The photosynthetic rate of higher plants is a critical component of plant-based atmospheric regeneration systems being proposed for long-duration space missions. The Photosynthesis Experiment and System Testing and Operations (PESTO) experiment is designed to directly measure photosynthesis in microgravity so that an informed decision on the feasibility and design of these systems can be made. The overall objective of this research is to determine the effect of microgravity on photosynthetic response of plant tissues developed in either gravity or microgravity. The specific objectives of this research are to:

• Determine the effect of microgravity on carbon dioxide (CO2) and light response curves of wheat.
• Determine the effect of microgravity on metabolism and electron transport processes associated with photosynthetic and respiratory gas exchange.
• Determine the effect of microgravity on carbohydrate partitioning in wheat.
• Determine the effect of microgravity on gas exchange, including water, over a range of atmospheric vapor pressure deficits.
• Utilize the knowledge gained to understand the response of plants grown under elevated CO2 conditions of commercial, controlled-environment crop production systems.

The PESTO experiment, which will be performed on the International Space Station (ISS), is one of the most complex plant experiments to be conducted by NASA. The experiment will be done in a special plant growth chamber, the Biomass Production Chamber (BPS) that is being built by Orbitec Inc. Photosynthesis and water movement of wheat will be measured for the duration of the experiment. A total of six harvests are anticipated on orbit. Postflight analysis of the tissue for primary photosynthesis parameters, including photosystem I, photosystem II, electron transport, and carbohydrate partitioning, will be made and correlated to in-flight data. These measurements of whole canopy gas exchange in microgravity will be used to understand the effects of microgravity on photosynthesis, to quantify the effects on metabolism, and to model the impact of microgravity on biological approaches to atmospheric regeneration for long-duration space missions.

A number of long-duration preflight experiments were conducted in the BPS during FY 2001, including a 24-day test to evaluate two dwarf wheat cultivars, a 10-day muffler test to evaluate elevated air temperatures, a 50-day Mission Verification Test (MVT) that incorporated all elements of the flight experiment, and intensive training of the ISS Increment IV astronauts. In addition to the experiments in the BPS flight hardware, experiments were performed to quantify the movement of water in the rooting media and the effects of elevated root zone temperatures on wheat growth and development.

Key accomplishments:

• Completed 24-day test of PESTO experiment protocols in the BPS hardware (December 2000).
• Completed 10-day Acoustic Muffler test in the BPS hardware (March 2001).
• Completed 50-day MVT in the BPS hardware (May 2001).
• Completed training of the ISS Increment IV astronauts.
• Completed seven 24-day experiments using Ground Control Root Modules.
• Published three manuscripts in peer-reviewed journals.
• Published one invited presentation in the Proceedings of International Conference.
• Presented five talks at international scientific meetings.