Effects of Supplemental LED Interlighting Irradiance Position on Photosynthesis and Aboveground Dry-Matter Weight Accumulation in Tomatoes Under Dense Planting

Abstract

Supplemental interlighting is a technique to improve horticultural light conditions. However, optimal methods for energy-efficient supplemental lighting are not yet established. Therefore, this study investigated the influence of supplemental light canopy position during the tomato fruit enlargement stage on photosynthetic function and aboveground dry-matter weight. A supplementary interlighting module was fixed at the initial irradiation position, then the irradiation position for three other modules were raised to 10 cm above each fruit truss at different developmental stages. These stages were the early enlargement stage (ES), from flowering until the first fruit reached a diameter of 10 mm, then the vigorously enlarging stage (VES), with tomato fruit diameter from 10 to 30 mm, and the late enlargement stage (LS) with a tomato fruit diameter greater than 30 mm. Cultivation was carried out using a D-tray system with a planting density of 5.5 plants·m⁻². The LED supplemental interlighting reduced specific leaf area (SLA), altering the plant canopy structure. This increased the canopy light transmittance from 40% to 70% at 20 cm from the canopy and from 20% to 40% at 40 cm from the canopy, especially during the ES. The total chlorophyll (Chl) content of leaves was higher under all irradiated treatments compared to the untreated control. However, Chl a/b ratios decreased for all treatments except in leaves under continuous LED irradiation. The maximum photosynthetic rate was higher in leaves closer to the supplemental interlighting exposure, but was lower in the 17th and 13th leaves at 6 μmol·m²·s⁻¹ and 4 μmol·m²·s⁻¹, respectively. Fruit dry-matter weight increased significantly to 143.2–156.5 g in all supplemental interlighting treatments compared with 119.6 g for the control. Interlighting treatment during VES achieved the highest yield and the greatest increase in fruit and total dry-matter weight. Therefore, VES-irradiation is most efficient to increase dry-matter weight.

Introduction

Plant growth and yield depend on photosynthesis, with available light a limiting environmental factor (Hao and Papadopoulos, 1999). Therefore, improved light environments are expected to increase crop yields.

In greenhouses, supplemental lighting is one approach to improve light conditions. For densely cultivated plants, improving light availability for the middle or lower plant canopy layers, where light is lacking, can also increase crop productivity. Furthermore, interlighting, the use of supplemental lighting within the canopy, can increase tomato fruit yield and quality (Tewolde et al., 2018). However, supplemental lighting requires energy input, and, for dry-matter production, optimizations are required to ensure energy efficiency. Poor supplemental lighting efficiency will likely lose benefits through sub-optimal growth improvements coupled with energy costs. Energy efficiency improvements in supplemental lighting require multi-focused evaluations of timing, photoperiod, position, intensity, and supplemental light wavelength (Merrill et al., 2016). Accordingly, we will optimize interlighting use for dry-matter production in the Japanese environment.

In previous studies, we compared the effects of day- and nighttime interlighting (Tewolde et al., 2016) as well as differences between illumination from below and from interlighting (Jiang et al., 2017). In a study evaluation timing in supplemental lighting, the highest fresh and dry fruit yields of densely planted single-truss tomato plants were obtained from plants with the greatest cumulative leaf irradiance from supplemental interlighting between the setting of fruit and the mature green stage (Lu et al., 2012a). Few reports are available on the effects of interlighting irradiance position for continuous cultivation.

In addition, the translocation and distribution of photosynthates are dictated by sink strength, determined by sink size and activity (Marcelis, 1996; Nakano et al., 2008). Source-sink relationships for each photosynthate-generating leaf are generally with the nearest non-photosynthetic organs (Yoshioka and Takahashi, 1984). Furthermore, the photosynthate distribution pattern is determined by competition and positional relationships between sinks, and is influenced by differences in the degree of development between sinks at each crop growth stage (Shishido et al., 1991). In tomatoes, after fruit set, the enlarging fruits represent the primary sink organs (Scholberg et al., 2000; Wardlaw, 1990).

In this study, the supplemental interlighting irradiation positions were varied according to fruit enlargement stage to improve dry-matter production energy efficiency for tomatoes under six-truss dense cultivation. These effects were evaluated by photosynthetic function and dry-matter partitioning in the canopy.

Materials and Methods

Test site and cultivation method

Plants were grown and experiments performed in the demonstration greenhouses (13.5 m wide and 40 m deep) at the Plant Factory Mie (Mie, Japan). Two low-volume, high-density culture plots (Masuda et al., 2013) were used as untreated and supplemental light treated plots.

On September 24, 2019, seeds of the tomato variety (Solanum lycopersicum L.) ‘CF Momotaro York’ (Takii & Co., Ltd., Kyoto, Japan) were sown, and seedlings were grown for 21 days in a closed seedling production system Naeterasu^TM (Mitsubishi Chemical Agri Dream Co. Ltd., Tokyo, Japan) with artificial light. After being raised in this closed system, seedlings were transplanted into D-trays (60 cm long, 20 cm wide, 10 cm high, 250 mL/pot, 2 rows of 10 pots) filled with rockwool for 10 days.

After transplanting, 160 seedlings were set in the untreated plot and then in four test sections of the supplemental lighting plot, 160 seedlings were each placed, for a total of 740 seedlings. All seedlings were set in the D-tray cultivation system (Zhang et al., 2015), with a bet spacing of 150 cm, row spacing of 45 cm, planting distance of 25 cm, and planting density of 5.5 plants·m⁻². The planting took place on October 25, 2019.

