Comparison of Supplemental LED Top- and Interlighting for Year-Round Production of Cherry Tomato

Abstract

Supplemental lighting is common in northern countries or during winter greenhouse tomato production. We investigated the effect of supplemental lighting treatments on cherry tomato (‘Jun-Ama’) yield, productivity (light-use efficiency (LUE) and energy-use efficiency (EUE)), and fruit quality under high irradiance (average greenhouse daily light integral (DLI) = 14.5 mol m⁻² d⁻¹). Supplemental lighting treatments contained average DLIs of 2.7, 4.9, and 7.6 mol m⁻² d⁻¹ for interlighting, toplighting, and inter- + toplighting, respectively. Supplemental LED lighting increased fruit yield by 18, 41, and 40% with inter-, top-, and inter- + toplighting, respectively, compared with the control. Interlighting increased fruit number (+11%), and top- and inter- + toplighting also increased the fruit number (+26%, +27%) and weight (+10%, +10%), respectively. LUE and EUE were comparable between inter- and toplighting, while inter- + toplighting decreased LUE by 21 and 38%, and EUE by 38 and 31% compared with inter- and toplighting, respectively. All LED supplemental treatments significantly increased total soluble solids compared with the control. Total acidity and lycopene content were unchanged in all treatments. In conclusion, LED supplemental lighting with inter- or toplighting improved cherry tomato yield and quality, but inter- + toplighting was inefficient under high irradiation.

1. Introduction

Light is a limiting environmental factor in year-round tomato production in protected cultivation. In general, a 1% light reduction reduces tomato production by 0.7–1% [1]. Therefore, supplemental lighting (SL) is commonly used in northern countries [2]. Moreover, the source-sink ratio of beefsteak, round, and cherry tomatoes is below one in greenhouse tomato production with SL; therefore, SL with high photosynthetic photon flux density (PPFD) promotes growth and yield, except in the early growth stage [3]. In fact, SL is also effective in areas with higher solar radiation conditions, such as the Mediterranean [4,5].

High-pressure sodium lamps (HPS) are used for toplighting in northern countries to improve the yield and quality [6] of the high-intensity output of photosynthetically active radiation [7]. Light has a strong gradient from the top of the greenhouse, and the high-wire plant canopy used in tomato production results in very low irradiance at the bottom of the canopy [8]. Many interlighting studies using LEDs were applied to overcome this disadvantage [4,7,8,9,10,11,12]. LED interlighting increases the assimilation of photosynthetic products of the whole plant by promoting photosynthesis in the lower leaves and preventing leaf senescence, resulting in higher yields [7,10,13,14]. This result was shown in relatively low-truss and high-density tomato cultivation [12,15,16,17].

Meanwhile, hybrid systems (HPS toplighting and LED interlighting) showed a 20% higher yield compared with only HPS toplighting [18]. However, the fresh yield did not differ in the three different treatments (1. HPS toplighting, 2. LED interlighting, 3. hybrid HPS toplighting and LED interlighting) under the same daily light integral (DLI) treatment: 3.8–12.5 mol m⁻² d⁻¹ [7,8]. Moreover, the fruit yield under LEDs was lower than that of HPS under different lighting treatments (PPFD 170 µmol m⁻² s⁻¹): 1. HPS toplighting, 2. LED toplighting, 3. hybrid HPS and LED toplighting, 4. hybrid HPS toplighting and LED interlighting [19]. Furthermore, the addition of LED interlighting to HPS toplighting decreased light-use efficiency [9]. However, these studies were experiments comparing HPS and LED, and no study measured the effect of SL using LED for both top- and interlighting. LED and HPS SL differentially affect photosynthetic capacity in young tomatoes [20]. Therefore, we examined the effects of different LED SL positions on cherry tomato fruit yield and quality under high irradiance.

2. Materials and Methods

2.1. Plant Material and Growth Condition

Cherry tomato cultivar ‘Jun-Ama’ (Suntory Flowers, Tokyo, Japan) was used in this experiment. The study was conducted at Kashiwanoha campus (Chiba University) between 1 September 2018 and 26 July 2019. The purchased grafted seedlings (rootstock; Green Force, Takii Seed Corporation, Kyoto, Japan) were transferred to a growth chamber (NAE Terrace; Mitsubishi Chemical Agri Dream Co., Ltd., Tokyo, Japan) with a temperature of 23/18 °C, CO₂ concentration of 1000 µmol mol⁻¹, light period of 14 h and light intensity of 280 μmol m⁻² s⁻¹ for 3 weeks. The seedlings without substrate were then transplanted into a greenhouse (54.0 m width, 41.5 m length, and 4.0 m height covered with polyolefin film from north and south building) at 3.5 plants per square meter. The irrigation system was a modified hydroponic technique (Spray-ponic, Kaneko seeds Co., Ltd., Gunma, Japan), and the plants were cultivated using commercial nutrient solution (OAT House fertilizer; OAT Agrio Co., Ltd., Tokyo, Japan; pH 6.3–6.9 and approximately 2.5 electrical conductivity). All lateral branches and leaves at lower positions were removed every week. The daily light integral (DLI, mol m⁻² d⁻¹) in the greenhouse was estimated based on a light efficacy of 2.2 μmol J⁻¹ and a facility light transmittance of 48%, determined by measuring solar radiation inside and outside the greenhouse before the experiment.

