Nighttime Supplemental LED Inter-lighting Improves Growth and Yield of Single

Greenhouses with sophisticated environmental control systems, or so-called plant factories with solar light, enable growers to achieve high yields of produce with desirable qualities. In a greenhouse crop with high planting density, low photosynthetic photon flux density (PPFD) at the lower leaves tends to limit plant growth, especially in the winter when the solar altitude and PPFD at the canopy are low and day length is shorter than in summer. Therefore, providing supplemental lighting to the lower canopy can increase year-round productivity. However, supplemental lighting can be expensive. In some places, the cost of electricity is lower at night, but the effect of using supplemental light at night has not yet been examined. In this study, we examined the effects of supplemental LED inter-lighting (LED inter-lighting hereafter) during the daytime or nighttime on photosynthesis, growth, and yield of single-truss tomato plants both in winter and summer. We used LED inter-lighting modules with combined red and blue light to illuminate lower leaves right after the first anthesis. The PPFD of this light was 165 μmol m^-2 s^-1 measured at 10 cm from the LED module. LED inter-lighting was provided from 4:00 am to 4:00 pm for the daytime treatments and from 10:00 pm to 10:00 am for the nighttime treatments. Plants exposed only to solar light were used as controls. Daytime LED inter-lighting increased the photosynthetic capacity of middle and lower canopy leaves, which significantly increased yield by 27% in winter; however, photosynthetic capacity and yield were not significantly increased during summer. Nighttime LED inter-lighting increased photosynthetic capacity in both winter and summer, and yield increased by 24% in winter and 12% in summer. In addition, nighttime LED inter-lighting in winter significantly increased the total soluble solids and ascorbic acid content of the tomato fruits, by 20 and 25%, respectively. Use of nighttime LED inter-lighting was also more cost-effective than daytime inter-lighting. Thus, nighttime LED inter-lighting can effectively improve tomato plant growth and yield with lower energy cost compared with daytime both in summer and winter.

Plant Material and Growth Conditions
Tomato (Solanum lycopersicum L., cv. Sanbi) seeds were sown in 128-cell plug trays filled with vermiculite. After 2 days in a dark room at a temperature of 26°C for germination, the trays were transferred to a walk-in type environment-controlled growth chamber (Nae Terrace, Mitsubishi Plastics Agri Dream Co., Ltd., Tokyo, Japan). Seedlings were cultivated using Otsuka nutrient solution with 2.0 ± 0.2 EC and 5.5 ∼ 6.0 pH supplied only once a day using ebb and flow system. In the growth chamber, temperature was 20/16°C day/night during the winter and 22/18°C day/night temperature in the summer, under cool white fluorescent lamps with 360 μmol m-2 s-1 PPFD and 1200 ppm CO2 concentration.

After 3 weeks in the growth chamber, the seedlings were transplanted in a greenhouse at a plant density of 10 plants per square meter (on 7 December 2013 and 1 August 2014 for winter and summer, respectively). The greenhouse was equipped with heat pumps to keep the day/night temperature of 22/17°C in winter, and pad and fan system were used to keep the day/night temperature at 26/20°C during the summer. The irrigation system was a modified nutrient film technique (NFT) designed to distribute nutrient solution evenly to each plant through longitudinal flow of the nutrient solution, rather than flowing from the start to the end of the growing bed (Zen-Noh Co., Ltd., Tokyo, Japan). Otsuka nutrient solution (Otsuka Chemical Co., Ltd., Osaka) was used for fertigation with an application schedule of 10 min with a subsequent 30 min break during the day and an hour break during the night. The pH was maintained at 6.0 at all times. Electrical conductivity was set at 2.5 dS m-1 at the time of transplanting and was increased progressively up to 12 dS m-1 until the harvest date in order to increase the total soluble solids content of the tomato fruits for commercial production. The tomato cultivation method was a high-plant-density single truss tomato production system in which each plant was allowed to develop only a single truss of fruit (Giacomelli et al., 1994). To achieve this, the apical meristem of each plant was pinched after the first anthesis, leaving two leaves above the first flowering truss and all flowers that set fruits were kept intact. To improve fruit setting, fully blooming flowers were sprayed once using 4-chlorophenoxy acetic acid with 15 mg L-1 and 7.5 mg L-1 concertation in summer and winter, respectively as described (Yoshida et al., 2014).

LED Inter-lighting
LED inter-lighting (Philips Green Power LED inter-lighting module DR/B, Philips, Eindhoven, the Netherlands) was used to illuminate the understory leaves. The lamp spectrum was red and blue combined (Supplementary Figure S1) with a PPFD of 165 μmol m-2 s-1 measured at 10 cm from the LED module. LED modules were positioned on both sides of the aisle at a distance of 50 cm from the stem (10 cm away from the mid-canopy leaves, on average), and at a height of 40 cm from the Styrofoam panel under which the root system was established (Figure 1).