The culture solution was supplemented with EC 1.0 to 2.5 dS·m⁻¹ (16 me·L⁻¹ N, 4 me·L⁻¹ P, 8.0 me·L⁻¹ K, 8 me·L⁻¹ Ca, 4 me·L⁻¹ Mg) depending on the growth. Irrigation was provided using the repurposed drainage method (Isozaki et al., 2006). Irrigation was supplied so that the volume of solution drained from the pots did not reach zero in a single irrigation round. The culture medium was refreshed once per day at night.

An Akisai (Fujitsu Limited, Tokyo, Japan) system was used to control the environment of the treatment plots. The greenhouse target temperature was set to a minimum of 12°C, a maximum of 25°C, and maintained at a minimum of 20°C during daytime. The CO₂ concentration was set at a target of 400 ppm after planting and supplied at 36 g·m⁻²·day⁻¹ after November 15, 2019, using a combustive CO₂ generator (Z04036; Furuta Electric Machinery Co., Ltd., Aichi, Japan). For environmental measurements, a temperature and humidity sensor (TC(1)-005; Eiritu Equipment Technology Co., Ltd., Fukuoka, Japan) was used for temperature and relative humidity, an integrated solar radiation sensor (RAD(1)-053; Eiritu Equipment Technology Co., Ltd., Fukuoka, Japan) for integrated solar radiation, and a CO₂ concentration sensor (K30; Senseair AB, Delsbo, Sweden) for CO₂ concentration. All measurements were performed at 5-minute intervals.

Figure 1A shows a schematic representation of a representative tomato plant used in this experiment.

Fig. 1

Diagram of tomato plants used in this experiment (A) and position of supplemental interlighting (B). ^z Leaf position on the main branch. ^y Fruit truss. * Photo (B) was taken on December 9, 2019. Supplemental interlighting modules are installed at the fruit truss positions shown. Tr2 (left) and Tr1 (right). The arrows indicate the positions of the installed supplementary interlighting systems.

Plants were managed under six-truss cultivation, with leaf removal carried out by oblique pulling after December 24, 2019. Eight leaves from the first flower node were removed by oblique pulling. On January 27, 2020, at the end of the harvest of the first fruit truss, leaves were removed to just below the second fruit truss. On January 10, 2020, the plants were pinched to two main leaves just above the sixth fruit truss. Fruit thinning was carried out to obtain up to four fruits per fruit truss. Fruit set was facilitated by treatment with 4-chlorophenoxyacetic acid.

Setting the positions of LED interlighting modules

LED illumination (2.5 m, 200 V, 79 W, light output: 220 μmol·s⁻¹; Philips Green Power LED Interlighting module; Signify, Eindhoven, Netherlands) was installed for supplemental lighting between plants in D-trays in the supplemental lighting treatment plots. In one section of the supplemental interlighting treatment plot, the illumination was set to 7.5 m using three LED modules, and 3 replications were performed.

Supplemental irradiance was carried out from 7:00 to 17:00 for 10 h. Irradiance treatments were performed from November 15, 2019, on the first flowering date of the first flower truss to April 8, 2020, at the end date of harvest.

Table 1 shows the supplemental lighting positions and repositioning dates for each treatment plot as well as flowering date and leaf numbers for each flower truss. A photo depicting the experimental system is shown in Figure 1B. Table 2 shows the enlargement stages of each fruit truss. The LED module was positioned 10 cm above the first flower truss (Tr1, approximately 80 cm above the D-tray) at the beginning of irradiance treatments.

Table 1

Irradiance position of supplemental interlighting, flowering stage, and leafing number.

Table 2

Status of enlargement stage in each fruit truss.

Four supplementary interlighting treatment plots were set up. One fixed-position (Fixed) supplementary interlighting treatment plot was arranged with the light apparatus in the initial irradiance position. The three-remaining supplementary interlighting treatment plots were set up according to the following three fruit enlargement stages: 1) an early stage (ES) in which the position was shifted to the immediately superior flower truss at 12–14 d after the first flowering of each flower truss (i.e., the stage from the flowering of the first flower until it reached a fruit diameter of 10 mm). 2) a vigorous enlargement stage (VES) in which the position was shifted to the immediately superior flower truss at 25–28 d after the first flowering of each flower truss (stage of fruit diameter growth from 10 mm to 30 mm), and 3) a late stage (LS) in which the position was shifted to the immediately superior flower truss at 39–41 d after the first flowering of each flower truss (fruit diameter growth beyond 30 mm). The LED modules were positioned 10 cm above each truss, except in the Fixed position plot, after leaf removal via oblique pulling. The LED modules in the Fixed plot were positioned behind the third truss after leaf removal via diagonal pulling. Supplementary interlighting illuminated approximately three leaves above and below the module (Fig. 1A). An untreated plot was used as control (Control). Repositioning of the interlighting apparatus continued up to the fourth fruit truss.

Measurement of light transmission within the plant canopy

To evaluate the plant canopy structure in each treatment plot, the relative light intensity at the central part of each leaf group was measured between 11:00 and 13:00 without supplementary interlighting. Two PPFD sensors (LI-190SA; LI-COR Biosciences, Inc., Lincoln, NE, USA) were used to simultaneously measure the light intensity incident on the upper part of the leaf canopy and the mean light intensity for 15 s near the back of each flower truss and the ground in the canopy. The value obtained by dividing the latter by the former was taken as the intracanopy light transmittance. The investigation was conducted on December 18, 2019, before leaf removal, and on January 23, 2020, after pinching with three locations measured in each plot.

