Materials and Methods

Commercial closed system with artificial lighting.

The cultural room of the commercial CSAL (hereafter referred to as commercial CSAL) developed by Mirai Co., Ltd. (Japan) was 21 m long, 15 m wide, and 4.5 m high. Nine basic modules with 10 shelves were installed inside the commercial CSAL. Each shelf was equipped with 31 or 47 fluorescent lamps and a culture bed (11 m long, 1.3 m wide, and 40 mm deep). In the culture beds, nutrient solution was circulated and Styrofoam boards were floated on the nutrient solution to support the lettuce plants. In the culture room, 90 culture beds were available in the commercial CSAL. In the present experiment, the operation rate, which is defined as the percentage of the culture beds with plants, was 33%. Six CO₂ cylinders were placed outside to supply CO₂ into the commercial CSAL. The CO₂ supply rate was adjusted by a controller to maintain the CO₂ concentration inside at a predetermined level. Seven heat pumps (total cooling and heating capacity: 154 and 145 kW; Daikin Industries Ltd., Japan) were installed in the commercial CSAL for controlling the air temperature.

Lettuce plants and growth conditions.

Lettuce plants (Lactuca sativa L. ‘Early Impulse’, ‘King Crown’, and ‘Cos Lettuce’) were produced in the commercial CSAL for 35 d at a planting density of 133 plants/m² for the first 25 d and 33 plants/m² for the next 10 d. Photosynthetic photon flux (PPF) on the Styrofoam boards was 100 μmol·m⁻²·s⁻¹ for the first 25 d and 130 μmol·m⁻²·s⁻¹ for the next 10 d of lettuce plant growth with a photoperiod of 17 h (Table 1). PPF was adjusted by moving the lettuce plants to different shelves. The air temperature and the CO₂ concentration were set at 20 °C and 1800 μmol·mol⁻¹ in the commercial CSAL, respectively. The electrical conductivity of the nutrient solution was maintained at 2.0 dS·m⁻¹.

Table 1.

Light environment in commercial closed systems with artificial lighting for producing lettuce plants.

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Production procedure.

Four workers entered the commercial CSAL at 0915 hr and 1315 hr sequentially at intervals of 15 min and left together at 1200 hr and 1800 hr every day. During the period from 0915 hr to 0945 hr, the lettuce plants including 192 plants of ‘Early Impulse’, 192 plants of ‘King Crown’, and 768 plants of ‘Cos Lettuce’ were harvested at 35 d after sowing. After harvesting, the corresponding number of seeds in each cultivar was sown, so a series of lettuce plants of different ages between 0 and 34 d after sowing were grown at the same time in the commercial CSAL.

CO₂ balance of the commercial closed system with artificial lighting.

In the present experiment, the CO₂ balance of the commercial CSAL was analyzed and used to calculate the CO₂ utilization efficiency.

The amount of CO₂ dissolved in the nutrient solution was constant during the experimental period because the amount of nutrient solution in the culture bed was not changed. Thus, the CO₂ balance of the commercial CSAL was unaffected by the dissolution of CO₂ and expressed by:

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where A: the daily amount of CO₂ assimilated by the lettuce plants per unit area of the culture bed with plants (mol·m⁻²·d⁻¹); S_C: the daily amount of CO₂ supplied by the cylinders per unit area of the culture bed with plants (mol·m⁻²·d⁻¹); S_W: the daily amount of CO₂ supplied by workers’ respiration per unit area of the culture bed with plants (mol·m⁻²·d⁻¹); I: the daily amount of CO₂ increase in air per unit area of the culture bed with plants (mol·m⁻²·d⁻¹); and L: the daily amount of CO₂ loss resulting from leakage per unit area of the culture bed with plants (mol·m⁻²·d⁻¹).

In the present experiment, the CO₂ utilization efficiency (CUE) was defined as the ratio of the amount of CO₂ assimilated by plants to the amount of CO₂ supplied into the commercial CSAL:

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The daily amount of CO₂ assimilated by the lettuce plants per unit area of the culture bed with plants (A) was estimated from the dry mass of the lettuce plants:

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where W: the increase in dry mass of the lettuce plants in the commercial CSAL (g·d⁻¹); m: the molecular weight of CO₂ (44 g·mol⁻¹); j: the coefficient for converting the CO₂ assimilated by plants into dry mass (0.68 g·g⁻¹; Van Henten, 1994); and B: the area of the culture beds with plants (434 m²).

Assuming that 1) the growth rate of the lettuce plants was constant in the commercial CSAL; and 2) the dry mass of seeds was small compared with that of the harvested lettuce plants 35 d after sowing, the increase in dry mass of the lettuce plants in the commercial CSAL was estimated by:

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where w_d: the dry mass of the lettuce plants “d” d after sowing (g).

The daily amount of CO₂ supplied by the cylinders per unit area of the culture bed with plants (S_C) was determined by:

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where F_t: the CO₂ supply rate at time “t” (mol·h⁻¹).

