2014 Volume 83 Issue 4 Pages 273-281
The purpose of this study was to clarify the plant growth and fruit quality of blueberry in a controlled room under artificial light. Cultivars used were a northern highbush ‘Blueray’, and two southern highbush, ‘Misty’ and ‘Sharpblue’. A comparative study was carried out of growth characteristics, photosynthetic potential and fruit quality analysis in different growing environments, in particular focusing on plants growing in a glasshouse under natural sunlight and plants in a controlled room under artificial light. Environmental conditions of the controlled room under artificial light were 15 to 25°C, 50 to 70% humidity, 150 to 350 μmol·m−2·s−1 light intensity, and a 10-hour photoperiod from the primary experiment. In these growing environments, normal fruits developed from all the tested cultivars by successful growth without decreasing plant vigor and leaf photosynthetic ability until fruit harvesting time compared to the cultivars grown in the glasshouse under natural sunlight condition. Moreover, it was confirmed that high-quality fruits could be harvested in a controlled environment to increase fruit production with high SSC % and high anthocyanin content but low acid % in ‘Blueray’ and ‘Misty’, but not ‘Sharpblue’. Finally, this report presents the possibility of high-quality blueberry production in a controlled environment under artificial light conditions with some cultivars.
In Japan, blueberries (family Ericaceae; genus Vaccinium spp.) were first introduced from the United States of America in 1951. From about 1995, blueberry culture and consumption increased rapidly because blueberries were attracting worldwide interest due to their high antioxidant content and their validated link to human health (Joseph et al., 1999; Kalt et al., 2001; Mainland and Tucker, 2002). Consequently, the demand for blueberries as a nutritional food and beverage has increased markedly. Many cultivars comprising four species, northern highbush blueberry, half-high highbush blueberry, southern highbush blueberry and rabbiteye blueberry, have been introduced mainly from the USA, Australia, and New Zealand, and planted throughout the country (from Hokkaido to Kyushu). Investigations were conducted on the character traits of those introduced cultivars and their adaptability to Japanese climate and soil conditions (Fukushima et al., 1989; Ishikawa and Koike, 2006). Blueberry growing may be divided into three zones in Japan based on air temperature: a) northern highbush and half-high highbush blueberries in the northern region (Hokkaido) and the highland areas of the main island (Tohoku), which are relatively cool during the growing season; b) three blueberry types (northern highbush, southern highbush and rabbiteye) in the central regions (Kanto, Kinki, Chugoku, and northern Kyushu) of the main island, which are the leading growing areas in Japan; and c) mainly rabbiteye and southern highbush blueberries in the southern regions (southern Kyushu and Shikoku), which are warmer during the winter.
Nevertheless, the harvest duration of blueberry fruits in Japan is limited to June to September (only four months) for open-field production from all suitable elevations by using a range of species. Due to the increased demand of Japanese customers, during the off-season (October to May), about 12000 tons of frozen blueberry fruits need to be imported from countries including the USA, Canada, China, Australia, New Zealand, and Chile at high cost (Ishikawa and Koike, 2006). As a result, some blueberry farmers are trying to extend the production period by artificial heating in plastic houses starting from February after the completion of endo-dormancy. This enables the blueberry harvest to commence from May. However, year-round production of blueberry fruit, including the off-season, is required in Japan. In 2011, an advanced plant factory focusing on year-round production of blueberry fruits was established at Tokyo University of Agriculture and Technology, Tokyo, Japan (Ogiwara and Arie, 2010). The factory consists of six rooms (three glasshouses under natural sunlight and three controlled rooms under artificial light) that can duplicate the four seasons in Japan. In order to produce blueberry fruits year round, it is necessary to use both glasshouses under natural sunlight and controlled rooms under artificial light. Therefore, it is essential to establish a cultivation system in a controlled room under artificial light, and the purpose of this study was to clarify the plant growth and fruit quality of blueberry in a controlled room under artificial light.
The research was carried out in an advanced plant factory, which is a two-storied building composed of a ground floor and an underground floor located in Tokyo University of Agriculture and Technology, Tokyo, Japan (Ogiwara and Arie, 2010). The ground floor includes the glasshouses under natural sunlight and the underground floor consists of controlled rooms under artificial fluorescent lights (FHF32EX-N-H; Panasonic Co. Ltd., Osaka, Japan).
