Evaluation of season, cultivar, and aeration on biomass production of greenhouse hydroponic lettuce

The growth characteristics of three cultivars of lettuce (Latuca sativa 'Rex', 'Magenta', and 'Cherokee' (Johnny's Seed Company, Winslow, ME)) were evaluated for the effect of dissolved oxygen of the hydroponic nutrient solution within a climate controlled greenhouse. Seedlings grown 14 days from seeding in 25 mm Jiffy Preforma pellets (Jiffy Products of America, Inc), were transplanted at a density of 43 plants m^-2, and were harvested (between 150-250 g) after 4-6 weeks within four identical 3600 L floating-raft, 45 cm deep water hydroponic systems. The nutrient water of two systems was continually aerated by pumping greenhouse atmosphere through submerged ceramic air stones, while the nutrient water was circulated. The water in the two other systems was not aerated or circulated. Water temperature and dissolved oxygen (DO) concentration were monitored. The effect of DO was evaluated for extreme low (1-2 mg L^-1 DO) to more moderate values (3-6 mg L^-1 DO) during the growth periods. Seasonal, DO, and cultivar dependent responses in fresh and dry mass of roots and heads were observed upon harvest for each of the four experiments between March and August 2014. All plant responses to the DO treatment exhibited statistically significant larger average masses than the non-aerated plants for each season.