CO2 Balance of a Commercial Closed System with Artificial Lighting for Producing Lettuce Plants

The CO 2 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 CO 2 was added to the air by gas cylinders and workers’ respiration, was analyzed. In the experiment, 95% of the CO 2 supplied from cylinders was apparently assimilated by the lettuce plants in the commercial CSAL, suggesting that the supplied CO 2 was used efficiently. The amounts of CO 2 assimilated by the lettuce plants and loss resulting from leakage, respectively, accounted for 78% and 22% of the total amount of CO 2 supplied. The amounts of CO 2 supplied by the cylinders and by the workers’ respiration, respectively, accounted for 83% and 17% of the total amount of CO 2 supplied. Based on the analysis, a relatively high CO 2 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 CO 2 utilization efficiency would reach 92%. These results showed that CO 2 supplied by the workers’ respiration helped to reduce the amount of CO 2 supplied by the cylinders and hence the CO 2 cost in a commercial CSAL.