Four- to 5-year-old plants of northern highbush ‘Blueray’, southern highbush ‘Misty’ and ‘Sharpblue’ grown in 30 L pots containing Kanuma soil:peatmoss:volcanic ash (1:1:1, v/v) were used in this experiment. Three replicate pots of each cultivar grown in an open field until May 2, 2012 after finishing anthesis were moved to two growing conditions, a glasshouse under natural sunlight or a controlled room under artificial light, in order to compare vegetative growth characteristics, photosynthetic potential and fruit quality analysis between the different growing environments. Automated drip irrigation with a conventional liquid fertilization system for blueberry was used (Otsuka AgriTechno Co. Ltd., Tokyo, Japan). By using pH reducing solution containing potassium phosphate (P2O4−) (Otsuka AgriTechno Co. Ltd.), pH was constantly controlled between 4 to 5 and electric conductance (EC) ranged from 0.7 to 1.2 mS·cm−1. Irrigation was provided at 1 to 1.5 L per pot per day and a water wash system was applied to plants if insect-like aphids were found, and no pesticides or insecticides were used. Fruit loading of each cultivar was set at the same level.
In the glasshouse under natural sunlight, day and night temperatures were controlled to 30°C and 18°C using a ventilator and heater, respectively. There was no setting for humidity control but daylength was controlled to 14 hours using a high pressure sodium lamp. In the controlled room under artificial light, temperatures were controlled to 25°C (lighting period) and 15°C (dark period) using an air conditioner, humidity was set at 50–70% during the light period using a dehumidifier. Light intensity was set around 300 μmol·m−2·s−1 at the top of the plants and the photoperiod was set at 10 hours. The minimum and maximum environmental conditions of temperature, humidity, light intensity and daylength of the two different growing conditions throughout the experiment are shown in Table 1. Temperature and humidity were measured by using a single block relative humidity and temperature microprocessor transmitter (HD 9009TRR; Delta Ohm S.r.L., Padova, Italy). Light intensity was measured with a sensor for photosynthetic photon flux density (MIJ-14PAR Type 2; Environmental Measurement Japan Co. Ltd., Fukuoka, Japan).
Environmental conditions of the glasshouse and controlled room throughout the experiment.
For each cultivar, 20 newly produced shoots were randomly selected and shoot lengths were measured on June 28 for ‘Misty’ and ‘Sharpblue’, and on July 4 for ‘Blueray’. Likewise, 20 leaves (upper, middle, and lower position of the plant) were selected on each cultivar and estimated leaf areas were determined by leaf length and width based on the method of Fallovo et al. (2008). Leaf length (cm) was measured from the lamina tip to the point of intersection of the lamina and the petiole, along the midrib of the lamina, whereas leaf width (cm) was measured from end-to-end between the widest lobes of the lamina perpendicular to the lamina midrib. Estimated leaf area (LA) was calculated by following the formula LA = 0.54 + 0.68 LW (length and width). Chlorophyll content was determined by using a chlorophyll meter SPAD 502 (Konica Minolta Co. Ltd., Tokyo, Japan). Leaf thicknesses (mm) were measured by a 0.01–10 mm dial thickness gauge (Model G; OZAKI MFG. Co. Ltd., Tokyo, Japan).
An A/Ci regression curve compared growth between the glasshouse under natural sunlight and the controlled room under artificial light for ‘Sharpblue’. Three replicate measurements were taken and data were collected on May 8, 12, 13, June 10, 13, 14, and July 3, 8, 10 using fully expanded leaves. A portable photosynthesis system (LI-6400; LI-COR, NE, USA) with an LED light source (LI-6400-02B; LI-COR) was used. The photosynthetic rate was measured under base conditions of humidity 50–70%, leaf temperature 20°C, light intensity 2000 μmol·m−2·s−1, and air flow 350 μmol·s−1. Ambient CO2 concentration (Ca) in the cuvette was controlled with a CO2 mixer across the series of 0, 50, 100, 300, 500, 800, 1000, 1500, and 2000 μmol·mol−1. The intercellular CO2 concentration (Ci) was calculated as follows.
In addition, the diurnal changes of the photosynthesis rate (one-hour interval) of ‘Blueray’ and ‘Misty’ under two growing conditions were analyzed under base conditions of humidity 60–70%, temperature 25°C, air flow 350 μmol·s−1, and CO2 concentration 350 μmol·mol−1; from 6:00–19:00 for the glasshouse under natural sunlight and 6:00–22:00 for the controlled room under artificial light. Finally, in order to understand the photosynthetic rates under different light intensity levels in the controlled room, the diurnal changes (7:00–23:00) under the same conditions were analyzed for ‘Sharpblue’ by measuring the leaves in different positions; bottom, middle, and top; of the plant depending on the availability of light intensity levels (171–323 μmol·m−2·s−1) for each position under artificial light.
