Friday, April 22, 2011
We are slowly learning that plants do communicate information and do make decisions. A huge amount of this activity takes place in the plants root space. Recall that the root system typically matches the mass of the above ground mass.
This all supports research into protocols that take advantage of this behavior for agriculture.
For instance, using so called hills rather than rows promises to be far superior as a growing method simply because it assures maximum communication between the plants.
The traditional three sisters mixed corn and beans in a hill with the odd squash plant thrown in to provide weed suppression. Obviously the beans provided nitrogen.
At present, that is the only clever combination that I know of and it would be good practice to reestablish it, provided there is a way to mechanize the laying down of the hills themselves. It certainly should be applied in the family garden and perhaps we need to think out a lot of other prospective combinations. The nitrogen equation alone needs to be better exploited.
Any given hill for any particular plant type can be set with a core of peas or beans to do that. Instead of growing radishes and beets and carrots in rows, we set them in the borders of hills that also have peas or beans at its core.
Then of course, while we are at it, we throw in a handful of biochar to help eliminate leaching. Have to get that pitch in there.
Heard it on the grapevine: The secret society of plants
29 March 2011 by Ferris Jabr
The botanical underground is a social network of powerful alliances and nepotism. Decoding its messages could lead to radical change in farms and forests
Every autumn swarms of dusty grey moths engulf the mountainside birch forests of northern
Scandinavia, laying their
eggs on twigs so that, come springtime, the newly hatched larvae can feast upon
budding leaves. It looks like a battle that the trees, with no natural
defences, are doomed to lose, but some have a secret weapon. They form an
alliance with a neighbouring plant, a kind of rhododendron, borrowing wafts of
its volatile insecticides as a sort of olfactory camouflage. "This kind of
interaction has never been observed in the field before," says Jarmo
Holopainen at the University of Eastern Finland in , who made the discovery ( New Phytologist, vol 186, p 722). His study is one
of the latest to demonstrate the unexpectedly complex relationships between
We've known for some time that plants respond to one another, but only now are we realising how subtle and sophisticated their interactions can be. Plants continually eavesdrop on each other's chemical chatter - sometimes sympathetically, sometimes selfishly. Some plants, like the Scandinavian rhododendron, assist their neighbours by sharing resources. Others recognise close relatives and favour them over strangers. And at least one parasitic plant homes in on its host's telltale chemical scent (see "Scent of a victim").
"Plants don't go out to parties or to watch the movies, but they do have a social network," says Suzanne Simard, a forest ecologist at the
University of British
Columbia in .
"They support each other and they fight with each other. The more we look
at plant signalling and communication, the more we learn. It's really
Since the development of time-lapse photography, it has been possible to document the dances and scuffles in densely populated plant communities: saplings on the forest floor compete for space to stretch their roots and shoots; fallen trees provide young ones with nourishment; vines lash around desperately searching for a trunk they can climb to reach the light; and wildflowers race each other to open their blooms in springtime and compete for the attention of pollinators. To truly understand the secret social life of plants, however, you must look and listen more closely.
A good place to start is underground in the rhizosphere - the ecosystem in and around plant roots. Beneath the forest floor, each spoonful of dirt contains millions of tiny organisms. These bacteria and fungi form a symbiotic relationship with plant roots, helping their hosts absorb water and vital elements like nitrogen in return for a steady supply of nutrients.
Now closer inspection has revealed that fungal threads physically unite the roots of dozens of trees, often of different species, into a single mycorrhizal network. These webs sprawled beneath our feet are genuine social networks. By tracing the movement of radioactive carbon isotopes through them, Simard has found that water and nutrients tend to flow from trees that make excess food to ones that don't have enough. One study published in 2009, for example, showed that older
transferred molecules containing carbon and nitrogen to saplings of the same
species via their mycorrhizal networks. The saplings with the greatest access
to these networks were the healthiest (Ecology, vol 90, p 2808).