The daily amount of CO₂ supplied by the workers’ respiration per unit area of the culture bed with plants (S_W) was estimated using a fixed respiration rate of workers and determined by:

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where R: the respiration rate of the workers (0.95 mol/h/person; Japan Air Cleaning Association, 2000); P_n,t: the cumulative working time of worker “n” at time “t” (h); and K: the number of workers in the commercial CSAL.

The CO₂ increase in air describes the increase in the amount of CO₂ in air resulting from the increase in the CO₂ concentration. The daily amount of CO₂ increase in air per unit area of the culture bed with plants (I) was determined by:

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where k: the reciprocal of the molar volume of CO₂ (41.4 mol·m⁻³); V: the volume of the CSAL (1379 m³); and C_i,t: the hourly average CO₂ concentration in the commercial CSAL at time “t” (μmol·mol⁻¹).

The daily amount of CO₂ loss resulting from leakage per unit area of the culture bed with plants (L) was estimated by:

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Measurements.

The air temperature and relative humidity were measured in the commercial CSAL using sensors (RTR 53-AL; T&D Corp., Japan). The CO₂ concentration was measured by infrared gas analyzers (GMP 222; Vaisala Oyj, Helsinki, Finland) inside and outside the commercial CSAL. The CO₂ supply rate from the cylinders was measured by a flow meter (FD-A600; Keyence Corp., Japan). The hourly averaged data were recorded and used for analyzing the CO₂ balance of the commercial CSAL. The analysis of the CO₂ balance was repeated three times.

Three lettuce plants 35 d after sowing in each cultivar were sampled and dried at 60 °C for 7 d to determine the dry mass. Another three ‘Early Impulse’ lettuce plants were sampled 14, 24, 28, 32, and 35 d after sowing and their second leaf (14 d after sowing, counting from bottom) or third leaf (24, 28, 32, and 35 d after sowing, counting from bottom) were used for measuring the CO₂ assimilation rate using a portable photosynthesis system (LI-6400; LI-COR, Inc., Lincoln, NE). At the inlet of the leaf chamber, the CO₂ concentration was set at 200, 250, 300, 400, 600, 800, 1000, 1500, or 2000 μmol·mol⁻¹. PPF and the leaf temperature were maintained at 200 μmol·m⁻²·s⁻¹ and 20 °C, respectively. The measurements were repeated three times.

Results and Discussion

The CO₂ concentration and the air temperature inside the commercial CSAL were 1800 ± 56 μmol·mol⁻¹ and 20 ± 0.4 °C, respectively. The relative humidity inside ranged from 68% to 80%. The amounts of CO₂ assimilated by the lettuce plants and loss resulting from leakage, respectively, accounted for 78% [= 0.27·100%/(0.29 + 0.06)] and 22% [= 0.08·100%/(0.29 + 0.06)] of the total amount of CO₂ supplied (Table 2). The amounts of CO₂ supplied by the cylinders and by the workers’ respiration, respectively, accounted for 83% [= 0.29·100%/(0.29 + 0.06)] and 17% [= 0.06·100%/(0.29 + 0.06)] of the total amount of CO₂ supplied. The amount of CO₂ increase in the air was less than one-hundredth of the other amounts.

Table 2.

CO₂ balance and CO₂ utilization efficiency of commercial closed systems with artificial lighting for producing lettuce plants.^z

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The air can leak out of the commercial CSAL through small gaps and opening of the door when workers enter and leave the commercial CSAL (Acock and Acock, 1989). This process can be affected by the variation in environmental conditions such as air temperature (Wheeler, 1992). As a result, part of the CO₂ was lost because of leakage.

In previous studies, the CO₂ concentration in the CSAL was first increased as a result of the plant respiration during the dark periods and decreased quickly to the CO₂ concentration set point because of the plant CO₂ assimilation at the start of the photoperiod (Wheeler, 1992; Yoshinaga et al., 2000). As a result, the amount of CO₂ increase in air increased during the dark period and decreased during the start of the photoperiod in response to the variation in CO₂ concentration in the CSAL. In this study, the CO₂ concentration in the commercial CSAL ranged from 1750 to 1900 μmol·mol⁻¹ during the experimental periods because the photoperiods were staggered. The daily amount of CO₂ increase in air is just 5% of the total amount of the CO₂ supplied, assuming that the CO₂ concentration in the commercial CSAL was increased by 150 (1900 to 1750) μmol·mol⁻¹ during a day. Besides, both the increase and decrease in the CO₂ concentration in the commercial CSAL during a day were included in the analysis of CO₂ balance. Thus, the daily amount of CO₂ increase in air was small and can be neglected.

In the present experiment, 94% of CO₂ supplied by the cylinders was apparently assimilated by the lettuce plants in the commercial CSAL, suggesting that the CO₂ supplied into the commercial CSAL was used efficiently. In the previous studies, all of the CO₂ supplied was from the cylinders because there were no workers in the experimental CSAL. However, in the present experiment, there were two CO₂ supply sources: the cylinders and the workers’ respiration. If no workers entered the commercial CSAL, the amount of CO₂ supplied by the cylinders would be increased by 21% to obtain the same daily amount of CO₂ assimilated by the lettuce plants. This suggests that workers’ respiration can replace part of the CO₂ supplied by the cylinders and thus decrease the CO₂ cost.

