December 7, 2012
Space Station Crew Grow Plants Without Gravity
Lee Rannals for redOrbit.com — Your Universe Online
An experiment reported in the journal BMC Plant Biology studied the effects of growing plants without gravity, helping to disprove one theory about how gravity kick-started plant behaviors on Earth.
Astronauts growing Arabidopsis plants on the International Space Station tried to determine what plant growth patterns could be influenced by gravity.
Plant roots exhibit characteristic behaviors called "waving" and "skewing," and scientists thought these behaviors could be gravity-dependent events. However, experiments on the space station have proved this theory wrong.
The "waving" consists of a series of regular, undulating changes in the direction of root tips during growth. Scientists thought this behavior depended on gravity sensing and responsiveness. "Skewing" is the slanted progression of roots growing along a near-vertical surface. They believed this was a deviation of the roots from the direction of gravity.
To test the hypothesis of the root growth, a research team from the University of Florida, Gainesville grew two types of Arabidopsis thaliana cultivars aboard the ISS.
The planets were grown in specialized growth units that combined a plant habitat with a camera system that captured images every six hours. Imaging hardware helped provide real-time data from the ISS, and comparable ground controls were grown at the Kennedy Space Center.
The authors found that with the absence of gravity, spaceflight roots remained strongly negative phototropic and grew in the opposite direction of the shoot growth. The path taken by the roots as they grew also retained the complex patterns of waving and skewing, which is a characteristic of Earth-grown roots.
The team also observed that the degree of waving exhibited by the plants in space did not match what would be predicted for roots showing an equivalent amount of skewing back on Earth. While in space, waving was more subtle. This result reinforces the idea that waving and skewing represent two separate phenomena, and that gravity is not a mechanistic part of the basic waving and skewing processes, according to the researchers.
"Although plants use gravity as an orientating tropism on the Earth's surface, it is clear that gravity is neither essential for root orientation, nor is it the only factor influencing the patterns of root growth," Lead authors Anna-Lisa Paul and Robert Ferl wrote in the journal. "It seems that other features of the environment are also required to ensure that a root grows away from the seed, thereby enhancing its chances of finding sufficient water and nutrients to ensure its survival."