For both growing conditions, fruits started to harvest on June 28 for ‘Misty’ and ‘Sharpblue’, and on July 9 for ‘Blueray’. Mature blueberries assessed by full color development of the fruit and blue at the pedicel base were harvested under each growing condition. For relative fruit size, the weight (g) and diameter (mm) of 20 randomly selected fruits were measured individually for each cultivar, ‘Blueray’, ‘Misty’, and ‘Sharpblue’. Fruit firmness was determined by measuring the force to penetrate (with 2 mm Φ) the fruit positioned on its side with a rheometer (RT-3005 D; RHEOTECH Co. Ltd., Tokyo, Japan). The maximum peak of the recorded force-time curve, measured in kilogram force (kgf), was taken as the firmness of the blueberry. Twenty replicates were taken for each measurement, and SDs were obtained. Soluble solids content (SSC) and titratable acid (TA) were determined using freshly prepared juice. One gram of pulp per five fruits was mixed with 3 mL distilled water and centrifuged at 13000 rpm for 10 minutes. The SSC (Brix %) was measured using a digital refractometer (model PR101; Atago Co. Ltd., Tokyo, Japan) standardized with distilled water. TA was determined by diluting each 1 mL aliquot of blueberry juice to 5 mL with distilled water, then titrating to pH 8.1 using 0.01 N NaOH. Acidity was expressed as g citric acid/100 mL juice. Three replications were taken for each result. In order to assess total anthocyanin contents, three 0.05 g sub-samples of fruit skin were extracted with 3 mL 2% TFA methanol (methanol containing 2% trifluoacetic acid). A 100 μL aliquot was diluted again with 1400 μL of 2% TFA solution to dilute 15-fold and total anthocyanin content was determined using a Shimadzu UV-visible spectrophotometer (UV-1200; Shimadzu Co. Ltd., Kyoto, Japan) at 530 nm and the results were expressed as the absorbance (Abs) value. In addition, the surfaces of fruit skins of ‘Blueray’ and ‘Misty’ were observed under an Olympus microscope (magnification: ×6, SZX-ILLK100; Olympus Optical Co. Ltd., Tokyo, Japan) to compare the differences in bloom formation of the fruits between different growing conditions.
Growth characteristics and fruit quality data under two different growing conditions were statistically analyzed by Student’s t-test. Differences with P values of < 0.001, < 0.01, and < 0.05 were considered significant.
Diurnal changes (10-minute intervals) of environmental conditions between different growing systems (glasshouse under natural sunlight and controlled room under artificial light) were monitored every day. However, only the data for diurnal changes on 10th May, June, and July are shown in Figure 1. The day and night temperatures under natural sunlight changed markedly with the highest day temperatures in July, June, and May, respectively. However, the temperatures under artificial light were 25°C with slight variations over 10 hours during the light period and 15°C during the dark period. Daytime humidity under natural sunlight ranged between 30% and 60%, and under artificial light it ranged from 50% to 70%. Under artificial light, the maximum humidity in May and June was similar. However, it was 100% in July from midnight to early morning because the artificial room was located underground and high humidity occurred in the rainy season, especially at night time. Although PPFD values under natural sunlight showed great variations, the values under artificial light ranged from minimum 335 μmol·m−2·s−1 to maximum 521 μmol·m−2·s−1 for 10 hours during the light period of 10:00 to 20:00. In sum, the growing environment under artificial light was in conditions of low temperature (15–25°C) and low light intensity (335–521 μmol·m−2·s−1) but high humidity (50–70%). The conditions under natural sunlight had more variation.
Diurnal changes of environmental conditions in the glasshouse under natural sunlight and in the controlled room under artificial light in the plant factory on 10th May, June, and July, 2012 at ten-minute intervals using a real-time environmental monitoring system.
All the tested cultivars (northern highbush and southern highbush) grew healthily in the controlled room under artificial light (Fig. 2). The comparison of growth characteristics between two different growing conditions (glasshouse under natural sunlight and controlled room under artificial light) are presented in Table 2. Regardless of the growth habit of each cultivar, all the tested cultivars except ‘Misty’ showed no significant difference in shoot length between the two growing conditions. Shoots growth of ‘Misty’ grown under natural sunlight (19.43 cm) was greater than under artificial light (7.53 cm), although the leaf area and leaf thickness of all cultivars were the same under different growing conditions. When grown under artificial light, Chl content changed in ‘Blueray’ with a slight decrease in plants grown under artificial light (38.38).
