2025 Volume 94 Issue 3 Pages 346-355
The rising demand for strawberries in tropical and subtropical regions underscores the need for effective cultivation techniques. However, the challenges posed by high temperatures in these areas have hindered successful strawberry cultivation. In this study, we investigated the impact of nighttime light-emitting diode (LED) supplemental lighting on the growth, yield, and fruit quality of three everbearing strawberry cultivars, ‘Koi-ichigo’, ‘KS33’, and ‘KS38’. They were cultivated in a controlled environment greenhouse with a crown and growth medium cooling system from August to December 2023 under high temperatures similar to tropical regions. LED lights were employed daily for 3.5–5.5 hours post-sunset to extend the day length to 16 hours per day. Under daily mean air temperatures exceeding 26°C until October, ‘Koi-ichigo’ under a natural day length underwent anthesis later than the other two cultivars. In the LED treatment, flower bud differentiation was stabilized in all cultivars due to increased photosynthetic photon flux density (PPFD). All three cultivars exhibited enhanced growth and yield under the LED treatment. Furthermore, the cultivars’ total shoot dry weight increased in response to LED supplementation. Although the LED treatment increased the number of harvested fruits, the fruit weight and soluble solid content remained comparable to those in the control. The leaf area index decreased in the LED treatment in ‘Koi-ichigo’ and ‘KS33’. At the same time, no difference in light-use efficiency was found between the control and LED treatment, and cumulative light interception surpassed that of the control in the LED treatment in all cultivars. These findings suggest that when growing everbearing strawberry cultivars in tropical and subtropical regions, nighttime LED supplemental lighting effectively stabilizes flower bud differentiation and increases the total shoot dry weight due to increased cumulative light interception, resulting in increased yield.
Strawberries (Fragaria × ananassa Duch.) are important fruits in high demand worldwide. Currently, most strawberry production areas are located in the northern hemisphere, and the highest quantities are produced during the late spring and early summer months, primarily in California, Spain, and southern Italy (Hancock, 2020). In East Asian countries such as Japan, South Korea, and China, strawberries are widely grown in greenhouses from winter to spring using June-bearing cultivars because floral differentiation occurs under a short-photoperiod and cool temperatures in autumn (Nishizawa, 2021). In Japan, some studies were conducted on summer cultivation with everbearing strawberry cultivars to meet the summer demand for strawberries (Hamano et al., 2012; Nishiyama et al., 2009, 2020; Yano et al., 2021). Strawberry cultivation in Southeast Asia and other tropical and subtropical regions remains rare; even in cooler highlands and mountainous areas, stable cultivation methods have yet to be established. Despite this, there is increasing demand for strawberries in tropical and subtropical regions, notably Southeast Asia, driven by population growth and economic expansion. In Southeast Asia, strawberry imports have surged significantly, increasing by 5.7 and 17.6 times by volume and value, respectively, since 2001 (FAOSTAT, 2023). To meet this increasing demand, developing strawberry cultivation techniques in tropical and subtropical regions is necessary.
Advancements in environmental control technologies for greenhouse cultivation have paved the way for the development of vegetable cultivation systems suited to the high temperatures and humidity typical of subtropical regions (Nakayama et al., 2021). Additionally, studies exploring strawberry cultivation using light-emitting diode (LED) lighting have been documented (Hidaka et al., 2013, 2014; Nestby and Trandem, 2013). Nakayama and Nakazawa (2023) demonstrated that employing an integrated environmental control system combined with LED lighting during the daytime improved both the yield and sugar content of June-bearing strawberries cultivated from October to May in a subtropical region. They attributed this to an increase in dry weight due to photosynthesis enhanced by the daytime LED lighting. The usefulness of LED supplemental lighting was confirmed under natural daylength from October to May in a subtropical region with low solar radiation. On the other hand, the mean air temperature in Jakarta, Indonesia, located in a tropical region, is 27–28°C throughout the year. Although there are differences in solar radiation levels due to wet and dry seasons, the daylength is almost constant in the 12h4m–12h10m range. Ito and Saito (1962) concluded that the flowers bud in a June-bearing strawberry ‘Robinson’ initiated under short-day conditions with daylength between 4 and 12 hours provided air temperature is below 24–26°C; however, at 30°C, the plants failed to produce flower buds even with daylengths of 4 or 8 hours.