As well as sharing food, mycorrhizal associations may also allow plants to share information. Biologists have known for a while that plants can respond to airborne defence signals from others that are under attack. When a caterpillar starts to munch on a tomato plant, for example, the leaves produce noxious compounds that both repel the attacker and stimulate neighbouring plants to ready their own defences.
Yuan Yuan Song of
South China Agricultural
University in and colleagues investigated whether
similar chemical alarm calls travel underground. They exposed one group of
tomato plants to a pathogenic fungus and monitored the response in a second
group connected to the first via a mycorrhizal network. The diseased plants
were sealed inside airtight plastic bags to prevent any communication above
ground. Nevertheless, the healthy partners began producing defence chemicals,
suggesting that the plants detect each other's alarm calls via their mycorrhizal
networks (PLoS One, vol 5, p e13324). Guangzhou
Another recent discovery, one which may be connected with Song's finding, is that some plants recognise members of their own species and apparently work together for the common good. Amanda Broz of
University in paired spotted knotweed plants
inside a greenhouse either with other knotweeds or with blue bunchgrass. She
then simulated an attack by spraying them with methyl jasmonate, a chemical
many plants release when wounded. The knotweed's response depended on its
neighbours. When growing near members of its own species, it produced leaf
toxins to boost its defences. But it chose to focus on leaf and stem growth
when its neighbours were bunchgrass (BMC Plant
Biology, vol 10, p 115). Fort Collins
Such discrimination makes sense because, in the knotweed's native environment, dense clusters of a single plant tend to attract large numbers of insects to an all-you-can-eat buffet. So cooperating with other knotweed plants helps stave off an attack. However, when knotweed is surrounded by bunchgrass, a better strategy is to leave defence to its neighbours and concentrate on aggressive growth - which might also help explain why knotweed is such an effective invasive species.
Broz's research was published just last year, and it remains unclear how knotweed, or any other plant, could be recognising members of its own species. However, one instance of a plant with family values has been more thoroughly explored.
In a landmark paper published in 2007, Susan Dudley from
in , reported the first case of
plants recognising and favouring their kin (Biology Letters, vol 3, p 435). Her studies of American
sea rocket, a scraggly weed that grows along the shorelines of the Great Lakes,
showed that a plant potted with an unrelated individual did not hesitate to
spread its roots and soak up as much water and nutrients as it could. However,
when Ontario, Canada Dudley planted sea-rocket siblings in the
same pot, they exercised restraint, taming their eager roots to better share
resources. Siblings and strangers that grew near each other but did not share
pots showed no differences in root growth, indicating that sea rocket relies on
underground chemical signalling to identify its kin. They don't seem to be
using mycorrhizal networks, though.
In subsequent research with Meredith Biedrzycki from the
University of Delaware
in Newark, Dudley
discovered that the signals take the form of "exudates" - a cocktail
of soluble compounds including phenols, flavonoids, sugars, organic acids,
amino acids and proteins, secreted by roots into the rhizosphere. How these
indicate relatedness is still a mystery, though (Communicative & Integrative Biology, vol 3, p 28).
In the past few years, kin recognition has been discovered in other plants, including the botanical "lab rat" Arabidopsis and a kind of Impatiens called pale jewelweed. This has led some botanists to argue that plants, like animals, are capable of kin selection - behaviours and strategies that help relatives reproduce. Kin selection has an evolutionary rationale because it increases the chances that the genes an individual shares with its relatives will be passed to the next generation, even if altruistic behaviour comes at a cost to one's own well-being. "There's no reason to think plants wouldn't get the same benefits from kin selection that animals do," says
Recognising siblings and restraining one's growth in response certainly looks like kin selection, but that still leaves the question of whether such interactions also improve the survival prospects of related plants. Research by Richard Karban at the
University of California, ,
goes some way to answering that. Davis
Karban studied a desert shrub called sagebrush, which emits a pungent bouquet of chemicals to deter insects. When he clipped an individual plant's leaves to simulate an attack, he found that it mounted a more robust defence if it was growing next to its own clone than if its neighbour was unrelated. What's more, for a period of five months afterwards, the neighbouring clones suffered far less damage from caterpillars, grasshoppers and deer than did unrelated neighbours (Ecology Letters, vol 12, p 502).
