang et al. (2012) where temperature-sensitive response to night-breaking fluorescent lighting (28 W) was found in red pitaya cultivated in Taiwan. However, light-stimulated fruit production during off-season period was lower than that in the inductive period and it was unclear whether the use of incandescent lamps or fluorescent lamps at higher capacity could improve off-season pitaya production. Similarly, Saradhuldhat et al. (2009) Tran et al. (2015) indicated that the response to artificial lighting might also vary depending on the species, regional cultivar, and flesh color. Most notably, the Vietnam origin white-flesh pitaya cultivar was unable to flower under 4-hr night-breaking irradiation with incandescent bulbs (100 W).
In general, the aforementioned studies was unable to show the flowering response of pitaya to different lighting conditions (e.g., lamp type, power capacity, and emission spectrum) and was conducted in different regions having varying temperature. As a result, obtained results are contradictory and carry little implication when being implemented in regions with different climatic conditions. Driven by such shortcomings and the high electricity consumption in areas with intensive dragon fruit cultivation in Vietnam, this study aims to rationalize a low-capacity compact fluorescent lighting (CFL) lamp that is specifically designed for controlling off-season flowering of dragon fruit. We first presented the results on the current measures for controlling off-season dragon fruit flowering in three typical areas in Vietnam, namely Tien Giang, Tay Ninh, and Binh Thuan Province. Then, some CFL configurations were proposed and analyzed for their light spectra. Lastly, the selected CFL lamps were then used in experiments carried out in these areas to assess their stimulation effectiveness on some growth indicators of dragon fruits of both red-flesh and white-flesh varieties. Current results are expected to contribute to alleviate electricity burden for agricultural activities in those areas and aid in further development of CFL lamps specialized for fruit irradiation.

| Survey of lamp use for off-season blooming control of dragon fruits
A questionnaire was designed aiming at collecting relevant information on dragon fruit farming and techniques for flowering stimulation from farmers in Vietnam. Some typical categories include type of lamp, irradiation process, irradiation duration, and bulb density per hectare. A total of 100 respondents were selected from the farmer lists prepared by the local authorities in three provinces, including Binh Thuan (50 households), Tien Giang (25 households), and Tay Ninh (25 households) according to probabilistic randomized sampling procedure. We also collected lamp samples that were being used for floral stimulation from local farmers for further analysis.

| Lamp spectrum analysis
A PG100N Handheld Spectral PAR Meter (UPRtek, Miaoli, Taiwan) connected to a computer was used to measure emission spectra and determine photosynthetically active photon flux density (PPFD), photon flux density in the red region (PFD R, 600-700 nm), and photon flux density in the far-red region (PFD FR, 700-780 nm) of lamps.
The testing lamp was positioned 30 cm in front of the instrument sensor in the dark. The procedure was carried out at Laboratory of Agricultural Biotechnology, Nguyen Tat Thanh University, Vietnam.

| Field experiments
Based on the survey results and spectrum analysis, we designed new CFL bulbs with spectra focused in the red and far-red regions.
The light bulbs were manufactured by Rang Dong Light Source & Vacuum Flask Joint Stock Company (Vietnam) and were screened before being used in field experiment.
In the field experiment, three experimental CFL bulbs (20 W) selected from the spectra analysis results were used as treatments (denoted from 1 to 3) and one commercial incandescent lamp (60 W) acted as the control bulb (denoted as control). No lighting and domestic CFL lighting were not included as controls due to zero or very low productivity resulted from the adoption of the two options. In addition, Yamada et al. (2008) also maintained that the use of domestic CFL lighting gave longer budding time and worse flowering efficiency.
The subject plants were mature white-flesh pitaya (Hylocereus undatus) and red-flesh pitaya (Hylocereus polyrhizus) that were normally bearing fruits with the age ranging from 5 to 8 years. The experiment was carried out in household fields located in three provinces with largest dragon fruit production in Vietnam, namely Binh Thuan, Tien Giang, and Tay Ninh (Table 1). In each province, one field was selected to perform the experiment. Of the three fields, the field that was located in Binh Thuan province and specialized

| Statistical analysis
Excel software was used in producing descriptive statistical analysis.
The experiments were calculated according to the method for field survey (Gomez & Gomez, 1984). Average monitoring parameters of each replication between experimental treatments were treated by the method of analysis of variance (ANOVA), then compared with Duncan's test at confidence level p ≤ .05 by using SAS software 9.1.

| Current status on bulb use for controlling offseason flowering of dragon fruit
Preliminary interviewing results on some farmers and daytime measurement in the three provinces that have largest commercial dragon fruit cultivation area in Vietnam (Binh Thuan, Tien Giang, and Tay Ninh province) confirmed that, from September to March of the following year, dragon fruit plants cultivated in these areas were unable to bloom under natural conditions. In fact, this is also the period in which the daytime is shorter than 12 hr. As a result, a majority of farming households adopted artificial irradiation as a measure to stimulate flowering of dragon fruit crops. Prior to 2012, off-season floral stimulation of dragon fruit often involved incandescent bulbs with the capacity of either 100 W, 75 W, or 60 W to supplement lighting during nighttime. In the years 2013-2014, there was a shift in the use of bulbs for dragon fruit flowering induction There is a discrepancy in bulb use between Tay Ninh and other two provinces. To be specific, the percentage of households using CFL for dragon fruit irradiation in Tay Ninh was 32%, which was higher than that recorded in both Tien Giang and Binh Thuan (4%).