Comparison of blueberry cultivars under two different growing conditions: plants in a glasshouse under natural sunlight (above) and plants in a controlled room under artificial light (below).
Growth characteristics of blueberry cultivars under different growing conditions (glasshouse and controlled room).
In both production methods (glasshouse under natural sunlight and controlled room under artificial light), the net photosynthetic (Pn) rate of ‘Sharpblue’ increased linearly between 0 to 400 μmol·mol−1 of intercellular CO2 concentration (Ci), slightly increased between 400 to 700 μmol·mol−1, and finally remained constant at 1500 μmol·mol−1 of Ci (Fig. 3). There were no differences in the Pn rate up to 400 μmol·mol−1 of Ci in both growing conditions for three months (from May to July). Even in the controlled room under artificial light with low PPFD value, the photosynthetic ability of ‘Sharpblue’ showed the same tendency as in the glasshouse under natural sunlight.
Monthly determination of the net photosynthetic rate (May to July) in response to intercellular CO2 concentration (Ci) of ‘Sharpblue’ under different growing conditions: (●) glasshouse under natural sunlight and (▲) controlled room under artificial light. Vertical bars are standard errors of the mean (n = 3). In some cases, the error bars are obscured by the datum symbol.
In the determination of diurnal changes of photosynthetic ability between the two growing conditions, both ‘Blueray’ and ‘Misty’ showed nearly the same pattern (Fig. 4). In the glasshouse under natural sunlight, the Pn rate of both cultivars increased in accordance with increasing sunlight and decreased with a low PPFD value according to sunlight (Fig. 4A, C). Therefore, Pn rates under natural sunlight were not constant and markedly changed depending on the sunlight intensity, although it had the highest peak value of 5.18 μmol CO2·m−2·s−1 for ‘Blueray’ and 6.85 μmol CO2·m−2·s−1 for ‘Misty’ during the day. In contrast, in the diurnal changes of photosynthetic ability in the controlled room under artificial light, the Pn rate was quite constant for ‘Blueray’ with a value of about 2 μmol CO2·m−2·s−1 with a PPFD value 182 to 193 μmol·m−2·s−1 (Fig. 4B). Likewise, the Pn rate was also constant for ‘Misty’ with a value of around 4 μmol CO2·m−2·s−1 with a PPFD value 193 to 279 μmol·m−2·s−1 (Fig. 4D).
Diurnal changes in the net photosynthetic rate (♦) and light intensity (○) of ‘Blueray’ (A and B) and ‘Misty’ (C and D) grown under different conditions.
In the findings of the Pn rate depending on different positions of the plant for ‘Sharpblue’ in the controlled room under artificial light, the Pn rate gradually increased from bottom to top positions depending on the nearness to the light sources (PPFD value 171 to 323 μmol·m−2·s−1) and all the results remained constant during the light period (Fig. 5). Moreover, the Pn rate of the leaves in the middle and top position of the plants were relatively high, about 5 μmol CO2·m−2·s−1 for the former case and about 6 μmol CO2·m−2·s−1 for the latter case.
Diurnal changes in net photosynthetic rate (♦) of different-positioned leaves in ‘Sharpblue’ grown in the controlled room under artificial light depending on available light intensities (○).
Fruit size from different cultivars varied under different growing conditions (Table 3). The larger fruit weights and diameters were produced by ‘Blueray’ under artificial light and ‘Misty’ under natural sunlight. No significant differences occurred in fruit firmness of all cultivars under different growing conditions; however, ‘Sharpblue’ (0.19 kgf) produced the firmest fruit under natural sunlight. Higher SSC (%) and anthocyanin contents (Abs value) combined with low acid were observed in ‘Blueray’ and ‘Misty’ under artificial light (Table 3). In contrast, ‘Sharpblue’ produced higher SSC (%), higher anthocyanin (Abs value), and lower TA (%) under natural sunlight. Among all the tested cultivars, cultivar ‘Blueray’ produced the largest fruit (2.58 and 3.12 g) and the highest anthocyanin content (1.18 and 1.60 Abs) in both growing conditions. ‘Sharpblue’ under natural sunlight has the lowest TA (0.16%). Moreover, much more bloom appeared on the surfaces of the fruits grown under artificial light than those grown under natural sunlight (Fig. 6). By checking ‘Blueray’ and ‘Misty’ under the microscope, the surfaces of both fruits produced under artificial light (Fig. 6B, D) had a larger amount of bloom than those under natural sunlight (Fig. 6A, C).