Everbearing strawberry cultivars are typically categorized as qualitative long-day plants at high temperatures (27°C) and quantitative long-day plants at intermediate temperatures (15°C and 21°C) (Sønsteby and Heide, 2007). Nishiyama et al. (2009) investigated the critical photoperiod for flower bud initiation of several everbearing strawberry cultivars grown at 30/25°C and reported a difference between cultivars between 12 and 16 hours. This suggests that everbearing cultivars may be more suitable for stable strawberry production in tropical and subtropical regions than June-bearing cultivars, which require a short-day environment. Therefore, we hypothesized that to increase strawberry yield in tropical and subtropical regions with high temperatures throughout the year, it would be effective to grow everbearing strawberry cultivars with LED lighting at night to extend the photoperiod to 16 hours, to ensure the day length necessary for flower bud differentiation. However, there are very few studies focusing on the cultivation of everbearing strawberry cultivars in these regions. We investigated everbearing strawberry cultivars grown on Ishigaki Island, located in a subtropical region, from summer to early winter under high temperatures similar to those in tropical regions. This study aimed to investigate the effects of nighttime LED supplemental lighting on the growth, yield, and fruit quality of everbearing strawberry cultivars cultivated in a greenhouse under controlled environmental conditions to develop strawberry cultivation technology for tropical and subtropical regions.
Three everbearing strawberry cultivars, ‘Koi-ichigo’, ‘KS33’, and ‘KS38’ bred by a private company in Japan, were tested. Since these cultivars have been approved for growing in Japan and overseas by their breeders, we used them to consider their potential for cultivation in tropical and subtropical regions such as Southeast Asia. However, little is known about the flower bud differentiation of these cultivars, and there have been no studies on growing these cultivars under high-temperature conditions, so also investigated the growth of these cultivars under a high-temperature condition.
The strawberry cultivation was conducted in a greenhouse at the Tropical Agriculture Research Front of the Japan International Research Center for Agricultural Sciences (24°34′N, 124°16′E) on Ishigaki Island, Okinawa, Japan. The greenhouse, constructed with a pipe frame, was 6 meters wide, 25 meters long, and 3.3 meters high. Various devices were used, including roll-up side vents (Kankit; Toto Kogyo, Tokyo, Japan), two ventilation fans (NK-74DGA; Panasonic, Osaka, Japan), a shading curtain with 65% transparency rate (WHR31; Innovex, Tokyo, Japan), a fog cooling system (Cool Pescon kit-A; H. Ikeuchi, Osaka, Japan), and a crown cooling system with a chiller (UWAP75A; Daikin Industries, Osaka, Japan). In the crown cooling system, a polyethylene pipe with a 16 mm outer diameter through which cold water flowed passed over the crown and folded back at the edge of the cultivation bed, and the pipes were arranged so that they passed through the center of the growing bed as well as the cooled surface of the growth medium. An integrated environmental control system (Smart Saien’s Cloud; Panasonic) managed these devices based on a preset program. For instance, when the greenhouse temperature exceeded 26, 28, and 30°C, the side vents were opened, ventilation fans were activated, and the fog cooling system was engaged, respectively. Throughout the cultivation period, water was cooled to 18°C by the chiller circulated continuously through pipes to cool the crown and growth medium. The greenhouse was divided into control and LED treatment sections along the north-south axis. LED lights (Growlightengine Generation 2.0; Arrant-Light Oy, Turku, Finland; 290 W) were suspended 0.9 meters above the cultivation bed surface, with a spacing of 1.5 meters between each LED light. In the LED treatment, the LEDs were activated daily for 3.5–5.5 hours post-sunset to ensure a 16-hour day length. Shading curtains were deployed between sunset and sunrise to separate the control and LED treatment areas. The photosynthetic photon flux density (PPFD) under the LED treatment was measured at 237 ± 25.6 μmol·m−2·s−1 at 30 cm above the surface of the cultivation beds when the LEDs were turned on at night, and the LED lighting spectrum was as in our previous report (Nakayama and Nakazawa, 2023).