2.2. LED Supplemental Lighting

LED toplighting (Philips Green Power LED toplighting module DR/W_MB, 200 W and interlighting (Philips Green Power LED interlighting module DR/B, 79 W) were used for the SL treatment. The photosynthetic photon flux density of the top- and interlighting LED was 100 and 55 μmol m⁻² s⁻¹, respectively. Plants were supplied with one of the following lighting treatments: (1) Control: natural light without LED SL, (2) LED interlighting (3) LED toplighting at approximately 1.0 m above the top of the plants (4) LED inter and toplighting (Figure 1). The SL was started on 1 December 2018, and the lights were turned on for a maximum of 18 h per day. The amount of SL treatment was changed according to the solar radiation (Figure 2D).

2.3. Measurements

2.3.1. Plant Growth, Yield and Fruit Quality

The stem diameter near the first flowering truss and the number of leaves were measured every week from 28 December to 1 March. The leaf area index (LAI) was calculated by destructing the plant on 22 March. The fresh fruit weight was recorded from 28 December 2018 to 26 July 2019. The soluble solid content (SSC) and the total acidity (TA) were measured on 10 December 2018, 10 January, 10 February, 10 March, 9 April, 17 May, 17 June, and 19 July 2019. Lycopene content was measured on 4 March, 9 April, 17 May, 17 June, and 19 July 2019. Each replicate consisted of five average red ripe fruits from ten plants for each treatment, and three replicates were used in each measurement. Samples used for measurements were homogenized with a handheld blender to prepare a uniform mixture. The SSC and TA were measured using a refractometer (PAL-BX, Atago Co., Ltd., Tokyo, Japan). Lycopene was measured using a previous method [21].

2.3.2. Light-Use Efficiency and Energy-Use Efficiency

Light-use efficiency (LUE) (g mol⁻¹) was calculated as the ratio between the increase in fresh yield with LED treatment and the cumulative amount of PPFD received by the plants. Energy-use efficiency (EUE) (g MJ⁻¹) was calculated as the ratio between the increase in fresh yield with LED treatment and the cumulative energy consumption in MJ.

2.4. Data Analysis

Data were analyzed using version 2202 of the XLSTAT software (Esmi Co., Tokyo, Japan). Tukey’s multiple comparisons test was used for plant growth, yield, and fruit quality, while the Kruskal–Wallis test was used for LUE and EUE to evaluate significant differences.

3. Results

The average daily temperature and relative humidity was 18.6 °C and 71.1%, respectively, during the initial cultivation (Figure 2A,B). The DLI increased from February to the middle of June, and the average DLI was 14.5 mol m⁻² d⁻¹ during this experiment.

The stem diameter did not differ between different treatments (

Table 1. Morphological and fruit changes in cherry tomato grown under different supplemental lighting treatments.

Treatment	Stem Diameter (mm)	Number of Leaves	LAI (m2/m2)	Yield (kg/Plant)	Number of Fruits	Fruit Weight (g/Fruit)
Control	7.4	26.1 c *	1.8	2.4 c	233 c	10.4 b
Inter	7.7	28.0 b	1.7	2.8 b	258 b	11.0 ab
Top	7.6	29.9 a	1.5	3.3 a	294 ab	11.4 a
Inter × Top	7.4	29.2 ab	1.8	3.3 a	297 a	11.4 a

* Different letters indicate significant differences between treatments at p < 0.05 using the Tukey–Kramer test. LAI: leaf area index.

). Meanwhile, the number of leaves in the control was significantly lower than that in SL treatments, and the treatments with toplighting had higher values than those with interlighting. No differences in LAI were observed between treatments. Inter-, top-, and inter- + toplighting SL treatments increased yield by 18, 41, and 40%, respectively, compared with the control. Moreover, the increase in yield with SL was significant at 120–220 days after transplanting (DAT) (Figure 3), and the values at the end of supplemental light treatment (207 DAT) were 31, 65, 70% higher in inter, top-, and inter- + toplighting, respectively, compared with the control. The increase in yield with SL treatments was strongly related to fruit number with interlighting, and to fruit number and fruit weight with toplighting. No difference was found in the number of double trusses between control and SL treatments (data not shown).

The cumulative DLI of SL treatments was 408, 742, and 1150 mol m⁻² in inter-, top-, and inter + toplighting, respectively. Toplighting LUE was significantly higher than that of inter- + toplighting (Figure 4). Inter- + toplighting LUE decreased by 21% and 38% compared with inter- and toplighting, respectively.

