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Katsumi Ohyama, Yoshitaka Omura, and Toyoki Kozai

Providing continuous light (24-h photoperiod) at a relatively low photosynthetic photon flux (PPF) is one possible way to reduce both initial and operational costs for lighting and cooling during transplant production with an artificial light. However, physiological disorders (i.e., chlorosis and necrosis) are often observed in several species under continuous light with a constant temperature. The objective of this study was to find an effective air-temperature regime under the continuous light to avoid such physiological disorders, and simultaneously enhance floral development, using tomato [Lycopersicon esculentum Mill.] as a model. The seedlings with fully expanded cotyledons were grown for 15 d at a PPF of 150 μmol·m–2·s–1, a relative humidity of 70%, and a CO2 concentration of about 380 μmol·mol–1 (atmospheric standard). Leaf chlorosis was observed when the air temperature was constant regardless of average air temperature (16, 22,or 28 °C). Neither leaf chlorosis nor necrosis was observed when the air temperatures were alternated [periods of high (28 °C) and low (16 °C) air temperatures of 16/8, 12/12, and 8/16 h·d–1]. Faster floral development was observed in the seedlings grown at lower average air temperatures. These results indicated that physiological disorders of tomato seedlings grown under continuous light could be avoided, and at the same time floral development could be enhanced, by lowering the average air temperature through modification of the periods of high and low air temperatures.

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Katsumi Ohyama, Junichi Yamaguchi, and Ayumi Enjoji

Labor productivity in terms of material value (weight of plants harvested per hour per person) was evaluated for 6 months from the start of operations in a research facility at Osaka Prefecture University (Osaka, Japan), serving as a model system for plant production with sole-source lighting (also referred to as a “vertical farm” or “plant factory”). The research facility is capable of producing lettuce plants (Lactuca sativa) at a maximum production rate of ≈5000 plants/day when the relative harvest rate (i.e., number of plants harvested/maximum potential number of plants harvested) is 100%. However, in the present study, the relative harvest rate at the research facility was in the range of 17% to 65% and labor productivity varied from 1.5 to 6.0 kg·h−1 per person. The evaluation results indicated that increasing the weight of plants harvested and increasing the relative harvest rate was necessary to maintain a high level of labor productivity. The processing time for harvesting was greatest among all plant operations, suggesting the need to reduce the time taken in this operation to increase the labor productivity in the research facility and in other plant production systems with sole-source lighting (PPSLs). This study demonstrates the importance of analyzing labor productivity for increasing the commercial feasibility of PPSLs. However, further long-term investigation with higher relative harvest rates is required for a more definitive conclusion.

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Hiromi Toida, Toyoki Kozai, Handarto, and Katsumi Ohyama

Fogging systems are still not widely used for greenhouse cooling, primarily due to their low fog: evaporation ratio, resulting in a low cooling efficiency and a high risk of pathogen expansion caused by excess wetness of plant foliage. The fogging is operated intermittently because of these problems. Consequently, the air temperature and relative humidity fluctuates inside the greenhouse. If nozzles with a high fog: evaporation ratio are employed, the fogging can be operated continuously. By continuous fogging, steady air temperature and relative humidity can be achieved inside the greenhouse. In our previous study under indoor conditions, a high fog: evaporation ratio was achieved by installing two small fans close to a conventional upright nozzle in order to obtain an upward air stream. The objective of this study was to draw a comparison between the environmental conditions inside a greenhouse with the continuous fogging with small fans and that with the conventional intermittent fogging without fans. Reduced fluctuation of the air temperature and relative humidity inside the greenhouse were observed in the case of continuous fogging as compared with that observed in the case of intermittent fogging. The air temperature inside the greenhouse during cooling was lower than that before cooling in both methods. In the case of continuous fogging, the adjustment of the amount of fogged water will provide the desired environmental conditions inside the greenhouse.

