The hourly CO2 assimilation rate of lettuce plants (A) in a closed system with artificial lighting (CSAL) for commercial production was estimated based on the CO2 balance. Constant values for workers' respiration rate and number of air exchanges per hour were used to simplify the estimation. A was varied during the experimental period due to the variation in the leaf area of lettuce plants under lighting (LAL). However, the estimates of A contain inaccuracies. The percent error in A due to the variation in number of air exchange per hour can range from - 8 % to 20 %. Besides, the variation in the workers' respiration rate also can cause inaccuracy of A. The results indicate that A can be estimated based on the CO2 balance. The number of air exchanges per hour and estimation of workers' respiration rate must be accurate to improve the accuracy of A.