Determination of chlorophyll concentrations

The 13th leaves (between the second and third fruit trusses) and the 17th leaves (below the fourth fruit truss) from individual canopies were collected in triplicate, and 8-mm diameter discs from central portions of each of the first, third, fifth, seventh, and ninth leaflets from the tip were excised using a leaf punch (Fujiwara Scientific Co., Ltd., Tokyo, Japan), avoiding the inclusion of the central leaf veins as much as possible. These leaf discs were placed in test tubes to which 6 mL of N,N-dimethylformamide (Fijifilm Wako Pure Chemical Corporation, Tokyo, Japan) was added, and then left standing for 24 h in a dark low-temperature storage room at 6°C to extract chlorophylls (Chl). Then, 1 mL of the resulting Chl extract was dispensed into a spectrophotometer (U-best, 30; JASCO Corporation, Tokyo, Japan) cell to measure absorbance at 646.8, 663.8, and 750.0 nm. Chl a and b contents per leaf area were calculated by substituting these absorbance measurements into the equation provided by Porra et al. (1989). This part of the study was conducted on January 20, 2020 (upon flowering of the sixth flower truss) with three replicates per plot.

Measurement of photosynthetic rates

Photosynthesis potentials were measured using the 13th (between the second–third fruit truss) and 17th leaves (just above the fourth fruit truss) inside the canopy, and the eighth and ninth leaflets from the apical leaflet (Nada et al., 2018). The maximum photosynthetic rate was measured using a photosynthesis analyzer (LCpro-SD; Eko Instruments Co., Ltd., Tokyo, Japan) at the center of three leaflets during cloudy weather between 10:00 and 15:00 on January 24 and 27, 2020. The environment inside the chamber during measurement comprised a PPFD of 1,000 μmol·m⁻²·s⁻¹, CO₂ concentration of 400 ppm, temperature of 25°C, and partial pressure of water vapor of 8 mbar.

Growth measurements

The harvest survey was evaluated by fruit yield, number of fruits, weight per fruit, and dry-matter weight for each fruit truss, with three replicates of eight plants per plot. Also evaluated was stem length, stem weight, leaf weight, leaf area, stem dry weight, and leaf dry weight with three replicates of five plants per plot. Leaf area was measured using a leaf area meter (LI-3100, LI-COR Biosciences, Inc., Lincoln, USA), leaf area index (LAI) was calculated by multiplying leaf area by planting density, and specific leaf area (SLA) was calculated by dividing leaf area by leaf dry-matter weight.

Statistical analysis

Statistical analysis was performed using R software version 4.1.2. Multiple tests for proportion data (light transmission rate and dry-matter distribution), were performed after applying inverse sine transformation.

Results

Greenhouse environment

The mean daily solar radiation, mean daily temperature, and mean CO₂ levels in the untreated and supplemental light-treated plots after planting were controlled at 2.9 MJ·m⁻², 17.4°C, 1,032 ppm, and 2.8 MJ·m⁻², 17.5°C, and 1,050 ppm, respectively.

No significant difference was found on the flowering date of each truss in each treatment (Table 1) and no significant difference was found on the harvest date of each truss in each treatment (not shown).

Plant canopy structure and light environment

The plant canopy percentage light transmittance is shown Figure 2. Supplementary LED interlighting increased light transmittance from 17% to about 65% of the Control at 125 cm (35 cm below the crown, third flower truss) and from 6% to about 19% at 100 cm (60 cm below the crown, second flower truss) in all plots treated with supplementary light on December 18, 2019, at the prior stage to stem pinching and leaf removal.

Fig. 2

Effects of different supplemental interlighting positions on percentage light transmission. (A: Before stem pinching and leaf removal, B: After stem pinching and leaf removal) The surveys were conducted on A: December 18, 2019 and B: January 23, 2020 without supplementary interlighting. 0 cm refers to the ground level, 160 cm refers to just above the plant canopy and the position between them refers to near the flower truss. * Means with different letters are significantly different (P < 0.05) as determined by the Tukey-Kramer test (n = 3). Control, no supplementary lighting treatment; Fixed, supplementary interlighting at a fixed position; ES, early stage; VES, vigorously enlarging stage; LS, late stage

On January 23, 2020, after the stem pinching and leaf removal stage, the percentage light transmission in the canopy during the ES in which the supplementary light apparatus had been repositioned upwards from about 40% to 70% at 140 cm (20 cm below the crown, sixth flower truss) and from about 20% to 40% at 120 cm (40 cm below the crown, fifth flower truss) compared with the canopy of the Control and other supplemental interlighting plots. There were no differences in percentage light transmission between the other supplemental interlighting plots and the Control.

Chl content

The total Chl content of the 13th and 17th leaves are shown in Figure 3. Total Chl content increased from 24.9 μg·cm⁻² to 28.9–31.3 μg·cm⁻² in the 17th leaves and from 23.0 μg·cm⁻² to 27.9–31.3 μg·cm⁻² in the 13th leaves compared with the plants in all other supplemental interlighting treatments as well as the Control. For the 13th leaf, plants subjected to the longest cumulative supplemental interlighting treatment in the Fixed treatment exhibited increased total Chl content of 31.3 μg·cm⁻² compared to 27.9 μg·cm⁻² for the ES treatment, which was exposed to the shortest cumulative supplemental interlighting treatment.

