permaculture

[Lawrence London <lfljvenaura@gmail.com>]

http://nautil.us/issue/6/secret-codes/learning-to-speak-shrub

Biology Environment
Learning to Speak Shrub
http://nautil.us/issue/6/secret-codes/learning-to-speak-shrub
Using molecular codes, plants cry for help, ward off bugs, and save each
other.
By Elizabeth Preston Illustration by Eric Nyquist October 3, 2013

Entomologist Richard Karban knows how to get sagebrush talking. To start
the conversation, he poses as a grasshopper or a chewing beetle—he uses
scissors to cut leaves on one of the shrubs. Lopping off the leaves
entirely won’t fool the plants. So he makes many snips around the edges and
tips of the leaves—“a lot of little bites.”

A few months later, Karban, a professor at the University of California,
Davis who studies plant defense communication, returns to the sagebrush and
examines its leaves, many of which now have damage from real grasshoppers
or beetles. However, within about two feet of the branches he clipped,
leaves have been spared the worst ravages of the hungry insects. That’s
because Karban’s cuttings convinced those damaged leaves they were under
insect attack, so they sent chemical alarms into the air. Neighboring
leaves intercepted and deciphered the code messages, and began prepping
their own defenses against the bugs.

If plants seem silent to us, it’s only because we’re oblivious to their
chatter—we are just beginning to tap into their cryptograms. Plants emit
codes into the air all the time, helping them defend against insects and
other threats, and in some instances serving as warnings to their
neighbors. Moreover, plants can send “SOS” calls for rescue missions and
summon predators to feed on insect invaders.

Plants speak in chemical codes—carbon-containing molecules called volatile
organic compounds (VOCs). Characterized by the ease with which they enter
the air, VOCs are a diverse group: plants alone make more than 30,000
varieties. Some VOCs produce familiar herbal or flower smells. Others are
released only in response to a specific cue. Within seconds of being
damaged, plants send out green leaf volatiles (GLVs), which we can detect
too—for example, as the smell of a newly mown lawn.

Plants can send “SOS” calls for rescue missions and summon predators
to feed on insect invaders.
Sapolsky_TH-F1
Also in Environment
The Last Word with Diane Ackerman

By Kirsten Weir

For three decades Diane Ackerman has brought us as close to nature as
we can get through words. Her books are not a field guide to the
natural world, they are an embodiment of it. As she showed with,
among...READ MORE

Humans don’t get much more information from VOCs. But the waves of
molecules produced by a plant carry packets of cryptic messages. And just
like any transmitted signals, plant-o-grams can be received, decoded,
eavesdropped on, and even scrambled.

Plants send out VOCs in response to physical damage or to chemicals in
insects’ saliva, vomit, or egg-laying fluids. Insect bites can activate
hormones within the plant, like jasmonic acid, ethylene, or salicylic acid,
which increase the activity of the plant’s defense genes. These hormones
can also be released as VOCs to alert the plant’s other leaves and branches
as well as its neighboring vegetation community. In particular, Karban
says, methyl jasmonate—a volatile form of jasmonic acid—seems to be “pretty
potent.” He also found that such communication is more effective between
genetically identical plants—those that come from the same parental bushes
or shrubs. And when Karban placed plastic bags over clipped sagebrush
leaves and tied them up to prevent VOCs from escaping, neither the rest of
its own leaves nor its neighbors were able to crank up their resistance.

VOC messages may be meant for self or family, but plants of other species
can sometimes intercept them. Sagebrush’s alerts can trigger defense
responses in both tomato and tobacco bushes, although it’s not clear how
many plants can hack into other species’ signals.

Moreover, plants may not always want their cries to be heard, scientists
say. “It’s not really in the best interest of a plant to actually tell its
neighbor it’s being attacked,” says Amy Trowbridge, a postdoctoral research
fellow at Indiana University, Bloomington. Adjacent plants are competitors,
and warning a neighbor helps that plant survive while the Good Samaritan
may succumb to an insect invasion. So why do plants shout anyway? Part of
it may be unavoidable: The “chemical weapons” plants use to fend off bugs
may inevitably leak into the air because they’re volatile—so other
vegetation evolved to eavesdrop. So did some predatory insects—they
essentially tune in for dinner calls. Apple trees that are chewed by spider
mites send out a message that attracts other mites that eat the attacking
spider mites. When female sawflies lay their eggs inside the needles of a
Scots pine, the tree’s VOCs summon parasitic eulophid wasps that kill the
eggs. Similarly, tobacco plants that are being gnawed by budworms summon
parasitic red-tailed wasps that lay their own eggs inside the caterpillars’
bodies, later to be eaten by larvae from the inside out.

VOC messages may be meant for self or family, but plants of other
species can sometimes intercept them. Sagebrush’s alerts can trigger
defense responses in both tomato and tobacco bushes.

While plants and insects evolved to exchange these chemical messages,
humans are only beginning to break the code. “We don’t really know how
these compounds are perceived,” Trowbridge says. Researchers don’t yet
understand how the plants collect VOCs from the air and what the detectable
concentration is. Nor do they know whether the molecules are absorbed
straight through the surface of the leaf, or enter through its pores,
called stomata. But they do know that the “listening” plants must not only
receive but also decode the message in order to set off chemical defense
reactions. “Just because a plant may take up a compound doesn’t actually
mean anything,” Trowbridge says. If the intercepted signal can’t be
decoded, it’s of no help.

Moreover, messages could be encoded into a combination of molecules. “The
bouquet that’s released when you clip sagebrush contains literally hundreds
of chemicals that you can measure,” Karban says. He gathers VOCs in plastic
bags filled with special fibers that collect the compounds and analyzes
them in a gas chromatograph. But, he says that “identifying the active
ingredients is really difficult.” Chris Jeffrey, an organic chemist and
chemical ecologist at the University of Nevada, Reno, thinks that to really
crack plants’ cryptography, scientists need to decipher the chemistry of
whole ecosystems at one time. “You’re detecting a very complex mixture of
molecules,” he says, likening the phenomena to our own sense of smell.
“It’s not a single molecule that results in a single response.”

To really crack plants’ cryptography, scientists need to decipher the
chemistry of whole ecosystems at one time.

Why should we bother to crack plant codes? For one thing, they help us
understand how plants will react to environmental changes—like those
expected with climate change. Scientists warn that climate change can
scramble communications, and destabilize ecosystems. Some signals may be
amplified while others are dampened or never detected.

“A lot of volatility is dependent on temperature,” Trowbridge says, so a
warming planet may let VOCs enter the air more easily. Higher temperatures
can also increase the activity of enzymes that manufacture VOCs. On the
other hand, plants trying to survive droughts will squeeze their stomata
shut to prevent water loss. Leaves with closed stomata take in less carbon
dioxide, which they need to manufacture VOCs. With less VOC communications,
plants might not detect alarm signals and become more vulnerable to
insects, or completely succumb to them, Trowbridge speculates. But with too
much VOC emission, plant populations might defend themselves too well—so
insects may seek new food sources, destroying other plants species and
changing ecosystems.

So the next time you are enjoying the silence in a garden, alone, remember
that the silence is an illusion. There is a riot of shouting going on, if
only you could hear it.

Elizabeth Preston is the editor of Muse, a magazine about science and ideas
for kids, and author of Inkfish, a blog about science and cephalopods for
everyone. She has also written for Slate and National Geographic.
2 <https://ello.co/permaculture/post/pt7_ocw5mwznc9dqrxj25w>1m
<https://ello.co/permaculture/post/pt7_ocw5mwznc9dqrxj25w>