Improvements in and actual performance of the Plant Experiment Unit onboard Kibo, the Japanese experiment module on the international space station

In 2004, Japan Aerospace Exploration Agency developed the engineered model of the Plant Experiment Unit and the Cell Biology Experiment Facility. The Plant Experiment Unit was designed to be installed in the Cell Biology Experiment Facility and to support the seed-to-seed life cycle experiment of Arabidopsis plants in space in the project named Space Seed. Ground-based experiments to test the Plant Experiment Unit showed that the unit needed further improvement of a system to control the water content of a seedbed using an infrared moisture analyzer and that it was difficult to keep the relative humidity inside the Plant Experiment Unit between 70 and 80% because the Cell Biology Experiment Facility had neither a ventilation system nor a dehumidifying system. Therefore, excess moisture inside the Cell Biology Experiment Facility was removed with desiccant bags containing calcium chloride. Eight flight models of the Plant Experiment Unit in which dry Arabidopsis seeds were fixed to the seedbed with gum arabic were launched to the International Space Station in the space shuttle STS-128 (17A) on August 28, 2009. Plant Experiment Unit were installed in the Cell Biology Experiment Facility with desiccant boxes, and then the Space Seed experiment was started in the Japanese Experiment Module, named Kibo, which was part of the International Space Station, on September 10, 2009 by watering the seedbed and terminated 2 months later on November 11, 2009. On April 19, 2010, the Arabidopsis plants harvested in Kibo were retrieved and brought back to Earth by the space shuttle mission STS-131 (19A). The present paper describes the Space Seed experiment with particular reference to the development of the Plant Experiment Unit and its actual performance in Kibo onboard the International Space Station. Downlinked images from Kibo showed that the seeds had started germinating 3 days after the initial watering. The plants continued growing, producing rosette leaves, inflorescence stems, flowers, and fruits in the Plant Experiment Unit. In addition, the senescence of rosette leaves was found to be delayed in microgravity.