Fig. 3

Effects of different positions of supplemental interlighting on total chlorophyll contents. This survey was conducted on January 20, 2020, and LEDs were positioned at Tr2 in the Fixed supplemental interlighting treatment, Tr4 in the ES supplemental interlighting treatment, Tr4 in the VES supplemental interlighting treatment, and Tr3 in the LS supplemental interlighting treatment. Bars indicate standard error of the mean (n = 3). * Means with different letters are significantly different (P < 0.05) as determined by the Tukey-Kramer test. Control, no supplementary lighting treatment; Fixed, supplementary interlighting at a fixed position; ES, early stage; VES, vigorously enlarging stage; LS, late stage

The Chl a/b ratios of the 13th and 17th leaves are shown in Figure 4. The 13th leaf selection received the longest cumulative supplemental interlighting treatment in the Fixed treatment and received supplemental interlighting treatment until 13 days prior to the survey date. The LS treatment increased total Chl a/b ratio to 2.7 compared to 2.4 for those in the Control and ES treatment groups. There was no significant difference in the Chl a/b ratio for the 17th leaf group.

Fig. 4

Effects of different positions of supplemental interlighting on chlorophyll a/b ratios. This survey was conducted on January 20, 2020, and the LEDs were positioned at Tr3 in the Fixed supplemental interlighting treatment, Tr4 in the ES supplemental interlighting treatment, Tr4 in the VES supplemental interlighting treatment, and Tr3 in the LS supplemental interlighting treatment. Bars indicate standard error of the mean (n = 3). * Means with different letters are significantly different (P < 0.05) as determined by the Tukey-Kramer test. Control, no supplementary lighting treatment; Fixed, supplementary interlighting at a fixed position; ES, early stage; VES, vigorously enlarging stage; LS, late stage

Photosynthetic rate

The maximum photosynthetic rates of the 13th and 17th leaves are shown in Figure 5. In the 17th leaf, the VES supplemental interlighting treatment increased the maximum photosynthetic rate to 6.4 μmol CO₂·m⁻²·s⁻¹, compared with 4.5 and 4.9 μmol CO₂·m⁻²·s⁻¹ for the Control and Fixed, respectively. For the 13th leaf group, the maximum photosynthetic rate increased to 4.1 μmol CO₂·m⁻²·s⁻¹ for the Fixed treatment, and 3.7 μmol CO₂·m⁻²·s⁻¹ for the LS treatment, compared with 2.3 μmol CO₂·m⁻²·s⁻¹ in the Control.

Fig. 5

Maximum photosynthetic rate of each leaf at different supplemental interlighting positions. The survey was conducted on January 24, 2020 and January 27, 2020, when the LEDs were positioned at Tr3 in the Fixed supplemental interlighting treatment, Tr4 in the ES supplemental interlighting treatment, Tr4 in the VES supplemental interlighting treatment, and Tr4 in the LS supplemental interlighting treatment. Bars indicate standard error of the mean for photosynthetic response measured on individual leaves (n = 3). * Means with different letters are significantly different (P < 0.05) as determined by the Tukey-Kramer test. Control, no supplementary lighting treatment; Fixed, supplementary interlighting at a fixed position; ES, early stage; VES, vigorously enlarging stage; LS, late stage

Growth survey

The means of stem length, LAI, and SLA are shown in Table 3. Stem length was 233.8–247.8 cm, significantly shorter in all supplementary interlighting treatments compared with 271.5 cm in the Control. LAI was significantly decreased to 6.9–7.5 m²·m⁻² in all supplemental interlighting treatments compared with 7.9 m²·m⁻² in the Control. The ES treatment showed the greatest reduction in LAI of the supplementary interlighting treatments. SLA, an indicator of leaf thickness, was decreased significantly to 190–198 cm²·g⁻¹ in all supplemental interlighting treatments compared with 239 cm²·g⁻¹ in the Control.

Table 3

Effects of supplemental interlighting at different irradiance positions on stem length, leaf area index, and specific leaf area.

Dry-matter weights of leaves, stems, and fruits, dry-matter distribution percentages, yield, the number of harvested fruits, and weight per fruit are shown in Table 4. Fruit-dry matter weight was significantly increased to 143.2–156.5 g in all supplemental interlighting treatments compared with 119.6 g in the Control. The VES supplemental interlighting treatment showed the greatest increase in fruit dry-matter weight (156.5 g) of the supplementary interlighting treatments. Stem dry-matter weight was significantly increased to 42.5–45.0 g in all supplemental interlighting treatments compared with 37.0 g in the Control. Leaf dry-matter weight was also significantly increased to 64.3–70.9 g in all supplemental interlighting treatments compared with 59.1 g in the Control. The LS supplementary interlighting treatment resulted in increased leaf dry-matter weight compared with the Fixed and ES treatments, and the VES supplementary interlighting treatment increased leaf dry-matter weight compared with the ES treatment. Thus, the total dry-matter weights of fruit, stem, and leaf were increased significantly at 254.8–271.4 g in all supplemental interlighting treatments compared with 215.7 g for the Control. Among the supplementary interlighting treatments, the VES treatment showed the greatest increase in total dry-matter weight at 271.4 g. The fruit dry-matter distribution ratio significantly increased to 58.0% in the ES treatment compared with 55.5–56.2% in all other treatments except for VES. The leaf dry-matter distribution ratio was significantly decreased to 25.3% in the ES treatment compared with 26.5–27.4% in all other treatments except VES. The VES treatment decreased the leaf-dry matter distribution to 25.8% compared with 27.4% and 26.9% in the Control and LS treatments, respectively. There were no significant differences in the stem dry-matter distribution ratios.

