When humans go to the moon or Mars, they'll
probably take plants with them. NASA-supported researchers are learning
how greenhouses work on other planets.
by Karen Miller
Confused? Then you're
just like plants in a greenhouse on Mars.
No greenhouses exist
there yet, of course. But long-term explorers, on Mars, or the moon,
will need to grow plants: for food, for recycling, for replenishing
the air. And plants aren't going to understand that off-earth environment
at all. It's not what they evolved for, and it's not what they're
But in some ways, it
turns out, they're probably going to like it better! Some parts
of it, anyway.
"When you get
to the idea of growing plants on the moon, or on Mars," explains
molecular biologist Rob Ferl, director of Space Agriculture Biotechnology
Research and Education at the University of Florida, "then
you have to consider the idea of growing plants in as reduced an
atmospheric pressure as possible."
There are two reasons.
First, it'll help reduce the weight of the supplies that need to
be lifted off the earth. Even air has mass.
Second, Martian and
lunar greenhouses must hold up in places where the atmospheric pressures
are, at best, less than one percent of Earth-normal. Those greenhouses
will be easier to construct and operate if their interior pressure
is also very low - perhaps only one-sixteenth of Earth normal.
artist's concept of
Greenhouses on Mars.
The problem is, in
such extreme low pressures, plants have to work hard to survive.
"Remember, plants have no evolutionary preadaption to hypobaria,"
says Ferl. There's no reason for them to have learned to interpret
the biochemical signals induced by low pressure. And, in fact, they
don't. They misinterpret them.
Low pressure makes
plants act as if they're drying out.
In recent experiments,
Ferl's group exposed young growing plants to pressures of one-tenth
Earth normal for about twenty-four hours. In such a low-pressure
environment, water is pulled out through the leaves very quickly,
and so extra water is needed to replenish it.
But, says Ferl, the
plants were given all the water they needed. Even the relative humidity
was kept at nearly 100 percent. Nevertheless, the plants' genes
that sensed drought were still being activated. Apparently, says
Ferl, the plants interpreted the accelerated water movement as drought
stress, even though there was no drought at all.
That's bad. Plants
are wasting their resources if they expend them trying to deal with
a problem that isn't even there. For example, they might close up
their stomata - the tiny holes in their leaves from which water
escapes. Or they might drop their leaves altogether. But, those
responses aren't necessarily appropriate.
experiment related to Ferl's: Lettuce growing in a low-pressure
dome at the Kennedy Space Center.
Fortunately, once the
plants' responses are understood, researchers can adjust them. "We
can make biochemical alterations that change the level of hormones,"
says Ferl. "We can increase or decrease them to affect the
plants' response to its environment."
studies have found benefits to a low pressure environment. The mechanism
is essentially the same as the one that causes the problems, explains
Ferl. In low pressure, not only water, but also plant hormones are
flushed from the plant more quickly. So a hormone, for example,
that causes plants to die of old age might move through the organism
before it takes effect.
Astronauts aren't the
only ones who will benefit from this research. By controlling air
pressure, in, say, an Earth greenhouse or a storage bin, it may
be possible to influence certain plant behaviours. For example,
if you store fruit at low pressure, it lasts much longer. That's
because of the swift elimination of the hormone ethylene, which
causes fruit to ripen, and then rot. Farm produce trucked from one
coast to the other in low pressure containers might arrive at supermarkets
as fresh as if it had been picked that day.
Much work remains to
be done. Ferl's team looked at the way plants react to a short period
of low pressure. Still to be determined is how plants react to spending
longer amounts of time - like their entire life - in hypobaric
conditions. Ferl also hopes to examine plants at a wider variety
of pressures. There are whole suites of genes that are activated
at different pressures, he says, and this suggests a surprisingly
complex response to low pressure environments.
growing onboard the International Space Station. Ferl's
research will improve greenhouses not only on other planets,
but also on spaceships
To learn more about
this genetic response, Ferl's group is bioengineering plants whose
genes glow green when activated. In addition they are using DNA
microchip technology to examine as many as twenty-thousand genes
at a time in plants exposed to low pressures.
Plants will play an
extraordinarily important role in allowing humans to explore destinations
like Mars and the moon. They will provide food, oxygen and even
good cheer to astronauts far from home. To make the best use of
plants off-Earth, "we have to understand the limits for growing
them at low pressure," says Ferl. "And then we have to
understand why those limits exist."
Ferl's group is making
progress. "The exciting part of this is, we're beginning to
understand what it will take to really use plants in our life support
systems." When the time comes to visit Mars, plants in the
greenhouse might not be so confused after all.