Effect of vapor pressure deficit on spinach growth under hypobaric conditions

The effect of vapor pressure deficit (VPD) of air during photoperiod on spinach growth under low total pressure was examined. Spinach plants grown under atmospheric pressure were transplanted into a pressure-reduced growth chamber and cultured hydroponically for eight days. The air temperature in the growth chamber was kept at 25 degrees C during the light period and 20 degrees C during the dark period. In the control, the total pressure was 101 kPa and the VPD was 0.95 kPa. The low pressure treatment had a total pressure of 25 kPa and a VPD of 0.95 kPa. The low pressure and high humidity treatment had a total pressure of 25 kPa, and a VPD of 0.48 kPa. The VPD during the dark period was consistent for all treatments at 0.44 kPa. O2 and CO2 partial pressures were constant at 21 kPa and 40 Pa, respectively. The photosynthetic photon flux density (PPFD) on a plant bed inside the growth chamber was 250 micromoles m-2 s-1 on a plant bed. Relative growth rate (RGR) of spinach in the low pressure and high humidity treatment was significantly greater than in the control. There was no significant difference in RGR between the control and the low pressure treatment. The low pressure and high humidity treatment also had increased leaf area. These results showed that the effect of pressure alone was not significant on RGR, but the combination of low pressure with high humidity was important.