May 15, 2002: When gardeners poke a seed into the ground, they never worry in which direction it lays. Give it enough water and food and care, and sure enough, its root will grow downward and its stem will sprout upward -- every time! Lay the seed upside-down, and the root and stem would still find their proper positions.
How do plants do it? We humans know up from down (even with our eyes closed) because we have a complex organ in our inner ear that senses gravity's pull and signals the brain. But plants have no such organ. It's a puzzle.
Above: The seeds of flax plants, like those shown here, will be sprouted in orbit to help figure out how plants sense gravity. Image courtesy Flax Council of Canada.
Everyone knows that plants grow toward light, but there must be more to it than that. Trees in northern forests, for example, grow straight up even though the Sun is never directly overhead, and the first stem emerging from a buried seed grows upward through dark soil.
It's clear that gravity must play some role, too. Indeed, scientists know that the direction of gravity's pull is behind many plant behaviors, such as corn crops righting themselves after being flattened by a storm. What's unclear is exactly how plants "feel" gravity and respond to it. What part of a plant senses the direction of gravity's pull? And how is that pull translated into a chemical response that alters the plant's growth?
No one knows the answers.
But scientists do know enough to suggest two possibilities. First, when the fluid contents of plant cells (called the "protoplasm") are pulled downward by gravity, the pressure exerted on the cell walls might serve as a signal that helps plants distinguish up from down. Second, plant cells contain starch grains which, like protoplasm, drift down when gravity is present. Scientists suspect this might act as a cue to plants, too.
But which is it? A novel experiment slated to fly aboard the space shuttle in July 2002 (STS-107) might reveal the answer.
Right: Seen under a microscope, the starch grains in these plants cells are visible as small dots. Image courtesy NASA.
Karl Hasenstein, principal investigator for the BioTube/Magnetic Field Apparatus experiment, explains: The shuttle will carry a payload of flax seeds to orbit. Once there, a computer-controlled dose of water will start them growing. Unlike flax sprouts growing on Earth, these won't feel the usual pull of gravity. The protoplasm and the starch grains within their cells will float rather than sink.
Plants have been grown in space before. But this experiment will be the first to subject plants to an "artificial gravity" created by magnets.
The experiment will have a high-gradient magnetic field in the plant growth chamber. Within the cells of the plants, the protoplasm will be essentially unaffected by the magnet, but the starch grains will feel the magnetic force. They will sink to the bottom of the cell as if drawn there by gravity.
Starch grains are not magnetic in the usual sense -- if you held one against your refrigerator it wouldn't stick. But the grains are "diamagnetic," which means they develop a weak magnetic field when other magnets are nearby. The diamagnet's field will naturally oppose that of the nearby magnet -- hence the prefix "dia" -- so the starch grains will be repelled. Although the effect is weak, this diamagnetic response allows researchers to use magnets to move the starch grains.
"By changing only the internal displacement of the starch grains, we can put one of these two arguments to rest," explains Hasenstein, a professor at the University of Louisiana at Lafayette. "If the starch grains are the gravity-sensing trigger, we should see the flax-seed roots curve along the magnetic gradient. And if the pressure on the cell walls triggers the curvature, we should see no response."
Above: This plant originally sprouted with the pot upright and was later turned on its side. The new stem growth curved to re-align with gravity. Image courtesy University of Wisconsin-Madison.
Infrared cameras will automatically photograph the germinating roots. Regular cameras can't be used because the chamber will be kept completely dark. The darkness allows scientists to know that the seeds are responding to the magnetic fields, not just growing toward a light source.
Don't bother trying this experiment at home with ordinary refrigerator magnets. Only special "high-gradient" magnetic fields will do. Hasenstein's experiment uses magnets about 50 times more powerful than a typical refrigerator magnet. The magnets have ferromagnetic wedges attached to them, which focus a strong magnetic field into a small area. Around that area, the strength of the field tapers off quickly, creating the "gradient" of field strength that moves the starch grains.
High-gradient magnetic fields will be used in two chambers of the experiment, while a third chamber will use a homogeneous magnetic field as a "control."
Right: The wedge-shaped ferromagnet on the left focuses the magnetic field lines, illustrated on the right. Starch grains sedimenting in a narrow tube only 0.3 mm wide are repelled by the high-gradient field. Image courtesy Oleg Kuznetsov, University of Louisiana at Lafayette.
The lessons learned won't only apply to flax seeds (which were chosen for their small size and their quick, reliable germination). All normal plants have these starch grains, so the results of this experiment will add to our basic understanding of plants in general.
Starch grains or protoplasm? No matter which proves correct, researchers will have lingering questions. For example: "how does the mechanical trigger (e.g., starch grains drifting downward) produce a biochemical response?" BioTube/MFA won't provide all the answers right away, but it is an important first step -- one that will teach us something fundamental about the leafy-green life all around us.
Shuttle flight STS-107 -- information about the shuttle mission that will carry the experiment discussed in this article.
Right: Related experiments conducted on the ground used a slowly rotating device called a "clinostat," shown here, to approximate weightlessness. Gravity pulls on the sprouting seeds from every angle as the device rotates, so the net effect is nearly zero. These earlier experiments were informative, but scientists can't be sure of their conclusions without running the experiment in the true weightlessness of Earth orbit.
Leafy Green Astronauts --Science@NASA article: NASA scientists are learning how to grow plants in space. Such far-outs will eventually take their place alongside people, microbes and machines in self-contained habitats for astronauts.
Teaming up on Space Plants --Science@NASA article: students, scientists, and astronauts join forces to learn more about how plants grow in space.
Classroom exercise -- laboratory exercise to teach children about plants' responses to gravity (from the American Society of Plant Biologists)
Classroom exercise -- demonstration of gravitropism suitable for elementary school students
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