THE air turns muggy, the sky darkens and lethargy descends on the office. A
thunderstorm is brewing. Lassitude is a common human response to the oppressive
pre-storm atmosphere. Plants, on the other hand, seem to react in the opposite
way.
One botanist thinks he knows why. Andrew Goldsworthy believes that
far-sighted foliage can detect the electricity that accompanies a storm and
shift a plant’s metabolism into a higher gear, ready to use the expected
downpour for a spurt of growth. Perhaps that’s why our gardens always look so
verdant after a thunderstorm, he says.
Goldsworthy is the first to admit that this idea is far from being firmly
established. But it does help explain several curious findings. Plant cells can
apparently sense small electric currents. They may also grow better when exposed
to artificial electric fields, but only when other environmental conditions are
favourable. There must be an ecological reason why plants have these powers,
says Goldsworthy, and that reason seems likely to involve thunderstorms. Other
botanists are intrigued by the idea, but some are not entirely convinced it
would work in practice.
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Primed for action
I met Goldsworthy recently in his office in London’s Imperial College, just
next door to the Royal Albert Hall. I soon realised that he is a man who likes
big, bold ideas. There’s a sort of restless enthusiasm about him, I decided,
which makes his conversation completely compelling. And Goldsworthy does have a
good argument to explain why plants might benefit from predicting rainfall.
Water is needed for growth, but if plants waited until rain ran across their
roots, the water would be seeping away before they could exploit it. So it makes
sense for plants to detect an approaching storm and then prepare for growth by
switching on the necessary biochemical machinery.
“Plants are very clever at sensing the environment and if there’s any signal
they could possibly use, my guess is they’ll use it,” says Goldsworthy.
In good weather there is normally a slight voltage gradient in the
air—around 100 volts per metre. “But this is weak by comparison with the
kilovolts per metre that you get in a thunderstorm,” says Goldsworthy. “If the
weather is thundery, even before the lightning starts, you’ve got very large
voltage gradients building up.” At times like this currents can pass into the
earth through plant tissue, as was confirmed decades ago when pioneering
thunderstorm researcher Basil Schonland dug up a tree and earthed it via an
ammeter, recording a current of around 4 microamps.
But do thunderstorms happen often enough, I wonder, for plants to benefit
from using these currents as a cue? “It depends what part of the world you’re
in,” insists Goldsworthy. Worldwide there are about 44 000 thunderstorms every
day and some eight million bolts of lightning, so perhaps he has a point. Still,
I can’t help thinking it would be better for plants to predict rain in general
not just the rain that comes with thunderstorms—perhaps by sensing low
pressure. That would be hard for them to do, replies Goldsworthy.
I try another tack and wonder whether plants really do flourish after storms,
or whether they simply look fresher and greener because they have just taken a
shower. Goldsworthy doesn’t offer any direct evidence, except the opinion of
gardeners who are adamant that their plants look particularly healthy after
thundery weather. This effect can’t be achieved with a sprinkler, he says. The
implication is that something more dramatic is going on during thunderstorms.
And Goldswothy does have scientific evidence to bolster this assertion.
First, there is a large body of research in the area known as
“electroculture”. One of the pioneers, Karl Selim Lemström, a physicist
from the University of Helsinki, published an English translation of his results
as long ago as 1904. Lemström carried out several field experiments in
which he exposed growing plants to electric fields from overhead wires, creating
a voltage gradient of about 10 kilovolts per metre. The wires weren’t directly
connected to the plants, but small currents could reach the plants via ions in
the air. The plants flourished under these conditions, producing a harvest
almost one-and-a-half times that expected.
In Britain, Vernon Blackman—a plant physiologist based, like
Goldsworthy, at Imperial College—set about staging similar experiments.
Between 1915 and 1920, he ran field trials on oats, barley, winter-sown wheat
and clover-hay mixtures in three different areas of the country. He charged
wires above his test plots to between 40 and 80 kilovolts for six hours each
day.
Blackman was convinced that the electricity was having an effect. Of his 18
field trials, 14 showed increased yield. Nine had yields over 30 per cent higher
than expected. Oats and barley were up 22 per cent compared with the control
plots. Tests on plants in pots seemed to confirm this, with maize and barley
plants flourishing under the wires. When Blackman made the wires negative
instead of positive, the effect persisted, just as it did when he substituted
alternating currents for direct currents. He recorded successes with currents as
low as 10 picoamps (10 x 10-12 amps) flowing through the plants, but
currents above 10 nanoamps (10 x 10-9 amps) reduced growth.
Although the tests now sound rather eccentric, they were taken seriously at
the time. Blackman was a distinguished professor and a Fellow of the Royal
Society, who was known for his meticulous eye for detail. His work also had
official approval and interest from the electroculture committee of the Ministry
of Agriculture and Fisheries.
Growing pains
So why aren’t we eating electrified crops today? The trouble with
electroculture was that it didn’t always work. When botanists in the US tried
similar tests they drew a blank. “There was a huge controversy about this and
people were at one another’s throats virtually,” says Goldsworthy. “The subject
died a natural death just before the war.”
And here the matter rests. On the one hand, the positive results could have
been caused by something other than electricity. Equally, the failures don’t
necessarily mean that electricity has no effect. In fact, if Goldsworthy is
right then mixed results are predictable. Electrified plants that have geared
themselves up for a soaking might well respond poorly if that soaking doesn’t
come, as must often have happened in the field trials with artificial currents.
The failure of the American experiments may also have been due to high
background voltage gradients from sandstorms, says Goldsworthy. These could have
stimulated the control plots and obscured any effect of the treatment.
