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Cos lettuce growth under pulsed light generated with full-wave rectification of 50 Hz sine-wave alternating-current power
2024 Volume 80 Issue 2 Pages 35-40
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2024 Volume 80 Issue 2 Pages 35-40
To drive LEDs for plant cultivation using an alternating-current (AC) power supply, full-wave rectification (FWR) is a reasonable method to supply a unidirectional forward current to LEDs because of the simple configuration and low energy loss of the rectification circuit. We grew cos lettuce hydroponically using a white LED light source that emitted continuous light, 100 Hz square-wave (SW) pulsed light, or pulsed light generated with FWR of 50 Hz sine-wave AC with the same averaged photosynthetic photon flux density of 150 µmol m-2 s-1. The results showed that shoot fresh weight, shoot dry weight, leaf area, and number of leaves did not differ significantly among the treatments. Plants grown under FWR pulsed light showed similar net photosynthetic rates under continuous light and SW pulsed light. Shoot fresh weight per power consumption was estimated to be significantly greater with FWR pulsed light than with continuous light, and we concluded that the use of FWR pulsed light without elaborated transformation to a flat waveform direct current is a promising lighting method to reduce the lighting cost.
Cultivation in plant factories with artificial lighting (PFALs) enables the production of crops in regions with a harsh climate and in urban areas where open-field farming is constrained. A PFAL can produce high-quality vegetables stably by optimizing the environment to suit plant growth. Moreover, plant production per land area can be greater than that achieved by field cultivation because cultivation beds can be stacked vertically (Kozai and Niu, 2019). However, the cost of the facilities and input electrical energy is much higher than outdoor production. Reducing these costs is therefore an important research and development goal.
LEDs are lightweight, small, and durable (Brown et al., 1995). Recent improvements in luminous efficiency have led to the use of LEDs as the main lighting device in PFALs. Given that LEDs can emit light with a narrow wavelength band, numerous studies have been conducted to control plant growth and development in PFALs through LED wavelength selection (for a review, see Jishi, 2018).
An additional characteristic of LEDs is that they can be turned on and off at extremely short intervals in contrast to conventional light sources, and their lifespan is not shortened by blinking. Therefore, pulsed light, which blinks with frequencies of 1 Hz-100 kHz, has been investigated for use in plant cultivation (e.g., Park et al., 2000). Pulsed light does not increase the net photosynthetic rate (NPR) of plants compared with that under continuous light of equal average photosynthetic photon flux density (PPFD), and pulsed light with a lower frequency even reduces the NPR (Sager and Giger, 1980; Kurata et al., 1984; Jishi et al., 2012). However, pulsed light could be used to reduce the energy required for LED lighting. When dimming LED light, rather than adjusting the plant-level PPFD of the continuous light, pulse-width modulation (PWM) control, which changes the duty ratio of the pulsed light with the same light-period PPFD, can reduce energy loss in the electrical circuit. Therefore, in general, LED lighting uses PWM to adjust the luminous flux.
In PFALs, there is little need to use PWM control for dimming. However, the full-wave rectification (FWR) of pulsed light described below may be cost-effective (Baba, 2009). A LED is a polarity electronic device to which a forward current supply is necessary to induce light emission. However, the waveform of the current supplied from power lines is sinusoidal, the reverse half-cycle of which does not induce light emission by the LED. To obtain light emission throughout the full cycle of the alternating current (AC) supply, the reverse half-cycle of the AC must be converted to the forward direction. FWR is the simplest mechanism to form a unidirectional current (DC) from an AC. If the waveform of the rectified forward current is regulated to be perfectly flat without ripples, continuous light is emitted. However, pursuing the flatness of the DC waveform increases the complexity and thus the cost of the power supply circuit. When a simply rectified current with conspicuous ripples is supplied, the LED emits intermittent light synchronized with the pulse period of the supplied current. However, little information is available on the effects of pulsed light emitted from LEDs driven by a ripple DC on plant growth and development.
The FWR of 50 Hz, the frequency of commercial power supply in eastern Japan, results in a fluctuating voltage of 100 Hz. Compared with the NPR under continuous light of equal average PPFD, that under square-wave (SW) pulsed light is reportedly smaller at lower frequencies but equivalent at higher frequencies above 100 Hz (Jishi et al., 2015). The FWR of 100 Hz pulsed light has also been reported to achieve NPR per plant comparable to that of continuous light (Fujiwara et al., 2023). Therefore, using FWR pulsed light is a promising option in PFALs to reduce the cost of system development and energy loss from a simplified electrical circuit. However, a similar NPR does not necessarily mean similar plant growth in long-term cultivation.
