The effects of CO2 on growth and transpiration of radish (Raphanus sativus) in hypobaria

Plants grown on long-term space missions will likely be grown in low pressure environments (i.e., hypobaria). However, in hypobaria the transpiration rates of plants can increase and may result in wilting if the water is not readily replaced. It is possible to reduce transpiration by increasing the partial pressure of CO 2 (pCO 2 ), but the effects of pCO 2 at high levels (>120 Pa) on the growth and transpiration of plants in hypobaria are not known. Therefore, the effects of pCO 2 on the growth and transpiration of radish ( Raphanus sativus var. Cherry Bomb II) in hypobaria were studied. The fresh weight (FW), leaf area, dry weight (DW), CO 2 assimilation rates (C A ), dark respiration rates (DR), and transpiration rates from 26 day-old radish plants that were grown for an additional seven days at different total pressures (33, 66 or 101 kPa) and pCO 2 (40 Pa, 100 Pa and 180 Pa) were measured. In general, the dry weight of plants increased with CO 2 enrichment and with lower total pressure. In limiting pCO 2 (40 Pa) conditions, the transpiration for plants grown at 33 kPa was approximately twice that of controls (101 kPa total pressure with 40 Pa pCO 2 ). Increasing the pCO 2 from 40 Pa to 180 Pa reduced the transpiration rates for plants grown in hypobaria and in standard atmospheric pressures. However, for plants grown in hypobaria and high pCO 2 (180 Pa) leaf damage was evident. Radish growth can be enhanced and transpiration reduced in hypobaria by enriching the gas phase with CO 2 although at high levels leaf damage may occur.