27 Heat and Gas Exchanges Between Plants and Atmosphere in Space Yoshiaki Kitaya Graduate School of Life and Environmental Sciences, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan Correspondence: * kitaya@envi.osakafu-u.ac.jp Keywords: barley, heat convection, infrared thermography, leaf temperature, microgravity, parabolic airplane flights, photosynthesis, sweet potato, transpiration ABSTRACT Fundamental studies were conducted to develop a facility having an adequate environmental control system for growing healthy plants over a long-term under microgravity conditions in space farming. 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 sweet potato and barley leaves were evaluated at gravity levels of 0.01, 1.0 and 2.0 g for 20 seconds each during parabolic airplane flights. Thermal images were captured using infrared thermography. The net photosynthetic rates were determined using a chamber method with an infrared gas analyzer. Mean leaf temperatures increased by 0.9 - 1.0C and decreased by 0.6C over 20 seconds as gravity decreased from 1.0 to 0.01 g and increased from 1.0 to 2.0 g, respectively. The increase in leaf temperatures was greater at the regions closer to the tip of the barley leaf and at most 2.5C over 20 seconds 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 μmol m -2 s -1 . 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. 1. INTRODUCTION Plant growth and reproduction in space have recently been of greater concern as the possibility of realizing manned space flight over a long term increases. The feasibility of achieving long term manned space missions is dependent on crops in bioregenerative life support systems or space farming that will play important roles in food production, CO2/O2 conversion and water purification. In space farming, scheduling of crop production and obtaining high yields with a rapid turnover rate are important. The space environment including such factors as altered gravity and cosmic rays may affect vegetative and reproductive growth of plants in various ways, but a few space-flight experiments have attempted to test the effect of the space environment on seed production and incidence of genetic aberrations (Merkies and Laurinavichyus 1983, Mashinsky et al. 1994, Salisbury et al. 1995). The failure in seed production and genetic aberrations may be due to either microgravity or increased cosmic rays in space or resulting from unfavorable growing conditions in the plant growing facility. However, we still do not know whether these alterations cause significant changes in morphology and function of whole plants and reproductive organs when grown in space for a long duration. These questions should be answered by long-term experiments conducting in space-flight facilities such as International Space Station. In space farming, the utilization of closed plant culture facilities was expected. In such facilities without any adequate air circulation systems, air movement will be extremely restricted compared with that under field conditions on the earth. Insufficient air movement around plants increases the resistance to gas diffusion in the leaf boundary layer and thus limits photosynthesis and transpiration of plants (Yabuki and Miyagawa 1970, Monteith and Unsworth 1990, Jones 1992), which would result in suppression of plant growth and development. Therefore the enhancement of gas exchange in leaves and growth of plant would be dependent on appropriate control of air movement. On the earth, free convection can easily occur with uneven temperature distribution. Air movements are induced by convection even in a closed chamber with no forced ventilation system. However very little free convection would occur under a microgravity condition in space. The limited free convection would reduce plant growth by limiting heat and gas exchanges on plant leaves. Control of air movement in a closed plant production system is thus essential to enhance the heat and gas exchanges between plants and the ambient air, and consequently promote growth of plants. To clarify the effects of gravity on heat/gas exchange between plant leaves and the ambient air, the leaf temperatures and net photosynthetic rates of plant leaves were evaluated at gravity levels of 0.01, 1.0, 1.5 and 2.0 g for 20 seconds each during parabolic airplane flights.