Crop managementThis study investigated three strawberry cultivars: ‘Koi-ichigo’, ‘KS33’, and ‘KS38’ (Nii bio, Tokushima, Japan). These cultivars are known to differentiate flower buds under long-day conditions, but the details are not clear because there have been no studies on critical day lengths. Seedlings, each with four true leaves, of these cultivars were transplanted into the greenhouse upon arrival on August 17, 2023. The greenhouse was equipped with an environmental control system and operated in a manner consistent with our prior study (Nakayama and Nakazawa, 2023). Seven seedlings of the same strawberry cultivar were planted in each cultivation bed (0.75 m (L) × 0.35 m (W) × 0.15 m (D)), resulting in a plant density of 7.59 plants·m−2. The growth medium predominantly consisted of cocopeat (BVB for strawberries; Toyotane, Aichi, Japan). A split-plot design was used, with the main plot representing the treatment with LED supplemental lighting and the subplot representing the different cultivars. Each treatment with LED supplemental lighting comprised 14 plants (two cultivation beds) for each cultivar, with four replications in a completely randomized design. Plastic mulch was applied to the cultivation beds one week after transplantation, and automated irrigation via timer was initiated. A commercial stock solution (OAT House-A treatment; OAT Agrio, Tokyo, Japan) containing 233 mg·L−1 NO3-N, 23 mg·L−1 NH4-N, 120 mg·L−1 P2O5, 405 mg·L−1 K2O, 230 mg·L−1 CaO, 60 mg·L−1 MgO, 1.5 mg·L−1 MnO, 1.5 mg·L−1 B2O3, 2.7 mg·L−1 Fe, 0.03 mg·L−1 Cu, 0.09 mg·L−1 Zn, and 0.03 mg·L−1 Mo was used to prepare nutrient solutions by diluting with water to an EC of 0.4–0.6 dS·m−1, adjusted according to plant growth. The daily irrigation volume was standardized across all treatments and cultivars, with a drainage rate of 30–60%.
The number of crowns was maintained under three throughout the cultivation period. Axillary buds, runners, and old leaves were removed weekly. Green blowflies (Phaenicia sericata Meig., bee fly; Japan Maggot Company, Okayama, Japan) were introduced into the greenhouse weekly during flowering to facilitate pollination.
Starting from October 31, ripe fruits were harvested three times a week. During each harvest, the number of fruits, fruit weight, and weight of marketable fruits—defined as those with a normal shape and a weight of ≥6 grams—were recorded for three plants from each treatment. Subsequently, the soluble solid content (°Brix) was measured using a refractometer (PAL-1; Atago, Tokyo, Japan) for one to three marketable fruits from each treatment.
The total shoot dry weight (TDW) was determined by weighing three plants, regardless of replication, at the time of transplantation, onset of flowering, and harvest. At the end of the cultivation period, the leaves, crowns, runners, and other plant parts from three plants in each treatment were oven-dried at 80°C until a constant mass was achieved, after which their dry weights were recorded. Regarding measurement of the trimmed plants, the leaves, runners, and other plant parts removed during the cultivation period were also dried together at the same time as the measurement, and they were added to the dry weight of the leaf, runner, and other plant parts, respectively. The dry weight of the fruit was calculated by multiplying the fresh weight by the average dry matter content of the fruit, as measured during harvest. The average dry matter content of the fruit varied by cultivar and treatment as follows: ‘Koi-ichigo’ was 0.084 in the control and 0.088 in the LED treatment, ‘KS33’ was 0.083 in the control and 0.085 in the LED treatment, ‘KS38’ was 0.078 in the control and 0.082 in the LED treatment. Leaf area was measured using an area meter (LI-3100C; LI-COR, Lincoln, NE, USA) concurrently with TDW measurement. The specific leaf area (SLA) was calculated by dividing the leaf area by the dry weight of the leaf. The leaf area index (LAI) at the time of TDW measurement was determined by multiplying the measured leaf area per plant by the planting density.
