Indoor vertical farms can provide local, fresh, and nutritious food to densely populated urban regions as an alternative approach to food production. They enable growers to precisely adjust cardinal environmental factors, including CO2, per crop requirements to enhance growth and nutritional quality. As the substrate of photosynthesis, CO2 enrichment can enhance crop growth; however, how crops respond to sequential CO2 changes is not yet well understood. In a randomized complete block design, we studied how pre- and/or post-transplant CO2 concentration influenced lettuce growth. Lettuce ‘Rex’ and ‘Rouxai’ were grown from seed in four growth chambers with CO2 concentrations set randomly at 400, 800, 1200, and 1600 µmol·mol−1, under the same photoperiod of 24 h, photosynthetic photon flux density of 180 µmol·m−2·s−1 (blue:green:red = 20:40:120), temperature of 22 °C, and relative humidity of 60%. On day 11, biomass and morphological data on seedlings were collected while three additional uniform seedlings per chamber per cultivar were transplanted in hydroponic units in each of the four growth chambers, creating 16 sequential CO2 treatments. On day 28, biomass and morphological data were collected on mature plants. Preliminary data showed that pre-transplant CO2 enrichment from 400 to 1600 µmol·mol−1 increased seedling shoot fresh and dry mass of ‘Rex’ by 53% and 37%, respectively. In contrast, pre-transplant CO2 enrichment from 400 to 1200 µmol·mol−1 maximally increased shoot fresh and dry mass of ‘Rouxai’ by 81% and 122%, respectively. In mature plants, pre-transplant CO2 enrichment did not influence final ‘Rex’ biomass. In contrast, pre-transplant CO2 enrichment from 400 to 1200 µmol·mol−1 increased final shoot fresh mass of ‘Rouxai’ by 25%, whereas enrichment from 400 to 800 µmol·mol−1 saturated the increase in final shoot dry mass by 22%. Post-transplant CO2 enrichment had greater influence on final biomass of both cultivars than pre-transplant CO2 enrichment. Post-transplant CO2 enrichment from 400 to 1200 µmol·mol−1 saturated the increase in final biomass by 22% to 32% in ‘Rex’ and by 58% to 60% in ‘Rouxai’. Final leaf number, length, and coloration were generally unaffected by pre- or post-transplant CO2 enrichment. In all cases, there was no interaction between pre- and post-transplant CO2 concentration. We conclude that pre- and/or post-transplant CO2 enrichment independently influenced lettuce growth, with no CO2 acclimation. Post-transplant CO2 enrichment determined final biomass, with 1200 µmol·mol−1 being the saturation point. Keywords: carbon dioxide, controlled environment agriculture (CEA), indoor vertical farming, elevated CO2 concentration, growth chamber, whole-plant photosynthesis
