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Supplemental lighting and CO 2  enrichment have been employed to promote plant growth in commercial plant production in greenhouses. In a semi-closed plant production system with a large number of plants at a high density, the relative humidity in the air around growing plants could be in excess of 80%. This research was initiated to determine the effects of CO 2  concentration and photoperiod on the growth of plants under relatively high humidity conditions. In the experiment, lettuce plants were grown for 13 days under eight combinations of two CO 2  levels (CO 2 , 0.38 and 0.76 mmol·mol -1 ), two photoperiods (PP, 16 and 24 h/day), and two relative humidity levels (RH, 80% and 90%) in growth chambers. The air temperature was 25 °C. Plants were illuminated with fluorescent lamps at a photosynthetic photon flux of 0.23 mmol·m -2 ·s -1 . The dry mass of lettuce shoots (leaves and stems) grown in 0.76 mmol·mol -1  CO 2 , 24 h/day PP, and 80% to 90% RH was greatest in all treatments and was five times the least value obtained in 0.38 mmol·mol -1  CO 2 , 16 h/day PP and 90% RH. The dry mass of lettuce shoots decreased to 40% as RH increased from 80% to 90 % under 0.38-0.76 mmol·mol -1  CO 2  and 16 h/day PP. Growth suppression by excess humidity was less significant in longer PP and higher CO 2 . Supplemental lighting and CO 2  enrichment would be more effective for promoting growth of plants grown under higher humidity conditions.

609 Effects of CO2 Concentration and Photoperiod on Growth of Lettuce under High Humidity Conditions