Environmental measurement in the greenhouseAir temperature was carefully monitored at the central area of each treatment, capturing readings at 1-min intervals using platinum resistance thermometers housed within multi-plate radiation shields (C-PTWP and CYG-41303; Climatec, Tokyo, Japan) throughout the growing period. The temperature of the growth medium between plants was also logged at a depth of 5 cm, using thermocouples set to record data at 5-min intervals. Additionally, PPFD was measured at 30 cm above the cultivation beds, above the plant canopy, at 5-minute intervals by two quantum sensors (SQ-110; Apogee Instruments, Logan, UT, USA).
Estimation of LAI, light interception, and light-use efficiencyDaily Leaf Area Index (DLAI) was estimated by fitting the LAI measurements from four destructive samplings to the following equation and determining the constants a, b, and c in the equation for each cultivar and treatment:
| (1) |
Here, ‘DAP’ represents the day after transplanting, and all cultivars and treatments exhibited high correlation coefficients (R2 = 0.95–0.99). This allowed the calculation of light interception by the plant canopy for each cultivar in both the control and LED treatment conditions throughout the growing period. Daily Light Interception (DLI) was computed using the equation developed by Monsi and Saeki (2005) and Higashide et al. (2014):
| (2) |
Here, ‘k’ represents the light-extinction coefficient, standardized at 0.7 for all cultivars in this study based on previous experience with other strawberry cultivars, and ‘DPPFD’ represents daily PPFD at 30 cm above the cultivation bed.
Cumulative light interception was determined by aggregating DLI over the growing period. Light-Use Efficiency (LUE) was then computed as the slope of the linear regression between cumulative light interception and TDW, following methodologies outlined by Higashide et al. (2014) and Mochizuki et al. (2024).
Experimental data underwent analysis of variance, and significant treatment means were distinguished using Tukey’s HSD method, implemented in R Version 4.3.2 (RStudio 2023.06.0). Equations for measured leaf area and DLAI, and DLI were entered into Python 3.9.8 (Jupyterlab Version 3.2.2) for calculations.
The daily mean air temperature in both the control and LED treatment remained above 26°C until October 2023, decreasing to 24°C in November 2023 and 22°C in December 2023 (Table 1). Both the daily mean and nighttime air temperatures were consistently higher in the LED treatment compared to the control. The mean growth medium temperature was lowered below 23.8°C using the crown and growth medium cooling system, and the growth medium temperature was consistently higher in the LED treatment than in the control. The highest growth medium temperatures in the growing period were 26.7°C and 27.0°C in the control and the LED treatment, respectively. The mean differences between the daily mean and the daily maximum of the growth medium temperature were 1.0°C and 1.4°C in the control and the LED treatment, respectively.
Air temperatures, growth medium temperatures, and mean daytime hours for each month.
Day length during the growing period on Ishigaki Island ranged from approximately 13.0 hours on August 17, 2023 (transplanting date) to 10.5 hours on December 25, 2023 (end of cultivation). Supplemental lighting was applied daily for 3.5–5.5 hours after sunset in the LED treatment, with the PPFD during this period averaging around 200 μmol·m−2·s−1 (Fig. 1). On clear days (October 23, 2023), the PPFD exceeded 600 μmol·m−2·s−1 for certain hours (Fig. 1a), resulting in an integrated PPFD of 9.4 and 10.9 mol·m−2·day−1 in the control and LED treatment, respectively. The control had a higher PPFD than the LED treatment from 8:00 to 11:00 am, while the LED treatment had a higher PPFD than the control from 2:00 to 4:00 pm due to the effect of direct light. Conversely, on cloudy days (October 28, 2023), the PPFD dropped below 200 μmol·m−2·s−1 for several hours (Fig. 1b), leading to an integrated PPFD of 3.4 and 6.0 mol·m−2·day−1 in the control and LED treatment, respectively.
Daily photosynthetic photon flux density (PPFD) in the control and LED treatments on sunny and cloudy days.