The CO₂ utilization efficiency of the commercial CSAL was 78%, which was lower than that reported by Yoshinaga et al. (2000). This was mainly because the operation rate was 33% as a result of supply and demand for the lettuce plants. In other words, only 30 culture beds were used for growing lettuce plants in the commercial CSAL. If all the culture beds were occupied with plants, the CO₂ utilization efficiency would be 92%, which is comparable to that reported by Yoshinaga et al. (2000). Increasing the CO₂ utilization efficiency by increasing the operation rate will decrease the CO₂ cost for producing lettuce plants in the commercial CSAL. In addition, the CO₂ utilization efficiency is 100% and is unaffected by the CO₂ concentration if the CSAL is completely airtight, although this is difficult to achieve in practice (Baker et al., 2004).

The CO₂ assimilation rate of a single leaf of a lettuce plant increased with the increase in the ambient CO₂ concentration (Fig. 1), indicating that the amount of CO₂ assimilated by the lettuce plants also increased with the increase in the CO₂ concentration in the commercial CSAL. Simultaneously, the increasing CO₂ concentration in the commercial CSAL results in an increase in CO₂ loss resulting from leakage, which may reduce the CO₂ utilization efficiency of the commercial CSAL. If the set point of CO₂ concentration in the commercial CSAL decreased from 1800 in the present study to 1000 μmol·mol⁻¹, the amount of CO₂ assimilated by the lettuce plants would decrease by 22% according to measurements of the CO₂ assimilation rate of single leaves of lettuce plants (Fig. 1). At the same time, the amounts of CO₂ loss resulting from leakage and CO₂ supplied by the cylinders also decreased by 44% and 32%, although the amount of CO₂ supplied by the workers’ respiration was constant. Therefore, the CO₂ utilization efficiency of the commercial CSAL will be 6.5% higher than that obtained in the present experiment. However, for the commercial production of lettuce plants in CSAL, promoting the growth of the lettuce plants by increasing the CO₂ concentration is occasionally more important than increasing the CO₂ utilization efficiency. Although the CO₂ assimilation rate of single leaves can be scaled up to canopy level, this scaling-up is not straightforward and may cause errors (Bugbee, 1992). In future studies, the canopy CO₂ assimilation rate should be measured for estimating the effects of CO₂ concentration on the CO₂ balance of the commercial CSAL.

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CO₂ assimilation rate (A_L) of the second leaf of ‘Early Impulse’ lettuce plants 14 d after sowing (A), the third leaf of the lettuce plants 24 (B), 28 (C), 32 (D), and 35 d after sowing (E) as affected by the CO₂ concentration at the photosynthetic photon flux (PPF) of 200 μmol·m⁻²·s⁻¹. Mean ± se.

Citation: HortScience horts 47, 9; 10.21273/HORTSCI.47.9.1257

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According to the American Conference of Governmental Industrial Hygienists (1991), the indoor CO₂ concentration should be lower than 5000 μmol·mol⁻¹ for an 8-h working day to guarantee the workers’ health. In the present experiment, the CO₂ concentration in the commercial CSAL was below 2000 μmol·mol⁻¹ and the working hour in each day is less than 8 h. Thus, the impacts of high CO₂ concentration on workers’ health can be neglected. However, the accumulation of other air pollution in the commercial CSAL should be a concern, because the air exchange was minimized.

In the present experiment, we assumed constant values for the workers’ respiration rate for analyzing the CO₂ balance of the commercial CSAL. The workers’ respiration rate was determined based on interviews about their work, which mainly consisted of seeding, transplanting, and harvesting the lettuce plants in the commercial CSAL. The respiration rate ranged between 0.67 mol/h/person when seated quietly and 2.9 mol/h/person during heavy work (Japan Air Cleaning Association, 2000). In future work, the workers’ respiration rate should be precisely determined for obtaining the CO₂ balance of the commercial CSAL more accurately. On the other hand, the CO₂ concentration below 20,000 μmol·mol⁻¹ does not elevate the workers’ respiration rate significantly (Rice, 2003). Thus, the respiration rate measured under normal conditions was used to estimate the daily amount of the CO₂ supplied by the workers’ respiration in this study.

Conclusions

In conclusion, 94% of CO₂ supplied by the cylinders was apparently assimilated by the lettuce plants in the commercial CSAL. This was because the amount of CO₂ supplied by the workers’ respiration was almost equal to that of CO₂ loss resulting from leakage (Table 2) and thus contributed to the relatively high CO₂ utilization efficiency (78%). Our results showed the important contribution of the CO₂ supplied by workers’ respiration to decreasing the amount of CO₂ supplied by the cylinders and hence the CO₂ cost in a commercial CSAL.