Table 4

Effects of supplemental interlighting at different irradiation positions on dry-matter weight, dry-matter distribution, yield, number of fruits, and single fruit weight.

Fruit yields showed the same trend as fruit and total dry-matter weight. There were no significant differences in the number of harvested fruits, but weight per fruit increased from 137.8 to 143.4 g in all treatments compared with 119.0 g in the Control. Fruit-dry matter weight by fruit truss is shown in Figure 6. Fruit dry-matter weights for the first, second, and sixth fruit trusses increased in all supplemental interlighting treatments compared with the Control. In the VES treatment, which had the highest fruit dry-matter weight, the fruit dry-matter weight increased to 25.7 g in the third fruit truss compared with 19.4 g in the Control, and increased to 27.3 g in the fifth fruit truss compared with 18.7 and 21.5 g in the Control and Fixed treatments, respectively.

Fig. 6

Effect of supplemental interlighting at different irradiation positions on fruit dry matter weight by fruit truss. * Means followed by different letters are significantly different (P < 0.05) as determined by the Tukey-Kramer test (n = 3). Control, no supplementary lighting treatment; Fixed, supplementary interlighting at a fixed position; ES, early stage; VES, vigorously enlarging stage; LS, late stage

Discussion

Relationship between supplemental interlighting irradiation position and leaf position

Regarding the leaves for which Chl and maximum photosynthetic rate were measured in the present study, it can be inferred from Figure 1A and Table 1 that the 13th leaf was exposed to supplemental interlighting from before leafing until it changed position to Tr3. The supplemental interlighting was considered to directly irradiate the leaves during the entire period from the start of supplemental interlighting (15-Nov) until measurement in Fixed, for ES from the start period until December 10, in VES from the start period until December 24, and in LS from the start period until January 7. Meanwhile, the 17th leaf was exposed to supplemental interlighting after module repositioning to Tr3. The supplemental interlighting was considered unlikely to have directly irradiated the leaves in Fixed, but was considered to have done so after December 10 in ES, after December 24 in VES, and after January 7 in LS.

Low light conditions and morphogenesis

The light transmission through the plant canopy is a limiting factor for plant growth in high-density planting as mutual shading between plants lowers transmission and decreases light levels (Okano et al., 2001a). This issue worsens in winter with low sunlight intensity and shallow angles (Gunnlaugsson and Adalsteinsson, 2006). The percentage of light transmission in a plant canopy under long-term multilevel cultivation conditions at planting densities of 2.5 plants·m⁻², typical of greenhouse vegetable management, decreases from the top of the plants to the ground and can be less than 40% at 100 cm below the top of the plant canopy (Hisaeda et al., 2007). In the present study, at 100 cm above ground and 60 cm below the top of the plants before pinching, the light transmission rate was less than 6% in the untreated plot and less than 20% in the supplemental interlighting treatment plots. After pinching, the percentage light transmission was about 10% at 100 cm above the ground and 60 cm below the canopy and was only 15% even in the plot with the highest percentage of light transmission, the ES treatment (Fig. 2). This decreased light transmission was likely due to the mutual shading of leaves caused by dense planting. In tomatoes under dense planting, light is very limited within the plant canopy, with the LED supplemental interlighting treatments reducing SLA (Table 3) and increasing light transmission compared with Control.

The percentage of light transmission was highest in the ES treatment, in which leaves at the undeveloped stage, up to the fourth or fifth flower trusses, were irradiated with supplemental interlighting.

Light intensity influences plant morphogenesis, increasing stem length and LAI under shaded conditions (Diaz-Perez, 2013; Kittas et al., 2012). The development of palisade cells and spongy mesophyll are weakened under PPFD below 300 μmol·m⁻²·s⁻¹ (Fan et al., 2013). In the present study, stem lengths shortened and LAI decreased with supplemental LED interlighting, an effect most clearly observed in the ES treatment. In addition, SLA was reduced by supplemental LED interlighting compared with the Control (Table 3). This suggests that supplemental light suppressed internode elongation and leaf size within the light-limited plant canopy. Thus, supplemental interlighting can alter plant canopy morphogenesis, thus influencing light transmission.

Effects of cumulative supplemental interlighting on photosynthetic function

Light limitation adversely affects photosynthesis with low light levels reducing photosynthesis (Naumburg and Ellsworth, 2002) and photosynthetic pigment decomposition (Aldesuquy et al., 2000). In tomato plants grown under long-term multi-stage cultivation, the leaf Chl a/b ratio decreased from the upper to lower layers of the canopy, reflecting the decreased photosynthetic capacity of individual leaves and demonstrating shade acclimatization of the middle and lower canopy layers (Takayama et al., 2006). The maximum photosynthetic rates of the 10th, 15th, and 18th leaves, counted from the uppermost shoot apex decreased to 76%, 37%, and 18%, respectively, compared with the 5th leaf (Xu et al., 1997). This decrease in the maximum photosynthetic rate of older leaves may be attributed to a decrease in content, rather than RuBisCO activity, as soluble protein content may be lower in older leaves than in younger ones (Xu et al., 1997).

The total Chl content of leaves from all supplemental interlighting treatments was higher than those of the Control, and the total Chl content of the 13th leaf group was higher in leaves (Fixed) exposed to supplemental light for a longer period closer to the light source compared with leaves in ES farther from the light source (Fig. 3). The Chl a/b ratios in the 13th leaf group in the Fixed and LS treatments were higher than those in the Control and the ES treatment (Fig. 4). However, even with supplemental interlighting under dense culture, the total Chl content was about 20% lower and the Chl a/b ratio was about 40% lower compared with the lower leaf layer in a tomato canopy grown continuously under long-term multilevel cultivation without supplemental light (Takayama et al., 2010).