Following transplantation, the mean anthesis dates varied by cultivar and treatment (Table 2). No significant difference was observed in the anthesis date between the control and LED treatments, but ‘Koi-ichigo’ was significantly later than other cultivars. Several ‘Koi-ichigo’ plants in the control started anthesis on October 18, but many other plants started in November. The LED treatment also started anthesis on October 26. ‘Koi-ichigo’ had a greater variation in anthesis than the other two cultivars. Yield during the growing period varied among cultivars; however, no interaction was observed between the LED treatment and cultivars regarding yield. The yield was significantly higher in the LED treatment compared to the control. Marketable fruit yield was also higher in the LED treatment compared to the control, with ‘KS33’ demonstrating significantly higher yields than ‘Koi-ichigo’. The number of harvested fruits was significantly greater in the LED treatment compared to the control, with ‘KS33’ showing significantly higher yields than ‘Koi-ichigo’. Conversely, fruit weight showed no significant difference between the control and LED treatment. Regarding cultivars, ‘Koi-ichigo’ fruits were significantly heavier than the other two cultivars. The fruit soluble solid content showed no significant difference between the control and LED treatment, with ‘KS33’ demonstrating the highest soluble solid content among cultivars, and ‘Koi-ichigo’ the lowest.
Anthesis date, yield, number of fruits, fruit weight, and soluble solid content (°Brix) in the control and LED treatment.
By the end of cultivation, there was no difference in TDW among cultivars; however, it surpassed the control in the LED treatment (Table 3). Fruit dry weight was significantly greater than the control in the LED treatment, but no significant differences were observed in the dry weights of other plant parts between the control and LED treatment. ‘Koi-ichigo’ had significantly greater crown and leaf dry weights compared to the other cultivars; however, it also had significantly lower fruit dry weights. An interaction between LED treatment and cultivar was observed for LAI. While the LAIs of ‘KS33’ were significantly larger in the control compared to the LED treatment, ‘Koi-ichigo’ and ‘KS38’ demonstrated no significant difference in LAI between the two treatments. SLA was significantly lower in the LED treatment compared to the control, with ‘KS38’ having a significantly higher SLA compared to the other two cultivars.
Dry matter weight of each part, leaf area index (LAI), and specific leaf area (SLA) at the end of cultivation in the control and LED treatments.
The observed LAI (LED_ob) increased in all treatments and cultivars from transplantation to the end of cultivation (Fig. 2). In ‘Koi-ichigo’ and ‘KS33’, the estimated LAI (LAI_est) increased more in the control than in the LED treatment following transplantation, and the difference between the control and the LED treatment increased until the end of cultivation. In the LED treatment of ‘Koi-ichigo’ and ‘KS33’, the LAI_est increased gradually from about 60 days after transplanting. Conversely, the LAI_ob of ‘KS38’ increased similarly in both the control and LED treatments.
Changes in leaf area index (LAI) and light interception (LI) during the cultivation period in the control and LED treatments; LAI:Ctrl_est: estimated daily LAI in the control; LAI:LED_est: estimated daily LAI in the LED treatment; LAI:Ctrl_ob: observed LAI in the control; LAI:LED_ob: observed LAI in the LED treatment; LI:Ctrl: calculated daily light interception in the control; and LI:LED: calculated daily light interception in the LED treatment.
Immediately after transplanting, the LAI of ‘Koi-ichigo’ increased in the control compared to the LED treatment. The difference in LI between the control and LED treatments up to 60 days after transplanting was smaller than for the other cultivars (Fig. 2). ‘KS33’ and ‘KS38’ demonstrated greater daily light interception in the LED treatment throughout the growing period compared to the control. The daily light interception during the harvest period showed similar trends among cultivars, with the lowest values ranging from 1.2–1.3 mol·m−2·d−1 in the control and 2.4–2.9 mol·m−2·d−1 in the LED treatment, while the highest values ranged from 5.0–5.4 mol·m−2·d−1 in the control and 5.8–6.7 mol·m−2·d−1 in the LED.
Cumulative light interception surpassed the control in the LED treatment in all cultivars (Fig. 3). LUE values, expressed as the slopes of the regression lines, for each treatment were as follows: 0.72 for ‘Koi-ichigo’ in the control; 0.78 for ‘Koi-ichigo’ in the LED treatment; 0.78 for ‘KS33’ in the control; 0.74 for ‘KS33’ in the LED treatment; 0.80 for ‘KS38’ in the control, and 0.79 for ‘KS38’ in the LED treatment.