In long-term cultivation until the harvest, the growth of lettuce plants may be affected by morphogenesis (Kitaya et al., 1998) as well as changes in photosynthetic characteristics (Hogewoning et al., 2010). Simulations indicate that, in environments where PPFD changes at intervals of several seconds to several tens of seconds, the total amount of photosynthesis is smaller because NPR increases slowly during the phase of photosynthetic induction at the start of light period (Murakami and Jishi, 2021). Plant growth under pulsed light with a high frequency, which does not reduce NPR, would be expected to be similar to that under continuous light, although this has not been tested in practice.
In this study, we investigated the effects of pulse lighting on the growth of cos lettuce. As types of pulsed light, 100 Hz SW pulsed light, which has been commonly used in conventional pulsed-lighting research, and 100 Hz FWR pulsed light, which is a promising lighting method in PFALs, were used. Continuous light with a flat waveform was used as a control lighting condition. The NPR per plant was measured after cultivation to determine whether the difference in growth could be attributed to differences in NPR.
Cos lettuce (Lactuca sativa) seeds were sown on gauze moistened with distilled water and placed in Petri dishes. After sowing, the Petri dishes were placed in a chamber for two days at 25 ± 1 °C under a 16 h light/8 h dark photoperiod during which a white LED panel (AL1411A-13S28P; Solidlite Corp., Taiwan) irradiated the Petri dishes at a PPFD of 150 µmol m-2 s-1. Germinated cos lettuce seedlings were then transplanted to hydroponic urethane cubes (22 mm×22 mm×28 mm) in cell trays and placed in polypropylene containers filled with nutrient solution (half-strength Otsuka A nutrient solution; Otsuka Agritechno Co. Ltd., Tokyo, Japan). The electric conductivity and pH of the solution were 1.3 ± 0.1 dS m-1 and 6.0 ± 1.0, respectively. The containers were placed in a temperature-controlled room at 25 ± 1 °C under a 16 h light/8 h dark photoperiod for four days. A PPFD of 150 µmol m-2 s-1 was provided using fluorescent lamps (FLR110H∙N/A/100; Toshiba Lighting and Technology Corp., Kanagawa, Japan). The relative spectral photon flux density distributions of light emitted from the white LED panel and fluorescent lamps were measured (Fig. 1) in advance using a spectroradiometer (MS-720; Eko Instruments Co. Ltd., Tokyo, Japan). Six days after sowing, the plants with urethane cubes were transplanted to polystyrene panels, floated on polypropylene containers filled with nutrient solution, and grown in the same temperature-controlled room for an additional six days. The nutrient solution was constantly aerated using an air pump. Twelve days after sowing, uniformly developed plants were selected for subsequent growth experiments.
Fig. 1. Relative spectral photon flux distribution of white fluorescent light (A) and white LED light (B).
For the growth investigation, three treatments were set up: continuous lighting (CL), SW pulsed lighting (SW), and FWR pulsed lighting (FWR). The frequency of the two types of pulsed lights was 100 Hz. The duty ratio of the SW pulsed light was 50%. Two white LED panels (AL1411A-13S28P) were used in each treatment, and the DC power supply (PAS80-4.5; Kikusui Electronics Corp., Yokohama, Japan) provided power directly to the LED panels for CL. For SW, a square-wave pulse signal with a frequency of 100 Hz and a duty ratio of 50% generated by a pulse generator (SFG-2104; Good Will Instrument Co., Ltd., Taiwan) was sent to a power amplifier (custom-made; Shin-Nihon Shomei Co., Ltd., Tokyo, Japan), which supplied the SW current to the LED panel. For FWR, a sinusoidal signal with a frequency of 50 Hz generated by the pulse generator was converted to a full-wave rectified pulse with a frequency of 100 Hz using a diode bridge (AM1510; Pan Jit International Inc., Taiwan). The full-wave rectified pulse was then supplied to a power amplifier (PWM2-60SW; Shin-Nihon Shomei Co., Ltd., Tokyo, Japan), which supplied the FWR current to the LED panel. The LED panels were cooled by air from a cooling fan placed 20 cm above the center of each panel. A timer (H5CX-A-N; Omron Corp., Kyoto, Japan) was used to control the 18-h light/6-h dark photoperiod.
