| Summary: | This study investigates and quantifies the integrative effects of CO<sub>2</sub> concentration (500, 1000 and 1500 µmol mol<sup>−1</sup>), illumination intensity (100, 200 and 300 μmol m<sup>−2</sup> s<sup>−1</sup>) and air speed (0.25, 0.50 and 0.75 m s<sup>−1</sup>) on the growth, gas exchange and light use efficiency of lettuce plants (<i>Lactuca sativa</i> L.) grown under artificial lighting. The results show that lettuce growth and gas exchange are closely related to CO<sub>2</sub> concentration and illumination intensity, while air speed enhances CO<sub>2</sub> transport during photosynthesis. The most influential two-way interactions were observed between CO<sub>2</sub> concentration and illumination intensity on the fresh and dry weights of lettuce shoots with effect sizes of 34% and 32%, respectively, and on the photosynthesis, transpiration and light use efficiency, with effect sizes of 52%, 47% and 41%, respectively. The most significant three-way interaction was observed for the photosynthetic rate, with an effect size of 51%. In general, the fresh and dry weights of lettuce plants increased by 36.2% and 20.1%, respectively, with an increase in CO<sub>2</sub> concentration from 500 to 1500 µmol mol<sup>−1</sup> and by 48.9% and 58.6%, respectively, with an increase in illumination intensity from 100 to 300 μmol m<sup>−</sup><sup>2</sup> s<sup>−1</sup>. The photosynthetic rate was found to be positively correlated with CO<sub>2</sub> concentration, illumination intensity and air speed. The transpiration rate and stomatal conductance increased by 34.9% and 42.1%, respectively, when the illumination intensity increased from 100 to 300 μmol m<sup>−2</sup> s<sup>−1</sup>. However, as CO<sub>2</sub> concentration increased from 500 to 1500 μmol mol<sup>−1</sup> and air speed increased from 0.25 to 0.75 m s<sup>−1</sup>, the transpiration rate decreased by 17.5% and 12.8%, respectively. With the quantified data obtained, we were able to adequately determine how CO<sub>2</sub> concentration, illumination intensity and air speed interact with their combined effects on the growth of lettuce plants grown in indoor cultivation systems with artificial lighting.
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