Studying kin selection and other plant interactions doesn't just improve our knowledge of basic plant biology and ecology. "There are a lot of people really interested in it, because it's not just an intellectually neat puzzle," says James Cahill at the
of Alberta in .
"There are many potential applications, especially for agriculture." Edmonton, Canada
One obvious area is in companion planting - the strategic positioning of different crops or garden plants so they benefit one another by deterring pests, attracting pollinators and improving nutrient uptake. This ancient technique, which traditionally relies on trial and error and close observation, can be highly effective. For example, beans fix nitrogen that boosts growth in some other plants, and when Europeans arrived in
in the 15th century, they
discovered that Native Americans used corn as a natural trellis for bean
plants. Our modern understanding of plant interactions suggests we could find
new, more subtle and potentially beneficial relationships, which could help us
overcome a major drawback of modern monoculture farming. Since a single
pathogen can wipe out an entire crop of genetically similar - and therefore
equally vulnerable - plants, farmers make heavy use of pesticides. But instead
of picturing an endless stretch of corn or wheat, imagine something more like a
jungle of diverse species that work together above and below ground. America
Cahill has another idea. "Fertilisers aren't always spread evenly," he says. "Maybe we could breed plants to cooperate more effectively with their neighbours to share fertiliser." Meanwhile, Simard thinks the recent discoveries about mycorrhizal networks have implications for both agriculture and forestry. Hardy old trees should not be removed from forests so hastily, she says, because saplings depend on the mycorrhizal associations maintained by these grandparent trees. She also suggests that farmers should go easy on fertilisation and irrigation because these practices can damage or destroy delicate mycorrhizal networks.
Clearly, we do not yet have all the information we need to start deploying such tactics. "What we want to do next is develop more advanced techniques to watch roots grow, to really see what they do with each other and how they interact in space,"
She also wants to figure out what genetic factors control plant interactions
and look at how they change survival and reproduction. "The molecular
aspects are perhaps the most challenging," she adds, "but we have
made some big leaps."
The idea that plants have complex relationships may require a shift in mindset. "For the longest time people thought that plants were just there," says Biedrzycki. "But they can defend themselves more than we thought and they can create the environment around them. It turns out they have some control over what is going on through this chemical communication." Passive and silent though plants may seem, their abilities to interact and communicate should not come as such a shock. "Some incredibly simple organisms - even one-celled organisms - can recognise and respond to each other," says Broz. "Why is it so bizarre to think that plants could have this same kind of ability?"
See gallery: "Plants that act like people"
Scent of a victim
Many of the social interactions of plants seem to involve a form of sharing or cooperation mediated by chemical signals. However, some chemical communication is far from benevolent, as research on a parasitic vine called dodder has found.
Dodder contains almost no chlorophyll - the green molecule that allows plants to produce sugars from sunlight, water and carbon dioxide. Instead, after sprouting as a leafless tendril, it searches for a victim into which it sinks its nozzles and sucks out the sugary sap. "We knew how it creates nozzles and gets resources from the host, but nobody knew how dodder found its host," says Consuelo De Moraes at
at . University Park
Some plants identify neighbours by sensing sunlight refracted off their leaves, but time-lapse video suggests that dodder uses a different technique. The footage shows that when the tendril searches for a host it twirls about like a snake tasting the air. Could it be searching for a chemical, wondered De Moraes?
To test this idea, she and her colleagues hid a variety of plants around a corner from a dodder tendril. If the vine were really using chemical sensing to find its victims, it should be able to home in on its hosts using the volatile chemicals they naturally produce.
That is exactly what they found. In fact, dodder even showed dietary preferences based on the different airborne chemicals, almost always choosing succulent tomatoes over twiggy wheat, and favouring healthy hosts by avoiding the chemicals given off by damaged plants (Science, vol 313, p 1964). "Not only does dodder use chemical cues to find a host," says De Moraes, "it can distinguish between hosts of different qualities. It knows which plants are healthier and goes after them."