GROWTH AND GAS-EXCHANGE BY LETTUCE STANDS IN A CLOSED, CONTROLLED ENVIRONMENT

Two studies were conducted in which 'Waldmann's Green' lettuce (Lactuc a sativa L.) was grown hydroponically from seed to harvest in a large (20-m2), atmospherically closed growth chamber for the National Aerona utics and Space Administration's controlled ecological life support sy stem (CELSS) program. The first study used metal-halide (MH) lamps [28 0 mumol.M-2-S-1 photosynthetic photon flux (PPF)], whereas the second used high-pressure sodium (HPS) lamps (293 mumol.m-2.s-1). Both studie s used a 16-hour photoperiod, a constant air temperature (22 to 23C), and 1000 mumol.mol-1 CO2 during the light period. In each study, canop y photosynthesis and evapotranspiration (ET) rates were highly correla ted to canopy cover, with absolute rates peaking at harvest (28 days a fter planting) at 17 mumol CO2/M2 per sec and 4 liters.m-2.day-1, resp ectively. When normalized for actual canopy cover, photosynthesis and ET rates per unit canopy area decreased with age (between 15 and 28 da ys after planting). Canopy cover increased earlier during the study wi th HPS lamps, and final shoot yields averaged 183 g fresh mass (FM)/pl ant and 8.8 g dry mass (DM)/plant. Shoot yields in the first study wit h MH lamps averaged 129 g FM/plant and 6.8 g DM/plant. Analysis of lea f tissue showed that ash levels from both studies averaged 22% and K l evels ranged from 15% to 17% of tissue DM. Results suggest that lettuc e should be easily adaptable to a CELSS with moderate lighting and tha t plant spacing or transplant schemes are needed to maximize canopy li ght interception and sustain efficient CO2 removal and water productio n.