Gardeners in Space
Scientists have been studying how plants grow in outer space to help sustain life in the microgravity environment. The Russian space station Mir has been a test-bed in low-gravity conditions.
Super Dwarf wheat (Triticum aestivum), the most widely-grown common variety, was sprouted in the Bulgarian/Russian growth chamber Svet on the orbiting space station Mir and in identical control units back on Earth at the Institute of Biomedical Problems in Moscow, Russia.
Wheat is a grass widely cultivated for its seed. The cereal grain is a global staple food.
The first NASA-supported trial began on August 13, 1995. Plants were sampled five times and harvested after 90 days, on November 9. Equipment malfunctions (3, then 4, of 6 lamp sets failed) caused low irradiance, instances of high temperatures, and occasional excessive substrate moisture.
Although plants grew for 90 days, no wheat heads were produced. Considering the low light levels, plants were surprisingly green, but natural biomass production was reduced. Plants were highly disoriented, attributed to low light and mirrored walls. Fixed and dried samples and the root module were returned on the U.S. Shuttle Atlantis on November 20, 1995.
Wheat seeds were planted aboard the Mir 19 and Mir21/NASA2 missions. Planting scheduled to occur during the Mir22/NASA3 mission was not performed.
After the first experiments, plants were grown on Mir with a new light bank and controller for 123 days in late 1996 and 39 days in 1996/1997.
Daily observations and photographs taken by the space station's crew tracked the seedling development. During the Mir21/NASA2 mission, video was also captured.
The American Space Shuttle brought back the plant samples and equipment to Kennedy Space Center (KSC) where U.S. and Russian researchers divided them for further analysis.
The Greenhouse investigation set out to learn how microgravity affects the productivity of a crop plant, specifically dwarf wheat. The researchers had several goals in mind when they started the agricultural experiments:
- Identify the chemical, biochemical, and structural changes in plant tissues induced by microgravity
- Determine microgravity's effect on plant processes such as photosynthesis and water use
- Evaluate current facilities for plant growth aboard the Mir Space Station
The Mir wheat trials shed new light and advanced our understanding of how microgravity disrupts the reproductive events in plants. Results from the Greenhouse experiments proved that both the knowledge and instrumentation to provide a good plant root environment in microgravity have been developed.
Wheat seedling emergence in space was measured at 56 percent and 73 percent in the two root-module compartments on Mir as compared to 75 percent and 90 percent germination rates on Earth for the two plantings.
Growth was characterized as vigorous, with about 1kg (2.2lb) dry mass produced. Individual plants produced 5 to 8 tillers (stalks or sprouts that emerge below the main stem) on Mir compared with 3-5 on the terrestrial controls.
Among the findings was that biomass production during the 123-day growth period far exceeded that of any other comparable experiment with plants in space, suggesting that plant growth is not harmed by microgravity. The second planting, harvested after 30 days and returned to Earth, confirmed these results.
The astrobiologists also examined atmospheric levels of ethylene before and during flowering. Results in Russia and on Mir were about the same - after emergence and before maturity, plant height, awn (hair- or bristle-like appendage) length, and the flag leaf (the last leaf produced by a wheat plant) were significantly shorter in the ethylene-exposed plants than in controls.
The most interesting observation noted was the complete lack of seeds normally expected in the 280 wheat heads produced in the 1996 experiments. A surprising 100 percent floret sterility was found.
Scanning-electron-microscopic (SEM) examination of ethylene-treated florets from Mir-grown and earth-grown plants showed that development ceased before anthesis (the period during which a flower is fully open and functional) and the anthers (pollen-bearing structures in the stamens - male organs - of the flowers) did not dehisce (burst open to release seeds).
Laser scanning confocal microscopic (LSCM) examination of pollen grains from Mir and ethylene-treated plants on earth exhibited zero, one, and occasionally two, but rarely three nuclei. Pollen produced in the absence of ethylene was always trinucleate, the normal condition.
Given the scarcity of trinucleate pollen, abrupt cessation of floret development prior to anthesis, and excess tillering in wheat plants on Mir and in ethylene-containing atmospheres on earth, the researchers argued persuasively that the ethylene on Mir caused the induced male sterility and other symptoms rather than microgravity.
A closed-system experiment carried out February-May, 2005, grew 3rd-generation Mir wheat seeds to the 4th generation. None of the wheat genes showed much difference between wheat with a history of spaceflight exposure and plants with no spaceflight exposure. This finding suggests that exposure to the spaceflight environment in low Earth orbit space stations does not cause significant, heritable, genetic changes in plants.