Fruit characteristics of blueberry cultivars under different growing conditions (glasshouse and controlled room).
Microscopic photos of blueberry fruits (‘Blueray’ and ‘Misty’) produced under two growing conditions: fruits under natural sunlight (A and C), fruits under artificial light showing bloom on the fruit skins (B and D).
As the plant factory for blueberries was established in 2011, it was necessary to perform studies on how to effectively control and adjust the suitable environmental conditions for the growing rooms. The study group performed some preliminary tests on various settings of temperature, daylength and photoperiod in order to understand the plant response under different conditions in the controlled room. Kameari et al. (2010) suggested that the most suitable temperature for maximum Pn on blueberry plants grown in a phytotron was 25°C. Furthermore, through research on the responses of plants, the optimal temperature was set at 15°C (dark period) and 25°C (lighting period) in the controlled room. Although the saturated point of Pn was reached under a PPFD value of 800–1000 μmol·m−2·s−1 (Kameari et al., 2010), the temperature increased if PPFD was set at 1000 μmol·m−2·s−1 in a controlled room and a lot of energy consumption was lost. Based on this result, PPFD was then set at around 300 μmol·m−2·s−1 at the top of the plants and the photoperiod was set at 10 hours. Low humidity might cause stomatal closure directly by causing water stress in the epidermal tissue and guard cells due to excessive loss of water by a high rate of transpiration (Dai et al., 1992; Loreto and Sharkey, 1990; Rawson and Begg, 1977). Finally, relative humidity was set between 50% and 70% during the light period in order to open more stomata. In the findings of monthly determination (from May to July) of leaf photosynthesis via A/Ci curve regression analysis, photosynthetic abilities of blueberry leaves in the glasshouse under natural sunlight and in the controlled room under artificial light showed no differences for three months (Fig. 3). Then, in the comparison of diurnal changes of photosynthetic rates between the two growing conditions, Pn values in the glasshouse varied depending on changes of sunlight intensity, although there were some peaks during the day (Fig. 4A, C). In contrast, diurnal changes of photosynthesis rates in the controlled room under artificial light were constant during the light period (Figs. 4B, D and 5). Although Pn values of bottom leaves in the controlled room were low, the values were quite constant for 10 hours during the light period and Pn values of middle leaves and top leaves were relatively high (Fig. 5). Therefore, it can be assumed that the same photosynthetic capacities of blueberry plants were obtained in the controlled room under artificial light throughout the growing period (only three months) compared to those plants in the glasshouse under natural sunlight. In addition, according to the results of environmental monitoring, the values of temperature, humidity, and light intensity under natural sunlight changed markedly depending on climate (Fig. 1), but the values under artificial light were under conditions of constantly low temperature and low light intensity, but high humidity, which may have helped to maintain the constant Pn rate (Fig. 1).
‘Misty’ and ‘Sharpblue’ are included among the recommended cultivars for forcing and/or heating culture in Japan (Ozeki and Tamada, 2006). In the report on the fruit ripening and quality profile of 64 cultivars in three species of blueberries grown in Tokyo (Che et al., 2009), the mean fruit sizes were 2.49 g in ‘Misty’ and 2.13 g in ‘Sharpblue’, respectively. Fruit weights in a recent study were a little lower than those reported in the study under field conditions by Che et al. (2009), which was thought to be because of the differences of the crop load. However, higher soluble solids content of fruit in ‘Misty’ (11.33% under natural sunlight and 15.73% under artificial light) and in ‘Sharpblue’ (14.80% under natural sunlight and 13.73% under artificial light, respectively) than those reported on both cultivars (9.67% in ‘Misty’ and 10.4% in ‘Sharpblue’) in the test of field conditions by Che et al. (2009) were observed in this study. In contrast, TA of the fruits in this study resulted in lower values (0.39% under natural sunlight and 0.26% under artificial light) for ‘Misty’ and (0.16% and 0.42% respectively) for ‘Sharpblue’ than those reported by Che et al. (2009); 0.76% for ‘Misty’ and 1.23% for ‘Sharpblue’. Beaudry (1992) demonstrated that a 0.1% decrease in acid concentration is known to be equivalent to a 1% increase in perceived sweetness in blueberry fruit and also suggested that blueberries should contain > 10% SSC and 0.3–1.3% TA. Based on these quality standards, fruit quality of all cultivars in this study except for TA % of ‘Misty’ (0.26%) under artificial light and that of ‘Sharpblue’ (0.16%) under natural sunlight showed acceptable SSC and TA values. As a whole, all cultivars grown under both growing conditions had high SSC %. However, the effect of environmental condition (natural sunlight and artificial light) on fruit quality was different among the tested cultivars. Since the environmental condition of the controlled room under artificial light was suitable for producing high-quality fruits of ‘Blueray’ and ‘Misty’, the condition in the glasshouse under natural sunlight was suitable for obtaining high-quality ‘Sharpblue’ fruits according to the results (Table 3). Krüger and Josuttis (2014) reported that differences in the accumulation of chemical components in berry fruits are related to genotype × environmental interactions. Moreover, they demonstrated that pre-harvest factors such as day and night temperature, light intensity and light quality, protected cultivation with various types of plastic film, irrigation and fertilization etc. can influence the contents and composition of the berry fruits. Therefore, in this study, the different results in fruit quality among the tested cultivars might be not only because of their genotype effects but also the external environmental influences, especially temperature and light during fruit development under different growing conditions.
