Svetlana Sapunova

Microgravity effects on water supply and substrate properties in porous matrix root support systems

The control of water content and water movement in granular substrate-based plant root systems in microgravity is a complex problem. Improper water and oxygen delivery to plant roots has delayed studies of the effects of microgravity on plant development and the use of plants in physical and mental life support systems. Our international effort (USA, Russia and Bulgaria) has upgraded the plant growth facilities on the Mir Orbital Station (OS) and used them to study the full life cycle of plants. The Bulgarian-Russian-developed Svet Space Greenhouse (SG) system was upgraded on the Mir OS in 1996. The US developed Gas Exchange Measurement System (GEMS) greatly extends the range of environmental parameters monitored. The Svet-GEMS complex was used to grow a fully developed wheat crop during 1996. The growth rate and development of these plants compared well with earth grown plants indicating that the root zone water and oxygen stresses that have limited plant development in previous long-duration experiments have been overcome. However, management of the root environment during this experiment involved several significant changes in control settings as the relationship between the water delivery system, water status sensors, and the substrate changed during the growth cycles.

Recent advances in the development of the SVET space greenhouse equipment

Providing optimal living conditions for crew members during future long-term missions to other planets has been one of the most important scientific and engineering tasks of our times. Plant crops grown onboard can supplement the food that is carried along from Earth with vitamin addition, regenerate the spacecrafts atmosphere and have a significant effect on the crew emotional frame creating psychological comfort for the astronauts during their long isolation. The paper describes the Bulgarian developed automated SVET Space Greenhouse (SG) equipment that is between the first ones in the world and with longest time of functioning in orbit. It also contains a brief report of the main experimental results obtained during the unique plant space experiments conducted onboard the MIR Space Station on Russian and American research programs during the period 1990-2000. They proved that there is not an obstacle of principle for normal plant development and reproduction under microgravity. Next step towards the development of future space greenhouses with a large growing area was to improve the equipment and biotechnology for plant growing under weightlessness and to work out experimentally some important questions concerning plant development in extreme environmental condition. The Bulgarian scientists developed a new concept for adaptive control of the plant environmental parameters for the next generation SUET SG with additional monitoring of some physiological processes which allow evaluating plant status and optimising growth conditions during the experiment. Methods and sensors for measurements of the plant leaf environment parameters (light intensity, air humidity and temperature, air flux velocity, air pressure, gas composition etc.) in SVET-3 SG are being developed. Special attention is paid to the problem of controlling substrate moisture and O/sub 2/ content in the plant root environment which is of great importance for normal plant growth. Results obtained in earth experiments imitating some processes that occur in microgravity are discussed.

Progress in the Svet-3 space greenhouse project: Temperature measurements; thermoelectric sensor calibration apparatus

The presented work is a part of the Bulgarian “Svet-3 Space Greenhouse” project for development of a plant growth facility with adaptive environmental control and automatic optimization of plant growth conditions during the experiment. The paper discusses the performance analysis, development and testing of a laboratory calibration and check calibration thermoelectric apparatus for preparing sensors for precise temperature measurements of plant environment. The apparatus uses an insulated chamber that accommodates a temperature-stabilized aluminum plate holding both the sensors to be calibrated and the feedback one. A thermoelectric module with heatsink-fan assembly is mounted outside the chamber and in thermal contact with the plate. The apparatus uses a standard Peltier module of TEC1-12703 type, an analogue PID control system and miniature stable thermistors of 10 kΩ at 20°C for feedback temperature control. The insulated chamber provides access for an external high-precision thermometer to monitor permanently the plate temperature. The operational characteristic of the apparatus is measured and the standard deviations of the steady state temperature for each set point are calculated. The results are depicted graphically and discussed.

Impact of Different Substrate Moisture Levels on Lettuce Plants during Ground Based Experiment in SVET-2 Space Greenhouse

Plant experiments carried out in space has proved that microgravity alters conditions in the plant growth facilities especially in the root modules and thus affects plant growth and development. Microgravity changes behavior of fluids and gases in the porous media used as plant growth substrates which causes problems with the control of water supply systems and this often leads to excess water input (overmoistening) and oxygen deficiency. The pattern of fluid and gas distribution in substrate medium in microgravity could not be repeated on Earth but some processes could be imitated. Overmoistening of the substrate medium and the subsequent oxygen deficiency could be replaced with the waterlogging on Earth. Ground experiment was carried out in the laboratory prototype of SVET-2 Space Greenhouse (SVET-2 SG) to study the effect of different root-zone moisture conditions and waterlogging on growth, photosynthesis and chlorophyll content of lettuce plants (Lactuca sativa L. cv. Lolo Rossa). The increase in height and biomass was suppressed while leaf dry matter increased during the waterlogging treatment suggesting assimilates accumulation in the leaves and slow translocation to the roots. Waterlogging caused a rapid decline in net photosynthetic rate (Pn). The reduction of Pn could not be attributed only to diffusion limitations resulting from stomatal closure but also to metabolic inhibition due to accumulation of assimilates in the leaves. The chlorophyll content decreased during the waterlogging and slowly recovered after termination of the waterlogging treatment. Lettuce plants showed decline in Pn and overall growth during waterlogging and demonstrated fast recovery after waterlogging removal. The results suggested that lettuce plants were waterlogging-resistant to a certain degree but yield is greatly affected.