If the effect was real, I ask, then surely we’d expect to find plants growing
particularly well near power lines. “People have occasionally reported greener
areas under power lines,” says Goldsworthy, with a mischievous grin, “but you
can’t be certain that it’s an effect of the electricity, because birds sit on
power lines and what do birds do when they’re sitting around?”
There is no doubt that plants respond to fertilisation by bird droppings, but
can they detect and respond to environmental electricity? There is evidence that
they can, says Goldsworthy, and this further supports his idea. For a start,
small electrical currents—carried by ions such as hydrogen, calcium and
potassium—flow through certain plant cells. These currents, which measure
around 0.1 microamps per square centimetre, appear to play a key role in plant
development.
Setting the direction
In developing seaweed eggs of the genus Fucus, for instance, an
electrical current defines the eventual axis of the growing plant. Calcium ions
flood into the cell at one particular site—and this region eventually
becomes an anchoring structure, called the rhizoid. In other plants, too,
including oat seedlings, cell currents seem to define the direction in which
growth occurs, with growth taking place parallel to the current.
“Electricity was first harnessed in evolution as a means of controlling
growth,” says Goldsworthy. “It’s more fundamental than the use of electricity by
animals in nerve impulses.” Electricity is important for cell growth in both
plants and animals and, in an intriguing series of experiments, Goldsworthy and
his colleague Minas Mina believe they have discovered how it might work.
They studied the currents flowing into and out of tobacco cells growing in
tissue culture using a device called a vibrating probe. This has a vibrating
head which picks up tiny voltage differences between two points in space. With
enormous ingenuity, the researchers modified an existing apparatus, using a
loudspeaker to produce the vibrations and a wooden kebab skewer to carry those
vibrations to the voltage sensor.
Vital ingredient
In this way they could make long-term measurements around individual cells,
monitoring currents of around 0.1 microamps per square centimetre. They then
exposed individual cells for a few hours to “artificial” currents an order of
magnitude bigger than the natural currents. After switching off the artificial
current, they tested the cell again. The current pattern had changed, with the
cell tending to become repolarised in line with the applied current. This could
explain the observation that plant cells apparently alter their own currents and
bring them into line with those of nearby cells, says Goldsworthy.
Further tests showed that cells only respond to an artificial current if
their environment contains calcium ions. One interpretation is that the response
is the work of voltage-gated calcium channels—special proteins that sit in
cell membranes and act as doorways for calcium. When they sense a change in
voltage across the membrane, they flip open and let calcium ions stream into
cells.
Goldsworthy’s point is that this mechanism could also be used to sense
atmospheric electricity, provided the currents induced by a thunderstorm were
similar in magnitude to those that cells can detect. “Having got an
electrosensing mechanism in the plant, it would be very surprising if evolution
hadn’t made use of it for other purposes,” says Goldsworthy. “It’s done it in
animals—you get all sorts of electroreceptors in fish—so why not in
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The fact that the mechanism depends on calcium could be highly significant.
Calcium is a ubiquitous cellular messenger that switches on enzymes within
cells, an effect that could underpin the increased growth that sometimes seems
to follow electrical stimulation. And the calcium connection may also explain
why electroculture experiments aren’t always successful. “Calcium stimulates the
cell to do whatever it’s programmed to do,” explains Goldsworthy. “I like to
think of it as being the `accelerator’ of the cell. And the response you get
will depend on what gear it’s in—forward or reverse.”
Goldsworthy has also tested the powers of electrical stimulation in his lab
by applying a current directly to cultured plant tissues. His aim was to induce
polarisation in groups of cells to increase the level of collaboration in
building structures such as shoots. Working with his colleague Keerti Singh
Rathore, he used tiny electrodes to deliver between 1 and 2 microamps, for long
periods, to collections of undifferentiated cells from tobacco plants. The
effect was extraordinary. Growth rates increased by around 70 per cent and the
cultures developed up to five times as many shoots.
The method was patented, but it hasn’t made Goldsworthy a rich man. Applying
the current directly using electrodes made the technique labour-intensive. And
when the researchers tried to achieve the same result using alternating currents
in metal grids above and below the cultures, the only result was a greening of
tissue.
What about other practical applications? If there were a revival of interest
in crop electroculture, for instance, might we end up sitting down to a meal of
supercharged salads and galvanised courgettes? Goldsworthy says it might be
possible, but could well be uneconomic because of the large capital investment
involved. And it might not always work.
“Where I think it might work,” he says tentatively, “is in seed treatment.”
Tests carried out in Russia, he explains, have shown that electric currents may
trigger germination of seeds such as cherry and barley. This could reflect a
natural mechanism by which seeds germinate after thunderstorms, when conditions
are right for growth. If seeds could be persuaded to germinate more uniformly by
treating them with electricity, there would be benefits for growers.
After our interview, I talked to half-a-dozen plant scientists about
Goldsworthy’s hypothesis. Some think it’s a neat idea, but—as you would
expect with any speculative notion—the general feeling is that more
evidence would be welcome. “There is something there,” says Paul Lynch of the
University of Derby, “we’re constantly surprised by what plants can do.”
Another reaction comes from Mark Tester of Cambridge University: “Plant
growth certainly appears to be affected by experimentally applied currents
passing through their tissues, and these currents may well be similar in
magnitude to those beneath thunderclouds. But this would be a highly
sophisticated response to what would be a very rare event for many species, so
it is unlikely to be a widespread adaptation.”
The last word, however, goes to Goldsworthy. “You should be able to set the
readers arguing among themselves anyway,” he says with a laugh.
- Further reading:
Electrical Manipulation of Cells
edited by P. Lynch and M. Davey (Chapman & Hall, 1996) - The electric compass of plants by A. Goldsworthy,
91av, 2 January1986, p 22