Furthermore, although low, the maximum photosynthetic rate was higher (Fig. 5) in leaves (the 17th leaves in ES and VES, the 13th leaves in Fixed and LS) exposed to longer cumulative irradiance closer to the supplemental interlighting instrument than those located further. This suggests that leaves in a light-limited canopy under dense planting experience low light conditions early in leaf development, and transformation to shade leaves occurs to acclimate to low-light conditions. Under these low-light conditions, supplementary interlighting may suppress shade leaf transformation, as evidenced by the increase in total Chl content (Fig. 3) and the decrease in SLA (Table 3). However, while these results applied to all supplemental lighting treatments, they are limited in the degree of shade leaf transformation control. In other words, irradiation timing is critical.

Effect of interlighting irradiance position on fruit dry weight

There is a linear correlation between light received and plant growth (Lu et al., 2012a, b). A model for dry-matter weight as a function of light received, temperature, CO₂ concentration, and LAI is also reported (Saito et al., 2020).

In the present study, fruit dry-matter weight was increased in all supplemental interlighting treatments compared with the Control. Among the supplementary interlighting treatments, the VES treatment showed the greatest increase (Table 4) in fruit dry-matter weight. For each supplementary interlighting treatment, the cumulative irradiance received by plants and interlighting light intensity were constant. For the ES treatment, the supplemental interlighting system was installed above the canopy at an early stage, so the proportion of light provided to the early leafing stage was high, with the cumulative supplemental interlighting transmitted through the canopy speculated to be comparatively high. However, fruit dry-matter weight increased in the ES treatment as well as the other supplemental interlighting treatments. This suggests that leaf canopy morphological adaptation to the light conditions may increase light usage efficacy, thus explaining the increased fruit dry-matter weight.

The increase in fruit dry-matter weight due to the supplemental light treatment was the result of an increase in fruit weight per fruit, not in the number of harvested fruits (Table 4). The amount of photosynthetic assimilates partitioned into fruits can be determined by fruit sink strength, number of fruits, and whole-plant carbon fixation (Higashide and Heuvelink, 2009). In conditions where light level is not a limitation, yield is limited by fruit number or size (sink strength) not by assimilate supply (source strength) (Ho, 1996a), and fruit size is determined by both cell number and cell size (Ho, 1996a). When assimilate supply is low, cell division is the main limiting factor for fruit growth, but fruit size is also affected by cell growth during subsequent fruit development (Bertin et al., 2002). Cell division in tomato ovaries is believed to continue for about 7–20 days after flowering (Bohner and Bangerth, 1988; Mapelli et al., 1978).

Supplementary interlighting irradiance was provided during the early stage of flowering to the leaves near the fruit trusses, during the ES treatment to the first to fifth trusses, during the VES of fruit enlargement to the second to fifth trusses, and during the LS treatment to the second to fifth trusses (Tables 1 and 2). The highest fruit dry-matter weight was obtained by irradiance during VES in the light-limited tomato canopy. This result is consistent with a tracer study of single-truss tomatoes cultivated under natural light conditions showing that most of the ¹³C (80%) fed to individual leaves at the VES was distributed to the first fruit trusses (Okano et al., 2001b). In the present study, observations of cells in the tomato fruit were not carried out. However, the cell number increase during VES treatment may be due to the supplemental interlighting of leaves near the fruit truss. The distribution of assimilates at flowering occurs, in order, to roots, unfolded leaves, and flowers, and after fruit set to fruits, unfolded leaves, and roots (Ho, 1996b). Considering this, after fruit set, the increased photosynthesis due to supplemental interlighting may contribute to increased cell size and number of cell divisions.

In addition to the source-sink relationship, leaf position may contribute to the distribution of assimilate products. The results for up to 15 leaves in tomato plants show that 2 to 4 leaves before and after the fruit truss provide 60% to 80% of the dry-matter production of the fruit (Shishido et al., 1991). In the present study, the fruit dry-matter weight of the third to the fifth fruit trusses was not significantly increased in the Fixed treatment compared with the Control (Fig. 6). These results showed that fruit dry-matter weight increased due to enhanced photosynthesis in leaves before and after the fruit truss, which then contribute substantially to the VES of fruit enlargement.

Photosynthetic rate and distribution

In the ES treatment, the fruit dry matter distribution ratio increased, while the leaf dry-matter distribution ratio decreased compared with the Fixed and the LS treatments (Table 4). This suggests that assimilate partitioning to fruit will be enhanced by increasing photosynthesis in the upper leaves, which have higher photosynthetic activity. Studies of photosynthetic rates and the translocation of photosynthetic products to other leaves show a high positive correlation between photosynthetic rate and translocation rate with the rate of photosynthetic product efflux that is proportional to leaf sucrose concentration (Hofstra and Nelson, 1969). In addition, when the carbon assimilation rate is above 2 mg·dm⁻²·hr⁻¹, the relationship between carbon assimilation rate and translocation is proportional (Ho, 1976). However, when it is below 1 mg·dm⁻²·h⁻¹, the translocation rate remains constant and proportionality is no longer observed (Ho, 1976). A similar process may lead to assimilate partitioning to fruits from the upper leaves with high photosynthetic rates in the present study, but further investigation is needed.