Relationships between cumulative light interception and aboveground dry matter weight in the control and LED treatments. The slopes of the regression lines indicate light-use efficiency (LUE). (a) LUE values (95% confidence intervals) for ‘Koi-ichigo’ in the control and LED treatments were 0.63–0.80 and 0.58–0.97, respectively. (b) LUE values for ‘KS33’ in the control and LED treatments were 0.71–0.85 and 0.59–0.90, respectively (c) LUE values for ‘KS38’ in the control and LED treatments were 0.75–0.86 and 0.70–0.87, respectively.
The daily mean air temperature observed in both the control (ranging from 22.2 to 28.6°C) and LED treatment (ranging from 22.6 to 29.4°C) (Table 1) surpassed the optimal range (15–26°C) recommended for leaf production in both everbearing and June-bearing cultivars (Hancock, 2020). Nishiyama et al. (2009) highlighted that the critical day length for flower bud differentiation in everbearing strawberry cultivars is activated under high temperatures, such as 30/25°C (day/night); moreover, flower bud differentiation is hindered under shorter day lengths, with variations among cultivars ranging between 12 and 16 hours. Despite all three cultivars developing flower buds in this study, ‘Koi-ichigo’ showed significantly later anthesis initiation and yielded fewer harvested fruits than the other cultivars (Table 2). Sønsteby and Heide (2007) concluded that everbearing strawberry cultivars are qualitative long-day plants at high temperature (27°C). In this study, the daily mean air temperatures were close to 27°C, and the nighttime air temperature exceeded 25°C until October, so all cultivars were considered qualitative long-day plants. In this study, a daylength of about 12 hours was sufficient for flower bud differentiation for ‘KS33’ and ‘KS38’ because anthesis was initiated in October, even in the control. ‘Koi-ichigo’ had a later anthesis date than the other two cultivars, especially in the control, and there was greater variation among the plants, resulting in unstable flower bud differentiation. ‘Koi-ichigo’ may need a longer daylength for flower bud differentiation than 12 hours. In this study, no significant differences in the anthesis date were observed between the control and the LED treatment. Hamano et al. (2012) reported that in an everbearing strawberry cultivar, the longer the lighting period per day, the stronger the flower induction; in fact, flower bud differentiation and the number of inflorescences showed a lower increase a lower number in the long-day treatment. Wang et al. (2020) investigated anthesis in June-bearing cultivars and an everbearing cultivar using LED supplemental lighting from 8:00 to 16:00. They reported that flower development was enhanced by increasing the light intensity and was delayed by shading. Although the LED supplemental lighting did not accelerate the anthesis date in this study, the number of harvested fruits was significantly higher in the LED treatment than in the control. This were probably due to the effects of longer day length and increased PPFD because of nighttime LED supplemental lighting.
The elevated daily mean and nighttime air temperatures in the LED treatment (Table 1) are ascribed to the heat generated by LED supplemental lighting during the night. Nakayama et al. (2024) noted that LED lighting raised leaf surface temperatures by 1–2°C, and Nakayama and Nakazawa (2023) reported that daytime LED lighting elevated air temperatures in greenhouses, particularly under overcast conditions with low greenhouse temperatures. Kadir et al. (2006) found that photosynthesis did not decline significantly even under moderately high temperatures (30/25°C day/night). Although the conditions in this study differed from these reports because the crown and growth medium were cooled and photosynthetic rates were not directly measured, strawberry cultivation in a high-temperature environment could likely proceed without a substantial reduction in photosynthetic activity. Therefore, the increased air temperature due to LED supplemental lighting in this study did not affect strawberry growth.
To mitigate the effects of high temperatures, crown and growth medium cooling was implemented in this study. This resulted in a stable mean growth medium temperature below 23.8°C and a difference of 1.4°C from the daily maximum temperature, which was deemed effective for stabilizing flower bud differentiation. Hidaka et al. (2017) observed flower bud differentiation of a June-bearing cultivar ‘Fukuoka S6’ even at high temperatures (30/27°C) with the introduction of crown cooling. Additionally, Iwasaki (2007) noted that cooling the medium during high-temperature periods increased the number of fruits per cluster and overall strawberry yield in both June-bearing and everbearing cultivars. In our previous study, when strawberries were grown without the crown and growth medium cooling, the daily mean air temperature was 25.3°C, and the nighttime air temperature was 22.8°C, resulting in a mean growth medium temperature of 26.5°C (Nakayama and Nakazawa, 2023). In the present study, the mean growth medium temperature did not rise above 27.0°C, even at time of higher air temperatures, indicating the effect of the crown and growth medium cooling.