Fruits with high SSC % and low TA % taste sweet and their quality is high. Interestingly, the point to consider here is why ‘Blueray’ and ‘Misty’ contained higher SSC % and lower TA % in the controlled room under artificial light. A possible reason is that, in the controlled room under artificial light, the growing environment was kept mainly under a constant temperature and light intensity, which might help to increase the accumulation of carbon in leaves that have a low respiration rate. SSC % of fruit could also be increased due to the constant Pn rate and due to the low respiration rate in the dark period in the controlled room under artificial light. However, TA % may have decreased due to the low respiration rate under low temperature and low light intensity in the controlled environment. Moreover, many studies have indicated that artificial irradiation by ultra violet (UV) fluorescent light increased fruit coloration and produced anthocyanin pigments in skin of apples (Ubi et al., 2005), sweet cherries (Arakawa, 1993; Kataoka et al., 2005), and grapes and peaches (Kataoka and Beppu, 2004; Kataoka et al., 2004). In this study, the UV length under artificial light was 380–430 nm, which may have increased the amount of anthocyanin in blueberry fruits. Finally, these overall conditions could produce “high-quality” fruits. However, the effect of the interaction between light intensity and temperature was not clearly differentiated and further study is still necessary to elucidate these effects. In addition, there was less fruit drop than in plants in the glasshouse under natural sunlight (data not shown). The air flow in the controlled room was created by a ventilator to cycle 80–100 cm·s−1 to increase the photosynthetic rate of blueberry leaves. In the open field, strong winds sometimes affect fruits, causing them to fall. In a recent study, there was no strong wind so fruit drop was low in the tested cultivars. In the future, this could be a benefit to increase the total harvest weight of blueberry plants by growing in a controlled environment compared to open-field production. In addition, the formation and amount of bloom on the surface of blueberry fruit may depend on genetic variability or environmental variables (Albrigo et al., 1980; Shepherd and Griffiths, 2006). In this study, it was assumed that the occurrence of more bloom on the surfaces of ‘Blueray’ and ‘Misty’ fruits (Fig. 6B, D) might be the effect of environmental conditions in the controlled room under artificial light. However, the different amounts of bloom between the two growing conditions could not be estimated in this study unfortunately. Therefore, bloom between the two growing conditions should be clarified by scanning electron microscopy in a future study.
In summary, it is possible to establish a new cultivation system for blueberry in a controlled room under artificial light by controlling conditions to a low temperature (15–25°C), high humidity (50–70%) and short daylength (10 hours) under constantly low light intensity (150 to 350 μmol·m−2·s−1) to produce a vigorous fruit tree by maintaining leaves with constant Pn activity in order to develop normal fruits with high quality. Finally, the study confirmed that successful continuous blueberry production is possible throughout the year, including the off-season by a combination of an open field, plastic houses, glasshouses, and controlled rooms.
For the successful completion of the present study, my heartfelt thanks go to Dr. Roderick Drew, School of Biomolecular and Physical Sciences, Griffith University, Australia, for kindly editing the English language even though he is very busy. Moreover, my special thanks also go to my colleagues in the blueberry group who have kindly supported me throughout this study.