TA B L E 1 Description of field experiments in the study
Tay Ninh is also the province in which many households adopted the combinational irradiation with both incandescent and CFL light bulbs. In addition, there were 5 out of 25 surveyed households in Tay  which are less energy consuming than incandescent counterparts, in their fields. This is reportedly due to delayed provision of electricity source reserved for agricultural activities and possibly reflects the inadequacy between grid capacity and production readiness of the electricity supplier and the recent, rising energy needs for dragon fruit cultivation in these areas.
Regarding lamp density, most growers allocated around 1,000- Overall, from the survey results, it is indicated that efficient control of off-season blossoming is an important concern in commercial dragon fruit farming and that possible improvements should be called for in efforts to conserve energy for such processes. Currently, all dragon fruit farmers adopted artificial irradiation as the main measure for controlling off-season flowering and ruled out other stimulation methods such as chemical intervention and foliar fertilization.
Moreover, the irradiation process was carried out based on farmers' experience, thus lacking considerations on standardizing critical aspects including bulb type, height of bulb hanging, bulb density, irradiation duration, and implementation of intermittent lighting.

| Spectrum analysis of currently used lamps for controlling off-season flowering of dragon fruit
We collected three types of lamps commonly used by dragon fruit farmers (60 W incandescent, 40 W incandescent lamp, and 20 W domestic CFL lamp) and measured their emission spectra. The results are shown as in Figure 1.
Examination of the spectrum of the three lamps shows that two incandescent lamps exhibited great intensity in the far-red and red region of their emission spectra, which coincides with the absorption spectrum of two main forms of phytochrome, namely P 660 and P 730 (Figure 2). Both of them are mutually transformable under appropriate irradiation and dark conditions and the ratio between the two determines blossoming state of the plant (Taiz & Zeiger, 1991).
The consistency between intensities of far-red and red regions of the incandescent lamp emission spectra and maximum absorption wavelength in P 660 and P 730 absorption spectra also sides with the widespread use of incandescent lamps with the capacity ranging from 60 to 100 W for dragon fruit flowering control in Taiwan and in Thailand (Saradhuldhat et al., 2009;Yen & Chang, 1997). However, the spectral regions that are not consistent with absorption spectra of phytochromes, including yellow, green, blue, and dark red areas, of incandescent lamps also exhibited high intensities. Most notably, the dark red region with the wavelength >730 nm, which is close to The spectrum of the 20 W domestic CFL lamp (Figure 1c) displayed high intensity in the green and yellow light, which is F I G U R E 1 Emission spectra of incandescent lamps with capacity of 60 W (a) and 40 W (b) and of CFL lamps (c) with capacity of 20 W that were currently in use engineered to suit the conventional lighting for human eye. The CFL lamp also has fair intensity in the far-red and red region but is comparatively lower than that in the absorption spectra. To further highlight the difference in red and far-red intensities between the two types of lamps, we measured the photon density of the two emission regions. The results are summarized in Table 3.
In terms of PPFD, the 60 W incandescent type exhibited the highest value, at 44.5 µmol m −2 s −1 , which is twofold higher than that

| Proposed CFL lamps and field experiment results on their effects on some growth indicators of dragon fruits
We obtained various improved CFL specimen lamps from Rang Dong Light Source & Vacuum Flask Joint Stock Company, one of leading manufacturers of light bulbs in Vietnam. All bulbs had the power capacity of 20 W and were specifically designed with increased intensity in red and far-red regions and reduced green and yellow intensity in their emission spectra. After measuring their spectra, we selected three CFL bulbs with highest total PFD and used them in the following field experiment. The spectra and analysis results are shown as in Figure 3 and Table 4.
Three improved CFL bulbs were used in field experiments con-  lamp with the capacity of 60 W was used as the control. The results are presented in Table 5.
Plant height, canopy diameter, and the number of stems per plant before and after the experiment 1 were not significantly different (See Table A1 in the Appendix A), confirming that the irradiation  (Be et al., 2014;Yen & Chang, 1997

TA B L E 5 (Continued)
Similar to experiment 2, experiment 6 was conducted under colder growing condition (24.9-25.9°C), resulting in comparatively poor growth and productivity. The treatment 2 gave plants with higher number of buds than other groups. In some plants in the control group, no budding was observed. Despite that, productivities and bud numbers of the control and treatment groups in this experiment are still higher than those in the experiment 2, confirming that red-flesh dragon fruit is more responsive to floral stimulation by artificial lighting. Both thermal and light sensitivity of the flowering process in red-flesh dragon fruit plant have been confirmed by a previous study where a temperature of at least 15°C was recommended for night lighting for the red-flesh cultivar (Jiang & Yang, 2015).

| CON CLUS IONS
In an effort to reduce energy consumption in dragon fruit farming, this study aimed to propose and evaluate new improved CFL lamps specialized for inducing light-stimulated off-season flowering in dragon fruit fields. The preliminary survey results indicated that the use of incandescent light bulbs was prevalent among farmers and that domestic CFL lights were generally inefficient in stimulating off-season flowering in both white-flesh and red-flesh dragon fruit cultivars. Emission spectrum measurement of currently used lamps suggested that the intensity in red and far-red regions of the lamp might be responsible for inducing phytochrome transformation, which is critical in flowering process of the plant. Field experiments adopting three improved CFL light bulbs in three provinces showed that one CFL light (treatment 2) resulted in better flowering in dragon fruit plants than others, which were demonstrated by better growth, blossoming indicators, and productivity. Future studies on combinational strategies in stimulating flowering in dragon fruits are recommended.

CO N FLI C T O F I NTE R E S T
The authors declare no conflict of interest.

DATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are available from the corresponding author upon reasonable request.