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Hiromi Toida, Katsumi Ohyama, Yoshitaka Omura, and Toyoki Kozai

The light and dark periods can be easily controlled by the use of artificial lighting. To understand the effects of alternation of light and dark periods on plant growth and development, we studied the growth and development of tomato (`Momotaro') seedlings under nonperiodic alternation of light and dark periods. Tomato seedlings grown under two nonperiodic alternation treatments of NF (NF-1 and NF-2) were compared with seedlings grown under a periodic alternation treatment (P treatment) with 12-hour light and dark periods. In all treatments, photosynthetic photon flux (PPF) during the light period was maintained at 280 μmol·m-2·s-1; the sum of each light period and the following dark period was 24 hours; and each of the integrated light and dark periods was 132 hours during 11 days of the experiment. In NF-1, the initial light and dark periods were 7 and 17 hours, respectively, and the light period was extended 1 hour per day, while in NF-2, they were initially 17 and 7 hours, respectively, and the light period was shortened 1 hour per day. At the end of the experiment, dry weight per seedling was greater and flower-bud initiation of the first flower truss was earlier in NF-1 than in NF-2 and P, even though the integrated PPF during the experiment was the same in all treatments. These results demonstrate that growth and development of tomato seedlings can be enhanced without any increase in electric energy consumption for lighting by gradually extending the light period or shortening the dark period.

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Katsumi Ohyama, Koji Manabe, Yoshitaka Omura, Toyoki Kozai, and Chieri Kubota

To evaluate the potential use of a 24-hour photoperiod for transplant production in a closed system, tomato (Lycopersicon esculentum Mill.) plug transplants were grown for 17 days either under a 24-hour photoperiod with a photosynthetic photon flux (PPF) of 200 μmol·m-2·s-1 or under a 16-hour photoperiod with a PPF of 300 μmol·m-2·s-1, resulting in the same daily integrated PPF (17.3 mol·m-2). Air temperatures were alternated between 28 °C during the first 16 hours and 16 °C for the subsequent 8 hours of each day. Fresh weight, dry weight and leaf area were 41%, 25%, and 64% greater, respectively, under the 24-hour photoperiod than under the 16-hour photoperiod. Physiological disorders (e.g., chlorosis and/or necrosis) were not observed under the 24-hour photoperiod, probably due to the alternating air temperature. Floral development of plants originating from both treatments did not differ significantly. Electric energy use efficiency of the closed system was 9% greater under the 24-hour photoperiod than under the 16-hour photoperiod. These results suggest that using a 24-hour photoperiod with relatively low PPF can reduce both initial and operational costs for transplant production in a closed system due to the reduction in the number of lamps.

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Ming Li, Toyoki Kozai, Katsumi Ohyama, Shigeharu Shimamura, Kaori Gonda, and Tetsuo Sekiyama

The CO2 balance of a commercial closed system with artificial lighting (CSAL), in which lettuce plants (Lactuca sativa L. ‘Early Impulse’, ‘King Crown’, and ‘Cos Lettuce’) were produced every day and CO2 was added to the air by gas cylinders and workers’ respiration, was analyzed. In the experiment, 95% of the CO2 supplied from cylinders was apparently assimilated by the lettuce plants in the commercial CSAL, suggesting that the supplied CO2 was used efficiently. The amounts of CO2 assimilated by the lettuce plants and loss resulting from leakage, respectively, accounted for 78% and 22% of the total amount of CO2 supplied. The amounts of CO2 supplied by the cylinders and by the workers’ respiration, respectively, accounted for 83% and 17% of the total amount of CO2 supplied. Based on the analysis, a relatively high CO2 utilization efficiency of 78% was observed in the experiment despite the operation rate of 33%, which is defined as the percentage of the culture beds with plants. If the operation rate could be increased to 100%, the CO2 utilization efficiency would reach 92%. These results showed that CO2 supplied by the workers’ respiration helped to reduce the amount of CO2 supplied by the cylinders and hence the CO2 cost in a commercial CSAL.