The PPFD was measured using a quantum sensor (LI-190SA, Li-Cor Inc., Lincoln, NE, USA) connected to a readout meter (LI-250SA, Li-Cor Inc.). The current supplied to the LED panels was adjusted so that the average PPFD was 150 µmol m-2 s-1 at the top of the growing beds in all treatments. Waveforms of the PPFD for each treatment were determined (Fig. 2) using a high-speed silicon photodiode (S1787-12; Hamamatsu Photonics K. K., Shizuoka, Japan) connected with an oscilloscope (TPS2024; Tektronix Inc., Beaverton, OR, USA).
Fig. 2.Waveforms of photosynthetic photon flux density (PPFD) at the plant base for continuous light (CL), square-wave pulsed light (SW), and full-wave rectification pulsed light (FWR) during the cultivation experiments.
Twelve-day-old cos lettuce seedlings were planted in 25 mm diameter holes spaced 170 mm apart in polystyrene panels for nutrient flow technique (NFT) cultivation (Fig. 3). Six plants were grown in a bench, and 18 plants were grown simultaneously using three benches for the three treatments. The nutrient solution (half-strength Otsuka A nutrient solution; electric conductivity: 1.3 ± 0.1 dS m-1; pH: 6.0 ± 1.0) was pumped from a polypropylene tank into a water passage (100 mm wide × 1,000 mm long × 50 mm high) using a water pump. The flow rate of the nutrient solution was controlled at 200 ml min-1 for each growing bed using a flow-regulating valve. The nutrient solution sent to the growing beds was returned to the nutrient tank, and the tank is shared among the three benches. The nutrient solution temperature was maintained at 20 ± 0.5 °C by a thermo-regulator (CTR-420; Iwaki Co., Ltd., Tokyo, Japan) that controlled the flow of cooling water through a plastic tube installed in the tank. The nutrient solution was added as it decreased. EC was adjusted with Otsuka A nutrient solution and tap water, and pH was adjusted with KOH and Hcl.
To prevent the cos lettuce plants from being exposed to light from other treatments, the growing beds were shaded with aluminum plates. Non-woven fabrics were laid on the bottom of the water passages to allow the nutrient solution to flow evenly. The indoor environment during cultivation was 25 ± 1 °C (light period) and 20 ± 1 °C (dark period), with no humidity control.
Fig. 3.Schematic diagram of the nutrient flow technique system used for the cultivation experiments. Arrows denote the nutrient flow direction.
The shoot fresh weight and shoot dry weight were determined using electronic balances after cultivation for 14 days. Shoot dry weight was measured after the shoot parts were dried in a constant-temperature dryer at 100 °C for one hour and then at 80 °C for three days. The total leaf area per plant was measured using an automatic area meter (AAM-9; Hayashi Denko Co., Ltd., Tokyo, Japan). The cultivation experiments were replicated for three times.
2.3 NPR investigationOther cos lettuce plants that were grown as described in section 2.2 were used for NPR measurements, but a hydrophilic polymer membrane (Hydro Membrane; Mebiol Inc., Kanagawa, Japan) was used instead of the non-woven fabric at the bottom of the nutrient solution passages. The polymer membrane facilitated the transfer of each whole plant, including its roots, from the NFT growing bed to the NPR measurement chamber. The NPR per plant grown in each lighting treatment was determined by measuring the rate of shoot CO2 absorption rate at a CO2 concentration of 400 µmol mol-1 using the open system assimilation chamber method as previously reported (Fujiwara et al., 2023). The NPR under CL was measured for plants grown under CL, SW pulsed light, and FWR pulsed light; in addition, plants grown under SW and FWR pulsed light were also subjected to NPR measurement under the respective light environment. The average PPFD during NPR measurements was 150 µmol m-2 s-1, the same as during cultivation.
2.4 Statistical analysesFor the growth investigation, six plants in each treatment were grown simultaneously in the same NFT growing bed in a row (Fig. 4). The measured values of the four inner plants were used for statistical analyses, considering that the plants at each end of the row had different light and airflow environments than those in the inner row. The mean values of the four plants were calculated and a Bonferroni test was performed, assuming that there was a correspondence among each of the three repetitions (n=3). Similarly, the shoot fresh weight per power consumption, calculated assuming power consumption rate from literature values, was also subjected to a paired t-test between CL and FWR.
For the NPR investigation, six plants in each lighting treatment were grown simultaneously in the same NFT growing bed. The NPR of the inner four plants were measured and the means were analyzed with Tukey’s multiple comparison test.