The total amount of energy for SL treatments was 586, 1483, and 2069 MJ m⁻² in inter-, top-, and inter- + toplighting, respectively. Interlighting EUE was significantly higher than that in inter- + toplighting. Inter- + toplighting EUE decreased by 38% and 31% compared with inter- and toplighting, respectively.

SL treatments had a positive effect on total soluble solids (TSS) and the values were approximately 10% higher in SL compared with the control (Figure 5). Total acidity and lycopene content were unaffected by SL treatments.

4. Discussion

4.1. Effect of Different Supplemental Lighting Position on Yield

The essential DLI in tomatoes is 13–17.3 mol m⁻² d⁻¹ for young tomato plants at daily average temperatures of 18–22 °C, and 30–40 mol m⁻² d⁻¹ for adult plants at daily average temperatures of 23–27 °C [22]. In this study, the average winter DLI was 10.5 mol m⁻² d⁻¹ with a daily average temperature of 16.3 °C (November to February), while the average spring and summer DLI was 17.8 mol m⁻² d⁻¹ with a daily average temperature of 20.7 °C (March to July). Supplemental lighting technology has not progressed in Japan because the DLI exceeds 30 mol m⁻² d⁻¹ on some days. However, due to the low winter radiation and positive SL effects at high radiation levels [4,5], the adoption of SL technology is likely to increase. The positive effect of SL on yield was confirmed in this study. The increase in yield due to SL was attributed to the increase in fruit number with interlighting and the increase in fruit number and fruit weight with toplighting (Table 1). Meanwhile, there was no difference in yield between toplighting and inter- + toplighting. The average DLI (natural light + SL) was 17.3 mol m⁻² d⁻¹ for toplighting and 18.8 mol m⁻² d⁻¹ for inter- + toplighting, which is lower than the essential DLI of 30 mol m⁻² d⁻¹ for medium tomatoes in previous studies [22]. In previous studies, SL increases fruit number [7,23]. Cherry tomatoes have a lower source-sink ratio than medium tomatoes, but source-sink ratio does not affect photosynthesis [24,25]. Furthermore, this study showed that stem diameter, (a measure of vegetative growth) was not significantly different among treatments, suggesting that other factors may have limited the inter- + toplighting results. Increases in solar radiation increase the optimal temperature [26], and planting density and carbon dioxide concentration are limiting factors for yield increase under high solar radiation [27].

4.2. Light- and Energy-Use Efficiency at Different Supplemental Lighting Positions

Energy for SL, including electricity, may constitute 50% of the total production cost [28]. Therefore, appropriate SL techniques are important for profitable year-round tomato cultivation. Light-use efficiency represents the ability to obtain dry-matter production per unit of light received in a plant, and varies with environmental factors, especially carbon dioxide concentration [29]. There was no difference in yield between HPS toplighting and LED interlighting in previous studies at the same DLI (9 mol m⁻² d⁻¹) [8]. Furthermore, no significant difference in LUE was observed between interlighting and toplighting in this study. Meanwhile, adding LED interlighting to HPS toplighting decreases LUE [9]. In this study, the LUE of inter- + toplighting decreased by 21% and 38% compared to inter- and toplighting, respectively, indicating that LUE decreases as the DLI of SL increases in the hybrid type.

In recent years, SL using HPS was replaced with LEDs due to their efficiency: HPS, 1.7–2.1 µmol J⁻¹; LED, 2.6–3.3 µmol J⁻¹ [30,31]. LED interlighting results in a higher EUE than HPS toplighting [8]. In this study, no difference in EUE was identified between inter- and toplighting with LEDs, possibly due to a lower LAI. In general, interlighting is more effective under high LAI conditions, such as high-wire cultivation. Past experiments reported a tomato cultivation EUE of 2.8–4.0 g MJ⁻¹ [32]. In our results, the EUE values ranged from 1.6 to 2.6 g MJ⁻¹, suggesting that the low EUE was due to low yields. In this study, the average yields with LEDs were 11.0 kg m⁻², which is approximately half of the average yield (21.6 kg m⁻²) in the same growing cycle experiment conducted in the Mediterranean [4]. Therefore, it is necessary to use high-yielding varieties to increase the EUE of SL from LEDs.

4.3. Effect of Different Supplemental Lighting Position on Quality

Tomato quality (sugar content, acidity, and carotenoids) is affected by light intensity and quality [33,34,35,36]. LED interlighting increases sugar content by 16% [37]. Meanwhile, in other studies, LED interlighting showed no effect on sugar content [5,38]. A meta-analysis of 38 observations revealed that LED interlighting increases sugar content by 6% [10]. Interlighting with LEDs showed an improvement in quality compared to toplighting with HPS [33]. In this study, quality was unaffected by the location of the SL and LED SL increased sugar content by approximately 10%.

5. Conclusions

In summary, supplemental lighting with LEDs enhanced fruit yield and quality (total soluble solids) compared with the control without SL. On the other hand, acidity and lycopene content were unaffected by SL treatments. In SL efficiency, there was lower light- and energy-use efficiency with inter- + toplighting than with inter- and toplighting based on the radiation level of the test region.