Coordinated management of daily PAR integral and carbon dioxide for hydroponic lettuce production

The interaction between daily integrated photosynthetically active radiation (PAR) and elevated aerial CO₂ concentration was studied during plant growth experiments with leaf lettuce (Lactuca sativa L., cv. Vivaldi) grown in a nutrient film technique system in a greenhouse. Accurate control of all environment parameters (except relative humidity) and four identical greenhouse sections constituted the experimental setup. Supplemental lighting (high pressure sodium lamps) was used to provide additional PAR to the lettuce on days when too little sunlight was available to reach the required daily light integral. Two experiments with four treatments each were performed to investigate six integrated PAR/CO₂ concentration combinations: 11/1500, 12/1250, 13/1000, 14/750, 15/530, and 16/400 (mol-m^-2-d ^-1/jimol-mol^-1). The target integrated PAR levels were accurately maintained during each treatment but the two highest CO₂ concentrations could not consistently be maintained because periodic ventilation was required for accurate temperature control. Lettuce plants were grown for 24 days under these conditions after being grown in a growth room under optimum conditions for 11 days. Periodic harvests during the greenhouse growing phase provided shoot dry-mass data. Shoot fresh-mass and number of leaves per plant were determined at the final harvest 35 days after seeding. Plant growth under the six different treatments was virtually identical and resulted in an average shoot fresh-mass of 190 g with a dry matter percentage of 3.7%. The results of the described experiments show a flexible management strategy regarding daily integrated PAR level and aerial CO₂ concentration can be employed for the most economical lettuce production.