Fundamental studies were conducted to develop a facility having an adequate air circulation system for growing healthy plants over a long term under microgravity conditions in space. To clarify the effects of gravity on heat and gas exchanges between plant leaves and the ambient air, surface temperatures and net photosynthetic rates of barley leaves were evaluated at gravity levels of 0.01, 1.0, and 2.0 g for 20 sec each during parabolic airplane flights. Thermal images were captured using infrared thermography at an air temperature of 22 degrees C, a relative humidity of 18%, and an irradiance of 260 W/m2. The net photosynthetic rates were determined by means of a chamber method with an infrared gas analyzer at an air temperature of 20 degrees C, a relative humidity of 50%, and photosynthetic photon fluxes (PPFDs) of 250 and 500 micromol/m2/sec. Mean leaf temperatures increased by 1.9 degrees C with decreasing gravity levels from 1.0 to 0.01 g and decreased by 0.6 degrees C with increasing gravity levels from 1.0 to 2.0 g. The increase in leaf temperatures was greater at the regions closer to the leaf tip and at most 2.5 degrees C over 20 sec as gravity decreased from 1.0 to 0.01 g. The net photosynthetic rate decreased by 20% with decreasing gravity levels from 1.0 to 0.01 g and increased by 10% with increasing gravity levels from 1.0 to 2.0 g at a PPFD of 500 micromol/m2/sec. The heat and gas exchanges between leaves and the ambient air were suppressed more at the lower gravity levels. The retardation would be caused by heat and gas transfers with less heat convection. Restricted free air convection under microgravity conditions in space would limit plant growth by retarding heat and gas exchanges between leaves and the ambient air.