Growth and Quality of Lettuce Cvs. Outredgeous and Flandria Under Hypobaric Conditions

Future space missions will likely include plants to provide fresh foods and bioregenerative life support capabilities.  Current space craft such as the International Space Station operate at 1 atm (101 kPa) pressure but future missions will likely use reduced pressures to minimize gas leakage and facilitate rapid egress (space walks).  Plants for these missions must be able to tolerate and grow reliably at these reduced pressures.  We grew two cultivars of lettuce, cv. Flandria (green bibb type), and cv. Outredgeous (red, loose-leaf type), under three pressures: 96 kPa (ambient control), 67 kPa (2/3 atm), and 33 kPa (1/3 atm) for 21 days in rockwool using recirculating NFT.  Each treatment was repeated three times using a different hypobaric chamber each time.  Lighting was provided with metal halide lamps at 300 µmol m^-2 s^-1 PPF for a 16-h photoperiod at 22°C.  Oxygen was maintained at 21 kPa (equal to 21% at 1 atm) and CO₂ at 0.12 kPa (equal to 1200 ppm at 1 atm).  Leaf area for cv. Outredgeous was reduced 20% and 38% at 67 kPa and 33 kPa, while shoot fresh mass was reduced 22% and 41% at 67 kPa and 33 kPa when compared to control plants at 96 kPa.  Leaf area for cv. Flandria showed no difference between 96 and 67 kPa but was reduced 31% at 33 kPa, while shoot fresh mass was reduced 6% and 27% at 66 kPa and 33 kPa compared to 96 kPa.  Previous studies with other cultivars of lettuce showed little change in growth across this range of pressures, suggesting responses may vary among genotypes and / or ancillary environmental conditions.  Collectively the findings suggest further testing is needed to understand atmospheric pressure effects on plant growth.