Fig. 4.Cos lettuce grown with the nutrient flow technique system for 14 days under continuous light (top row), square-wave pulsed light (middle row), and full-wave rectification pulsed light (bottom row).
No significant difference was observed in shoot fresh weight, shoot dry weight, leaf area, or number of leaves among the three treatments (Fig. 5). The results showed that there was no apparent disadvantage in using FWR pulsed light for plant cultivation in the PFAL. In recent years, rectification methods that lead to less energy loss than bridge diodes, with a theoretical conversion efficiency of 97.5% or higher, have been proposed, although the number of components and cost must be greater to increase the conversion efficiency (Kim et al., 2013). Realistically, the energy efficiency of AC/DC conversion appears to be approximately 92.5% at best (Sekine et al., 2022). If the conversion efficiency for CL irradiation is 92.5% of that for FWR pulse light irradiation, then the shoot fresh weight per energy expenditure is significantly higher under FWR pulsed light than CL (Fig. 6). Regardless of the rectification method, it would not be necessary to use a completely flat waveform DC to drive LEDs for plant cultivation in a PFAL. The omission of strict waveform regulation is expected to reduce development costs and reduce energy losses by several percent by simplifying the electrical circuit.
Fig. 5.Shoot fresh weight, shoot dry weight, leaf area, and number of leaves of cos lettuce plants grown under continuous light (CL), square-wave pulsed light (SW), and full-wave rectification pulsed light (FWR). Bars and error bars represent the mean ± S.E. (n = 3). No significant differences among the treatments were observed (Bonferroni’s paired significant difference test, P < 0.05).
Fig. 6.Relative shoot fresh weight of cos lettuce plants grown under continuous light (CL) or full-wave rectification pulsed light (FWR) per energy consumption. The conversion efficiency to emic CL was assumed to be 92.5% of that to emit FWR. Bars and error bars represent the mean ± S.E. (n = 3). Asterisk shows significant difference (paired t-test, P < 0.05).
The cos lettuce plants used to measure NPR also grew similarly in each treatment to those used to measure the growth parameters (data not shown). The NPR per plant under continuous light after cultivation also did not differ significantly among the treatments (Fig. 7). In our previous study, SW pulsed light at 100 Hz resulted in a lower NPR in cos lettuce grown under continuous light compared with those under continuous and FWR pulsed light at an equal average PPFD (Fujiwara et al., 2023). However, in the present study, plants grown under SW pulsed light had a comparable NPR under both CL and SW pulsed light. Plants grown under SW pulsed light might be acclimated to the light to compensate for the lowered NPR. In any case, all treatments in this study had similar NPR and growth, probably because the long-term responses, such as morphogenesis, were also similar among the treatments.
Fig. 7.Net photosynthetic rate (NPR) under continuous light (CL) of the aerial portion of cos lettuce plants grown under CL, NPR under CL and square-wave (SW) pulsed light of cos lettuce plants grown under SW, and NPR under CL and full-wave rectification pulsed light (FWR) of cos lettuce plants grown under FWR. Bars and error bars represent the mean ± S.E. (n = 12). No significant differences among the treatments were observed (Tukey’s honestly significant difference test, P < 0.05).
It should be noted that these results were likely due to the high frequency of 100 Hz. Under pulsed light with a high frequency, photosynthesis proceeds during the dark period with the same efficiency as that under continuous light by utilizing the photosynthetic intermediates produced in the light period (Jishi and Fujiwara, 2021). However, NPR declination has been reported at frequencies below 100 Hz (Jishi et al., 2012). The reason for the reduced photosynthetic efficiency has been explained previously in that photosynthetic intermediates are saturated during the long light period and a greater proportion of the absorbed light energy is dissipated as heat (Tennessen et al., 1995; Jishi et al., 2018). Pulsed light with lower frequencies risks a smaller NPR (Jishi et al., 2015). However, morphological and/or physiological acclimation of plants to pulsed light may result in a useful technique in plant production with artificial lighting. This is a suitable topic for future research.
Cos lettuce growth under pulsed light emitted from white LEDs, to which full-wave-rectified 50 Hz AC power was supplied, was comparable to that under continuous light with an equal average PPFD (150 µmol m-2 s-1). When using commercial AC power for LED lighting, the use of full-wave-rectified pulsed light without elaborated transformation to a flat waveform DC is a promising lighting method to reduce the cost of the power-supply system and to reduce energy loss in the electrical circuit.