Yield and fruit qualityStrawberry yield demonstrated a significant increase in the LED treatment compared to the control across all three cultivars (Table 2), aligning with several reports indicating that LED supplemental light enhances yield (Hidaka et al., 2013, 2014; Nestby and Trandem, 2013). Marketable fruit yield also experienced an increase. In addition, the marketable fruit rate, which is marketable yield divided by total yield, was higher for ‘KS33’ than for the other two cultivars. The rise in harvested fruits with LED treatment implies that the augmented yield stemmed from a greater fruit count, consistent with findings on a June-bearing cultivar in the same subtropical setting (Nakayama and Nakazawa, 2023). However, fruit weight showed no significant difference between the control and LED treatment. Various studies have documented reductions in fruit weight under high temperatures (Menzel, 2019; Miura et al., 1994). Kumakura and Shishido (1994b) asserted that the duration of photosynthate translocation and distribution is a crucial determinant of fruit weight. In this study, the daily mean air temperature difference between the control and the LED treatment was less than 1°C throughout the growing period (Table 1), and this temperature difference did not seem to affect fruit weight. Hidaka et al. (2013) and Nestby and Trandem (2013) noted an increase in fruit soluble solid content with LED supplemental light, but here the results did not show a similar rise in soluble solid content. Wang and Camp (2000) reported that high temperatures decrease soluble solid content, with Mackenzie et al. (2011) concluding that rising temperature is a major factor responsible for the decline of soluble solid content in strawberry fruits in a subtropical production system. Kumakura and Shishido (1994a) suggested that lower mean air temperatures and extended ripening days contribute to higher soluble solid content levels, emphasizing that the duration of photosynthate supply determines soluble solid content accumulation in fruits. In this study, the daily mean air temperature difference between the control and the LED treatment did not affect soluble solid content accumulation in fruits. Although light interception was increased in the LED treatment (Fig. 2), the soluble solid content was not improved. The daytime PPFD (6:00–18:00) of the control was larger than that of the LED treatment on a clear day (Fig. 1a), and as a result, the difference in daily integrated PPFD was small even with the nighttime LED supplemental lighting, which may not have been enough to increase soluble solid content.
Dry matter production and LUEThe elevation in TDW observed in the LED treatment stemmed from an increase in fruit dry matter weight (Table 3), attributable to a rise in the number of fruits because of LED supplemental lighting (Table 2). The ‘KS33’ LED treatment had the highest dry matter distribution to fruit compared to the other two cultivars. Hidaka et al. (2013, 2014) reported that LED supplemental lighting facilitated enhanced carbohydrate allocation to bud primordia, thereby bolstering flower numbers in June-bearing strawberries. They concluded that high carbohydrate levels are pivotal for flower bud differentiation and development. In this study, the LED supplemental lighting operated for 3.5–5.5 hours post-sunset at approximately 200 μmol·m−2·s−1. Although Mochizuki et al. (2013) and Nakayama et al. (2024) examined a different cultivar, they reported photosynthetic rates of approximately 5–10 μmol CO2·m−2·s−1 at a PPFD of about 200 μmol·m−2·s−1 under base conditions of leaf temperature 25–30°C and CO2 concentration 350–400 μmol·mol−1 in greenhouses. Hence, the nighttime LED lighting in this study was determined to be sufficient to boost photosynthate levels, even after sunset. Although TDW did not differ significantly among cultivars, ‘Koi-ichigo’ had significantly higher crown and leaf dry weights (Table 3), likely due to delayed anthesis that reduced the fruit number (Table 2) and fruit dry weight.
