
			Society for Amateur Scientists The Clinostat

The Clinostat

Scientific Tool for the Gravitational Biologist

By Jeff Smith

This monograph is supplemental information to "The Amateur Scientist," Scientific American, February 1996.

All life one Earth has evolved under the constant influenceof gravity. Plants, in particular, have developed strong shoots, trunks and branches that stretch upward and outward, providing ridged support in defiance of gravity's relentless pull. What would life be like without gravity? Would trees need trunks? Would they grow onward and upward forever? No one knows. And what if life had evolved on the Moon with 1/6th the gravity of Earth, or on Mars where gravity is 2/5th that of Earth? I like to envision Martian forests with giant, spindly trees having canopies that could stretch over several football fields. Less gravity would mean trunks could hold trees higher and branches could stretch over wider spaces. What about on the Moon? With such a small gravitational pull, plants might not even need to make rigid stems, trunks or branches. Instead of committing so much energy to making cellulose and lignin, the main structural components of plant cell walls, future lunar gardens might be full of soft-bodied trees with giant, paper-thin leaves.

As the number of Space Shuttle missions increases every year and plans for an International Space Station become a reality, for the first time in history, we have started to learn what plants do in the true absence of gravity. Unfortunately, with the high cost of boosting even small experiments into orbit (several thousand dollars per pound) the going is slow. That's why scientists have turned to the clinostat as a tool for studying plant growth and development under altered gravity conditions. It is true, clinostats may not provide a perfect means to mimic the weightlessness of space or the low gravity environments of the Moon or Mars, but they are the best and only way for Earth-bound scientists (and amateur scientists!) to learn about how upsetting normal gravitational cues can affect plant growth and development.

The number and types of experiments that can be done with a clinostat, like the one depicted in "The Amateur Scientist" [Scientific American, February 1996] are virtually unlimited; and they can provide valuable insights for future space experiments as well as bring us all closer to understanding exactly how plants sense and respond to gravity. Here are a few additional ideas for experiments you can do that are novel and useful.

Germination

Most plants are very sensitive to gravity, even as they are germinating, but whether or not gravity actually affects germination is still open for debate. When performing any other experiment, it only takes a few minutes to note how many of your seeds have germinated. Check for an emerging shoot or root from the seed. In order to determine what a germinated seed looks like, try placing the seeds on or between sheets of moist paper towel in a sealed container (like a Tupperware container). Check them every day and note which ones look like they are germinating. Then, on the next day, check the same seeds again to see if they have actually continued to grow. It doesn't matter as much how you score germination as long as you are consistent and you are sure germinated seeds will continue to grow. Some species of plants may display gravity-dependent germination characteristics while others may not. You might make a new discovery.

Plant Length

Plants move by growing. When a plant curves in response to gravity, it happens when cells on one side of the shoot or root elongate faster than those on the other side. Take the root, for example. When a vertically-growing root is turned on its side, curvature usually occurs at a point several millimeters behind the tip of the root. Cells on the top side of that part of the root get a signal to elongate while cells on the bottom side don't. The result--as cells on top elongate and cells on the bottom don't--is a root that turns down in response to gravity. Since this elongation process is the only way plants can orient their growth with respect to gravity, plant growth is closely related to the ability to respond to gravity. Plants elongate too slowly to watch, but the total length of a seedling is a good measure of how fast that seedling is growing. Try measuring total seedling length along with angularity. Just place the sections of the seedling end to end along a ruler to get the total length of the plant.

Is the total length of the plant related to its angularity? This is one area where the clinostat may not perfectly mimic the effects of zero-gravity. Plant curvature in response to gravity is thought to be due to an inhibition of elongation. For the root, this means downward curvature is due to normal cell elongation on the top side of the root and inhibited cell elongation on the bottom side of the root. Therefore, plants having a high angularity should be shorter than those that grow straight. On a slowly rotating clinostat, plants may try to respond to the continually changing gravity vector by inhibiting their growth on all sides. In space, where there is no gravity signal to tell the plant how to curve, elongation may be normal or even increased.

Do Nutrients Help Plants Sense Gravity?

For a plant to sense and respond to gravity it has to be healthy. Studies have shown that starving plants, or plants which are fed specific hormones or drugs become insensitive to gravity. When plants germinate, they all start out with a certain amount of stored energy and nutrient reserves in their seeds. As those reserves are used up, the plant must find new nutrients from the soil in order to survive. Measure the gravity threshold for seedlings grown on the clinostat with and without extra nutrients. For this experiment, the best thing to do is use an inert material like Vermiculite (at your favorite garden shop) that has no nutrients in it for plants. Then grow some of the seedlings in vermiculite wetted with water while growing others in vermiculite wetted with a plant fertilizing solution (Rapid Grow is a favorite plant food and is found in most garden shops). Are the plants grown with nutrients more sensitive to gravity than the others? If not, try growing the seedlings for a longer period. The plants which weren't given any nutrients will begin to starve and have more trouble sensing and responding to the changing gravity on the clinostat.

Does Clinostat Speed Change Plant Growth and Gravity Sensitivity?