In conclusion, these results demonstrate that the optimum supplemental interlighting irradiation method for dry-matter production in dense tomato cultivation is irradiation of leaves with during the VES in which the diameter of fruit increases from 10 to 30 mm.

Literature Cited

•  Aldesuquy,  H. S.,  G. M.  Abdel-Fattah and  Z. A.  Baka. 2000. Changes in chlorophyll, polyamines and chloroplast ultrastructure of Puccinia striiformis induced ‘green islands’ on detached leaves of Triticum aestivum. Plant Physiol. Biochem. 38: 613–620.
•  Bertin,  N.,  H.  Gautier and  C.  Roche. 2002. Number of cells in tomato fruit depending on fruit position and source-sink balance during plant development. J. Plant Growth Regul. 36: 105–112.
•  Bohner,  J. and  F.  Bangerth. 1988. Cell number, cell size and hormone levels in semi-isogenic mutants of Lycopersicon pimpinellifolium differing in fruit size. Physiol. Plant. 72: 316–320.
•  Diaz-Perez,  J. C. 2013. Bell pepper (Capsicum annum L.) crop as affected by shade level: Microenvironment, plant growth, leaf gas exchange, and leaf mineral nutrient concentration. HortScience 48: 175–182.
•  Fan,  X. X.,  Z. G.  Xu,  X. Y.  Liu,  C. M.  Tang,  L. W.  Wang and  X. L.  Han. 2013. Effects of light intensity on the growth and leaf development of young tomato plants grown under a combination of red and blue light. Sci. Hortic. 153: 50–55.
•  Gunnlaugsson,  E. and  S.  Adalsteinsson. 2006. Inter-lighting and plant density in year-round production of tomato at northern latitudes. Acta Hortic. 711: 71–76.
•  Hao,  X. and  A. P.  Papadopoulos. 1999. Effects of supplemental lighting and cover materials on growth, photosynthesis, biomass partitioning, early yield and quality of greenhouse cucumber. Sci. Hortic. 80: 1–18.
•  Higashide,  T. and  E.  Heuvelink. 2009. Physiological and morphological changes over the past 50 years in yield components in tomato. J. Amer. Soc. Hort. Sci. 134: 460–465.
•  Hisaeda,  K.,  K.  Takayama,  H.  Nishina,  K.  Azuma and  S.  Arima. 2007. Studies on improvement of tomato productivity in large-scale greenhouse: Analysis of vertical distribution of light intensity and net CO₂ fixation in tomato plant canopy. J. SHITA 19: 19–26 (In Japanese with English abstract).
•  Ho,  L. C. 1976. The relationship between the rates of carbon transport and photosynthesis in tomato leaves. J. Exp. Bot. 27: 87–97.
•  Ho,  L. C. 1996a. The mechanism of assimilate partitioning and carbohydrate compartmentation in fruit in relation to the quality and yield of tomato. J. Exp. Bot. 47: 1239–1243.
• Ho, L. C. 1996b. Source-sink relationships. p. 709–728. In: E. Zamski and A. A. Schaffer (eds.). Photoassimilate distribution in plants and crops: Marcel Dekker Inc., New York.
•  Hofstra,  G. and  C. D.  Nelson. 1969. A comparative study of translocation of assimilated ¹⁴C from leaves of different species. Planta 88: 103–112.
•  Isozaki,  M.,  H.  Kojiya,  N.  Konishi,  K.  Kuroda,  N.  Sato,  K.  Furuta,  H.  Suzuki,  K.  Tanaka and  A.  Tomikawa. 2006. Verification of availability of drained solution-reusing rockwool system for commercial production of tomato. Hort. Res. (Japan) 5: 135–140 (In Japanese with English abstract).
•  Jiang,  C. G.,  M.  Johkan,  M.  Hohjo,  S.  Tsukagoshi,  M.  Ebihara,  A.  Nakaminami and  T.  Maruo. 2017. Photosynthesis, plant growth, and fruit production of single-truss tomato improves with supplemental lighting provided from underneath or within the inner canopy. Sci. Hortic. 222: 221–229.
•  Kittas,  C.,  N.  Katsoulas,  N.  Rigakis,  T.  Bartzanas and  E.  Kitta. 2012. Effects on microclimate, crop production and quality of a tomato crop grown under shade nets. J. Hortic. Sci. Biotechnol. 87: 7–12.
•  Lu,  N.,  T.  Maruo,  M.  Johkan,  M.  Hohjo,  S.  Tsukagoshi,  Y.  Ito,  T.  Ichimura and  Y.  Shinohara. 2012a. Effects of supplemental lighting within the canopy at different developing stages on tomato yield and quality of single-truss tomato plants grown at high density. Environ. Control Biol. 50: 1–11.
•  Lu,  N.,  T.  Maruo,  M.  Johkan,  M.  Hohjo,  S.  Tsukagoshi,  Y.  Ito,  T.  Ichimuira and  Y.  Shinohara. 2012b. Effects of supplemental lighting with light-emitting diodes (LEDs) on tomato yield and quality of single-truss tomato plants grown at high planting density. Environ. Control Biol. 50: 63–74.
•  Mapelli,  S.,  C.  Frova,  G.  Torti and  G. P.  Soressi. 1978. Relationship between set, development and activities of growth regulators in tomato fruits. Plant Cell Physiol. 19: 1281–1288.
•  Marcelis,  L. F. M. 1996. Sink strength as a determinant of dry matter partitioning in the whole plant. J. Exp. Bot. 47: 1281–1291.
