Question

What do we know so far about growing plants in space?

So far, studies of plants and their responses to the space environment have concentrated on three main themes. The first is space horticulture, the study of how to grow plants successfully in space, either for experimental purposes or for human use. This involves learning the conditions needed for optimal crop yield, the best plants to grow in space, and the problems inherent in growing plants in a zero-gravity and totally enclosed environment. The second theme is the necessity for gravity, or whether there is any facet of a plant's growth, development, and metabolism that is impaired if there is no gravity. Are there plant processes where the mere presence of gravity is essential? Finally, there is the response of plants to specific directions of gravity by altering their pattern of growth and development. For example, a seedling’s roots will always orient themselves towards the direction of gravity (gravitropism). While NASA has supported research along these themes for over a decade, we still lack enough knowledge to successfully grow plants in space for human use.

Studies of plants on the Space Station and other orbiting spacecraft have shown us that space horticulture isn’t so easy. For example, water in microgravity spreads out evenly in the soil-like material around their roots, which makes it harder for both air and water to reach them. Researchers have to choose the size of the granules in the "soil" very carefully. Also, there is less natural air circulation in an orbiting outpost, and plants can therefore suffocate on their own "exhaled" oxygen! Designers have had to provide fans to keep the air moving. Another major problem has been “closing” the plant growth environment. In order for plants to be useful for future human missions, the technology to recycle water, nutrients, and gases is critical. Plants in space must be able to go through a complete life cycle with little or no waste. A number of attempts at this have been made, generally with rather limited success. For example, wheat plants were recently grown on the Mir space station, but while many heads were produced, none contained seeds. The reasons for failure were attributed to low light intensities, inability to control atmospheric contaminants, and problems with nutrient and water delivery. Clearly, our understanding of growing plants in space is limited, and we need much more research in order to make space horticulture a feasible option.

The above answer is based on a longer article posted on the National Academies website for the sciences at http://www.nationalacademies.org/ssb/csbmch4.htm Please see the link above for more detail.
February 22, 2002