In 1997, NASA grew Brassica rapa on Mir for 122 days to test a full life cycle; seed-to-seed cycles succeeded, but space-grown seeds were smaller

For decades, growing a plant from seed all the way back to seed in space seemed almost impossible. Plants either died before flowering or flowered without producing seed. Finally, the puzzle was solved, according to the study, ‘Gravity independence of seed-to-seed cycling in Brassica rapa,’ published in the journal Planta by a NASA-supported researcher, Mary E. Musgrave, and her team. In 1997, they spent 122 days aboard the Russian space station Mir growing Brassica rapa, a fast-growing relative of Arabidopsis from the mustard family, through two full generations. Plants bloomed and set seed. But the seeds grown in space were smaller and took longer to grow than seeds grown on the ground.

Why scientists wanted a fast-growing plant

Plants are not simply decorations on a spacecraft. They take in carbon dioxide and emit oxygen, expel water vapor that can be trapped and re-used, and can even be eaten. For astronauts on long voyages away from Earth, a crop that could continue to produce itself, generation after generation, would be a great advantage. The Musgrave team had previously conducted shorter tests on the space shuttle using Arabidopsis thaliana, a small plant in the same mustard family. Those initial flights produced something unexpected. The flowers inside the sealed growth chambers were dying, not because of the weightlessness itself, but because of the lack of airflow to bring fresh carbon dioxide to the plants. When the engineers fixed the ventilation problem and restored carbon dioxide flow, the shuttle-grown flowers were as healthy as their ground-based counterparts and produced normal seed. This finding comes from Musgrave, Kuang, and Matthews's 1997 study in Planta, 'Plant reproduction during spaceflight: importance of the gaseous environment,' which documented that later, better-ventilated flights showed normal reproductive development and seed production matching ground controls. That success gave the team confidence to try a much longer experiment aboard Mir.

Say hello to the first farmer in space

For the Mir experiment, the team turned to Brassica rapa, a close relative of Arabidopsis that grows faster and larger. Unlike Arabidopsis, which pollinates itself, Brassica rapa cannot fertilize its own flowers. Each flower required help, which meant researchers could observe the exact moment of pollination and see if the seeds that followed were developing on time. Astronaut C. Michael Foale had that job. The shuttle carried seed strips that were planted in a specially built, well-ventilated growth chamber aboard Mir. When the flowers opened, Foale hand-pollinated each with a “bee stick,” the dried body of a bee glued to a toothpick to transfer pollen from flower to flower. The pollinated plants produced seed pods, and when the pods ripened and dried, Foale collected the seeds and planted them again, starting a second generation grown, pollinated, and harvested entirely off Earth. Musgrave later referred to him as “the first farmer in space.”

Seeds that sprouted, but never really kept up

The first generation of plants grown from seed sent up from Earth looked perfectly normal. The size of the plants and the number of seeds in the pods were as expected. But looking closer at those seeds told a different story. Kuang, Xiao, McClure and Musgrave examined the internal structure of the space-grown seeds in a study titled ‘Influence of microgravity on ultrastructure and storage reserves in seeds of Brassica rapa L’ published in Annals of Botany and found that they had less than 20 percent of the cotyledon cell number of the ground-control seeds. The space-grown seeds retained starch longer than normal instead of shifting to protein and fat storage, as a healthy ripening seed should. The second generation grown from these smaller, less mature seeds also turned out smaller than the ground controls.

Separate test: Does pollination itself require gravity?

At about the same time, Musgrave and her team conducted a related but separate experiment on space shuttle mission STS-87 to test whether pollination itself required gravity. Kuang, Popova, Xiao and Musgrave found in the study, 'Pollination and embryo development in Brassica rapa L. in microgravity,' published in the International Journal of Plant Sciences, that the same hand-pollination technique could be used to collect and transfer pollen normally in microgravity. After 16 days in orbit, the embryos lagged Earth-grown counterparts by about 12 days in development, based on observations taken 15 days after pollination. Notably, this delay was specific to that first generation of embryos; the study found no difference between spaceflight and ground-control embryos in the generation that followed. The team found that not a single stage of Brassica reproduction is totally dependent on gravity, although embryo quality could still be influenced in microgravity.

Why it still matters today

Together, these experiments led Musgrave to conclude that gravity is not necessary for any stage in a plant’s life cycle. Plants can flower, be pollinated and set seed without it, but the final seed quality does seem to be affected if it is not there for spaceflight. That finding could be valuable beyond space travel. Delayed seed maturity is a familiar headache for farmers here on Earth. Understanding exactly how the environment inside a ripening seed pod shapes the final seed could help both future space farmers and farmers back home, long before anyone plants a crop on the Moon.