•  Masuda,  M.,  M.  Isozaki,  K.  Suzuki and  N.  Konishi. 2013. Research centers for intelligent plant production systems in Japan (10) MIE Plant Factory. J. SHITA 25: 70–76 (In Japanese with English abstract).
• Merrill, B. F., T. Kozai, T. Maruo, N. Lu, M. Takagaki, T. Yamaguchi and W. Yamori. 2016. Next evolution of agriculture: A review of innovations in plant factories. p. 723–740. In: M. Pessarakli (ed.). Handbook of Photosynthesis 3rd Edition. CRC Press, Boca Raton.
•  Nada,  K.,  S.  Kitade and  S.  Hiratsuka. 2018. Evaluation of tomato photosynthetic potential based on the chlorophyll fluorescence of leaflets sealed with transparent film or vaseline. Environ. Control Biol. 56: 7–12.
•  Nakano,  Y.,  Y.  Kawasaki and  K.  Suzuki. 2008. Distribution of ¹³C-photosynthates and changes in bleeding rate under waterlogged conditions at two developmental stages in tomato plants. Hort. Res. (Japan) 7: 209–213 (In Japanese with English abstract).
•  Naumburg,  E. and  D. S.  Ellsworth. 2002. Short-term light and leaf photosynthetic dynamics affect estimates of daily understory photosynthesis in four tree species. Tree Physiol. 22: 393–401.
•  Okano,  K.,  Y.  Nakano and  S.  Watanabe. 2001a. Single-truss tomato system—A labor-saving management system for tomato production. JARQ 35: 177–184.
•  Okano,  K.,  Y.  Sakamoto and  S.  Watanabe. 2001b. Source-sink relationship of ¹³C-photosynthates in single truss tomato. Bull. Natl. Inst. Veg. Tea Sci. 16: 351–361 (In Japanese with English abstract).
•  Porra,  R. J.,  W. A.  Thompson and  P. E.  Kriedemann. 1989. Determination of accurate extinction coefficients and simultaneous equations for assaying chlorophylls a and b extracted with four different solvents: verification of the concentration of chlorophyll standards by atomic absorption spectroscopy. Biochim. Biophys. Acta 975: 384–394.
•  Saito,  T.,  Y.  Kawasaki,  D. H.  Ahn,  A.  Ohyama and  T.  Higashide. 2020. Prediction and improvement of yield and dry matter production based on modeling and non-destructive measurement in year-round greenhouse tomatoes. Hort. J. 89: 425–431.
•  Scholberg,  J.,  B. L.  McNeal,  J. W.  Jones,  K. J.  Boote,  C. D.  Stanley and  T. A.  Obreza. 2000. Field-grown tomato: growth and canopy characteristics of field-grown tomato. Agron. J. 92: 152–159.
•  Shishido,  Y.,  C. J.  Yun,  T.  Uhashi,  N.  Seyama and  S.  Imada. 1991. Changes in photosynthesis, translocation and distribution of ¹⁴C-assimilates during leaf development and the rate of contribution of each leaf to fruit growth in tomato. J. Japan Soc. Hort. Sci. 59: 771–779 (In Japanese with English abstract).
•  Takayama,  K.,  Y.  Ishigami,  E.  Goto,  K.  Hisaeda and  H.  Nishina. 2006. Analysis of vertical distributions of chlorophyll fluorescence parameter (Fv/Fm), SPAD value and chlorophyll concentrations within tomato plant canopy in a large-scale greenhouse. J. SHITA 18: 277–283 (In Japanese with English abstract).
•  Takayama,  K.,  H.  Nishina,  K.  Hisaeda,  T.  Sueki and  S.  Harada. 2010. Spatial distribution analysis of photosynthetic function within a tomato plant canopy in a large-scale greenhouse. J. SHITA 22: 175–180 (In Japanese with English abstract).
•  Tewolde,  F. T.,  N.  Lu,  K.  Shiina,  T.  Maruo,  M.  Takagaki,  T.  Kozai and  W.  Yamori. 2016. Nighttime supplemental LED inter-lighting improves growth and yield of single-truss tomatoes by enhancing photosynthesis in both winter and summer. Front. Plant. Sci. 448: 1–10.
•  Tewolde,  F. T.,  K.  Shiina,  T.  Maruo,  M.  Takagaki,  T.  Kozai and  W.  Yamori. 2018. Supplemental LED inter-lighting compensates for a shortage of light for plant growth and yield under the lack of sunshine. PLoS ONE 13: e0206592. DOI: 10.1371/journal.pone.0206592.
•  Wardlaw,  I. F. 1990. The control of carbon partitioning in plants. New Phytol. 116: 341–381.
•  Xu,  H. L.,  L.  Gauthier,  Y.  Desjardins and  A.  Gosselin. 1997. Photosynthesis in leaves, fruits, stem, and petioles of greenhouse-grown tomato plants. Photosynthetica 33: 113–123.
•  Yoshioka,  H. and  K.  Takahashi. 1984. Studies on the translocation and accumulation of photosynthates in fruit vegetables VII Source-Sink units in tomato plants. Bull. Veg. Ornam. Crops Res. Stn. A 12: 1–8 (In Japanese with English abstract).
•  Zhang,  Y.,  Y.  Kiriiwa and  A.  Nukaya. 2015. Influence of nutrient concentration and composition on the growth, uptake patterns of nutrient elements and fruit coloring disorder for tomatoes grown in extremely low-volume substrate. Hort. J. 84: 37–45.