Evidence suggests that plants can sense gravity within a few seconds of turning. For this reason, a slowly rotating clinostat (usually considered to be less than 1 rotation per minute) doesn't trick a plant into thinking it's in the microgravity of space. Instead, the plant can sense its constant state of tumbling but it can't respond fast enough to keep up with the rotation of the clinostat. If you could rotate your clinostat very very slowly--say, one rotation per day--your plants could sense the slow tumble and respond by curving in a spiral with each rotation of the clinostat [some plants are have better gravity sensors than others so you might have to play with the rotation rate to get it just right]. But what about speeding up the rotation rate? Fast clinostats (typically 40-60 rpm) have also been used on plants, and at such a fast rotation rate the plants probably can't sense a change in gravity, much less try to respond so quickly. Unfortunately, as anyone who has ridden in the center and on the outer edge of a merry-go-round will attest, centripetal forces increase greatly as you move from the center of rotation further and further outward. For the fast clinostat, this means a plant's gravity sensing cells must not be located more than a few millimeters from the center of rotation or the plant will be able to sense a centripetal acceleration and the force it exerts.

What's the best speed for a clinostat? No one knows for sure. It depends mostly on the species of plant being used (how sensitive to gravity it is) and the kind of result you want to get. Once you have your clinostat running and you've had a chance to test a few batches of seedlings at one rotation rate, try doubling or halving the rate and see what happens. Is there a relationship between any of germination, angularity, and length with the rotation rate of the clinostat? For rotations that are very slow, you should get plants that can both sense and respond to Earth's 1-g pull. Based on the theories of gravity sensing and response, these plants should be shorter than normal and have a large angularity. For fast rotations, you should get plants that are responding to the centripetal acceleration which is dependent on the distance from the plant to the center of clinorotation. If the rotation rate is too fast, the g-forces at the outer edges of the clinostat will be too much for the plant to handle. At an average force of about 10 g (which corresponds to a distance of 25 cm from the center of the clinostat rotating at 60 rpm) the plants may have trouble growing at all. Now you're using your clinostat to test the effects of hypergravity (more than 1-g) on plant growth. Here, angularity should be low (maybe even less than normal) and plant length should be shorter than normal as well. At what rotation rate do plants grow the longest? That's the rotation rate that may best correspond to the real microgravity of outer space.

How exactly do plants sense and respond to gravity?

There are plenty of detailed scientific reviews that cover what is known about this complex process, right down to the subcellular structures and molecules that make things happen. Scientific American has a great review by Michael Evans, Randy Moore and Karl-Heinz Hasenstein called "How Roots Respond to Gravity." [Scientific American, December 1986]. For a more recent review, but also more technical try Fred Sack's "Plant Gravity Sensing" [International Review of Cytology, volume 127: pp. 193-252].

More on clinostats and how well they mimic the microgravity of space

The American Society for Gravitational and Space Biology (ASGSB) has an entire volume of its Bulletin devoted to clinostats and centrifuges and their use, value and limitations in gravitational biological research [ASGSB Bulletin, volume 5, number 2]. Chapter 7: "How Well Does the Clinostat Mimic the Effect of Microgravity on Plant Cells and Organs?" [pp. 69-75] by Andreas Sievers and Zygmunt Hejnowicz is specifically devoted to plant research. The Bulletin probably won't be in your local library, but you may write to ASGSB at PO Box 12247, Rosslyn, VA 22219. Or you might try visiting the ASGSB home page at: baby.indstate.edu:80/asgsb/

Current research with clinostats

Clinostats have been used to mimic the microgravity conditions that plants might experience in space, but they have recently been used to induce gravitational stresses on plants that seem to affect overall plant growth and development. Here is a small list of current publications by scientists who use clinostats for mimicking microgravity and for studying other processes of plant growth such as starch metabolism and the production of cancer fighting drugs under altered gravity conditions!

CS Brown, and WC Piastuch (1994) "Starch metabolism in germinating soybean cotyledons is sensitive to clinorotation and centrifugation". Plant, Cell and Environment, 17:341-344.

DM Obenland, and CS Brown (1994) "The Influence of Altered Gravity on Carbohydrate Metabolism in Excised Wheat Leaves". Journal of Plant Physiology, 144:696-699.

Hoson T, Kamisaka S, Masuda Y, and Yamashita M (1992) "Changes in Plant Growth Processes Under Microgravity Conditions Simulated by a Three-Dimensional Clinostat". The Botanical Magazine, 105:53-70.

Hoson T, Kamisaka S, Miyamoto K, Ueda J, Yamashita M, and Masuda Y (1993) "Vegetative Growth of Higher Plants on a Three-dimensional Clinostat". Microgravity Science and Technology, 6:278-281.

Hoson T, Kamisaka S, Yamamoto R, Yamashita M, and Masuda Y (1995) "Automorphosis of maize shoots under simulated microgravity on a three-dimensional clinostat". Physiologia Plantarum, 93:346-351.

Kaufman PB, Thompson A, Partide D, Bernal J, and Haidle A (1995) Role of Micro-G and Hyper-G in "Synthesis of Secondary Metabolites of Medicinal Importance in Plants". ASGSB Bulletin, 9:25.

Lorenzi G, and Perbal G (1990) "Root Growth and Statocyte Polarity in Lentil Seedling Roots Grown in Microgravity or on a Slowly Rotating Clinostat". Physiolgia Plantarum, 78:532-537.