A similar trend in leaf area was observed for ‘Koi-ichigo’ and ‘KS33’; LAI was greater in the control than in the LED treatment, and the difference increased with days after transplanting (Fig. 2). There was no difference in LAI for ‘KS38’ between the control and LED treatments. Lyu and Li (2014) reported that a June-bearing strawberry cultivar plant had a slightly greater leaf area because of flower thinning. The lower number of fruits in the control group than in the LED treatment group in this study may have influenced the LAI. Hidaka et al. (2014) conducted a study comparing June-bearing strawberries with LED supplemental lighting at 24-, 16-, 14-, and 12-h daily and under natural daylength (control). They reported no positive effect of supplemental lighting on leaf area; the leaf area of the plants in the 24- and 12-hour photoperiod treatments tended to be lower than those of plants in other treatments, including the control, from October to May in Kyushu, Japan. Conversely, leaf dry weight did not differ between the control and LED treatment, and the SLA at the end of cultivation was significantly greater in the control than in the LED treatment in this study (Table 3), aligning with previous findings (Hidaka et al., 2013; Nakayama and Nakazawa, 2023). SLA serves as a crucial index of leaf structure and thickness (Menzel and Smith, 2014). Nobel et al. (1975) reported that when the illumination was raised from 900 to 42,000 lux, the leaves more than tripled in thickness as the mesophyll cells increased in size in Plectranthus parviflorus Henckel. Hidaka et al. (2014) concluded that the lower SLA in plants cultivated under supplemental lighting resulted from leaf thickening caused by the increased accumulation of assimilation products originating from accelerated photosynthesis. Although leaf thickness was not measured in this study, it was assumed that the increased accumulation of assimilation products in the leaves in the LED treatment resulted in lower SLA.
Although the LAI increase during the growing period varied among cultivars, cumulative light interception surpassed that of the control in the LED treatment (Fig. 3). LUE did not differ between the control and LED treatment and was unaffected by the photoperiod or light interception. Therefore, TDW increased in the LED treatment because of the cumulative light interception. LUE is influenced by the photosynthetic rate, and high LUE helped produce a large amount of TDW and a high fruit yield (Mochizuki et al., 2024). Increased TDW has been reported to increase fruit yield in tomatoes (Higashide and Heuvelink, 2009), cucumbers (Maeda and Ahn, 2022), and sweet peppers (Homma et al., 2023) and is an important factor in increasing yield. In strawberry canopies, the photosynthetic rate at 400 μmol·m−2·s−1 of PPFD amounted to approximately 82% of that at 1,200 μmol·m−2·s−1, the light saturation point (Hidaka et al., 2013), suggesting increasing PPFD to 200 μmol·m−2·s−1 using nighttime LED supplemental lighting was sufficient to achieve an adequate photosynthetic rate. Hidaka et al. (2014) investigated the photoperiod of LED supplemental lighting on a June-bearing cultivar. They reported that flower bud initiation on the second inflorescence was inhibited by exceeding 12 hours per day of lighting. They concluded that the inhibition caused a decrease in leaf photosynthesis, leading to excessive carbohydrate accumulation in the leaves, resulting in a lower total fruit yield than that from plants in the 12 hour photoperiod treatment. To increase yield in strawberry cultivation, it is important to maintain a high rate of photosynthesis and stable flower bud differentiation. In this study, flower bud differentiation of ‘Koi-ichigo’ was not stable until November, even using the crown and growth medium cooling; increased PPFD and extended day length because of nighttime LED supplemental lighting stabilized flower bud differentiation and increased fruit number. Flower bud differentiation in ‘KS33’ and ‘KS38’ was stable under the high-temperature environment of Ishigaki island using crown and growth medium cooling, while LED supplemental lighting increased the TDW and fruit number, which resulted in a higher yield, due to the effect of increased PPFD. ‘KS33’ exhibited a higher marketable fruit rate and fruit partitioning compared to ‘KS38’. Further study is needed to determine the effectiveness of nighttime LED supplemental lighting on strawberries over a long period; even so, when growing everbearing strawberry cultivars in tropical and subtropical regions, nighttime LED supplemental lighting effectively stabilizes flower bud differentiation and increases TDW, resulting in increased yield.
The author is grateful to Ms. Kazuko Nishime of the Japan International Research Center for Agricultural Sciences for her help in creating the graphical abstract.
