How do plants talk to each other?

A  In 1983, plant scientist Jack Schultz and Baldwin reported that the leaves of young maple trees increased their defense systems when exposed to maples that had been damaged by plant-eating (herbivores). The injured trees, they suggested, were warning neighbors to the presence of a predator by releasing chemical signals into the air. But the plant research community did not accept this. The results were difficult to replicate, critics pointed out. Many also questioned how it could be evolutionarily stable if it benefited neighboring plants but not the plant releasing the signal. By the late 1980s, most ecologists thought that Schultz and Baldwin’s idea had been discredited.

B  A decade later, however, a number of more carefully designed experiments began to yield convincing indications to the contrary. In 2000, evolutionary ecologist Richard Karban showed that wild tobacco plants became resistant to herbivores when grown in close proximity to sagebrush plants whose leaves had been damaged by cutting. This change appeared to be in response to chemicals known as volatile organic compounds or VOCs--released by the sagebrush plants. Other researchers soon reported similar VOC-induced defense responses in several other plants, including lima bean, broad bean, barley, and corn. And in 2006, Karban showed that VOCs from sagebrush induce herbivore resistance in plants growing at distances of up to 60 cm, well within the range of sagebrush neighbors.

C  But the question remains: Why should a plant waste valuable resources on a function which has no obvious advantage for itself? One hypothesis is that communication channels are merely an extension of within-plant signaling. In sagebrush, lima bean, and poplar, VOCs from damaged parts of a plant induce resistance in undamaged sections of the plant, suggesting that each individual plant uses the signals to coordinate its own physical responses to protect itself. Karban agrees, saying "The interplant signaling we see may be a result of plants co-opting this process." Alternatively, VOC-based signaling between plants may have been favored because it enhances the inclusive fitness of the sender by aiding related plants of the same species: a strategy known as kin selection.

D  Over the past few years, a team led by Ariel Novoplansky of Ben-Gurion University of the Negev in Israel seems to have found proof that distress signals can be passed through plant roots. They planted garden pea plants in rows and subjected the first in each row to conditions similar to those experienced in a drought. They then evaluated the response by measuring the microscopic holes on leaves, known as pores, which react when there is a shortage of water. After ten minutes, the stressed plant was seen to be closing its pores, followed by all of its neighbors, one by one. Importantly, in a control setup where root contact between neighboring plants was blocked, pores stayed open. Meanwhile, David Johnson’s team at the University of Aberdeen in Scotland have been studying the labyrinths of hair-like fungi that curl around the roots of most plants. These fungi are involved in an important two-way relationship: in exchange for sugars, they provide plants with much-needed phosphorus and nitrogen. Research in which broad bean plants were infested with aphids--small insects--revealed that these networks also served as a channel for warning signals.

E  Monica Gagliano of the University of Western Australia believes that plants may even use sounds to alert their neighbors. In one study, she demonstrated that chilli plants growing next to fennel plants developed more quickly than seedlings grown with other chilli plants. Gagliano and her colleagues suspect the chilli plants were compensating for the presence of the fennel, which is known to release chemicals that inhibit the growth of other plants. Remarkably, however, when plant communication pathways--via VOCs, root contact, and common fungal networks--were blocked, the results begged for an alternative explanation. "We think this other channel of communication might be acoustic," says Gagliano. But behavioral ecologist Carel ten Cate of the University of Leiden in the Netherlands points out that taking advantage of such benefits would require mechanisms yet to be described in plants.

F  Researchers believe that knowledge of this phenomenon could eventually be applied to agriculture, and lead to the cultivation of hardier crops. But all agree that there is much work to be done. "Applying our limited knowledge of plant-communication mechanisms to agriculture is a big jump," says Johnson, "but it is definitely on the horizon." Nevertheless, a message has emerged loud and clear from those studying this new area of botanical interaction: despite not possessing eyes, ears, or a nervous system, plants are anything but uncommunicative. "When I started my PhD [in the late 1980s], all this stuff was considered very weird," recalls Ariel Novoplansky. "Today, there is no doubt. We now recognise that plants are capable of some very sophisticated exchanges of information with other plants. This idea is not strange anymore."

Power from the Sea

A  In the north of Scotland there is a hazardous stretch of water called the Pentland Firth. Every day, twice a day, a mass of the Atlantic Ocean rushes into this 22.5-kilometre channel long known to sailors as ‘Hellsmouth’ between the Orkney Isles and the Scottish mainland. The vast torrent of water crashes into the North Sea, and is then sucked into reverse motion, starting another whirlpool cycle. The currents in this massive tidal action can reach 22km/h and some people think there is enough energy in these flows alone to supply up to 10 per cent of Britain’s electricity. Finally, after years as the most overlooked source of renewable energy, two experimental marine turbines have now begun to exploit the power of the sea.

B  People first started harnessing the sea’s powers over a thousand years ago, with water mills placed over tidal rivers to grind wheat. But when the industrial revolution came along, the sea lost out to coal and oil. But things are now changing. Faced with the diminishing resources of fossil fuels and concern over climate change, many governments are considering pumping more money into exploring the sea’s potential.

C  Tidal currents are caused as the oceans bulge in response to the gravitational pull of the Moon and Sun. The continuous flow of water speeds up as it’s squeezed between landmasses. But this is not the only way in which the oceans carry exploitable power. There’s also the energy in waves created by the wind blowing over the water. In fact, on a global scale, wave energy represents a massive resource -- because seas cover 70 per cent of the world’s surface. At this time, engineers are tackling the key problem of the survivability of wave energy devices that operate in the harsh environment of sea and air.

D  Over thirty years ago, a Scottish professor, Stephen Salter, led efforts to exploit wave energy. The ‘Salter Duck’ was a vast 300-tonne floating machine with a generator powered by the motion of the waves. But his much-admired invention was abandoned when it was (wrongly) calculated that the cost of the energy produced would be too high.

E  Other engineers have chosen tidal currents as the most efficient sea power option. Tides have a key advantage over most other renewable energy resources: they are almost continuous and entirely predictable. And when there is a slowing in the speed of an ocean current as the tide is turning, backup power from other sources can be fed into the power grid to meet demand.

F  The first schemes produced by these engineers centred on building barrages across the mouths of estuaries to trap water at high tide, letting it escape through turbines in order to drive generators. In the mid-1960s, one such barrage began operation in France, and it still supplies most of Brittany’s electricity. Another has been proposed for Canada’s Bay of Fundy, which has the largest tidal range in the world. But such schemes can cause serious environmental problems. They prevent fish migration and destroy highly sensitive marine habitats, located between high and low tide lines. And harm done to plant and animal life isn’t the only drawback of barrages. The building of a barrage was proposed for the Severn Estuary, which has the world’s second highest tidal range. But the plan has been abandoned because the estuary route is one of the main outlets for waste from central England and Wales. Blocking it could produce the world’s dirtiest stretch of water.

G  Yet the potential for sea power has continuing appeal and, while developments continue on a number of wave energy converters, in the last three years both government and private financiers have finally made up their minds to back tidal energy projects. A number of different technologies are being considered, but arguably the most advanced and perhaps the most elegantly straightforward is the marine current turbine.

H  This is essentially an underwater wind turbine -- a propeller mounted on a tower. The principal advantage is that the underwater environment is calm, even in storms. And as water is 800 times denser than air, these turbines can capture much larger volumes of energy than land turbines can. Furthermore, while on- and offshore wind farms face huge opposition from those who see them as an eyesore, the first marine turbine designs present minimal intrusion on the view. Crucially too, they appear to impact little on the environment: the blades move so slowly that marine creatures can easily swim through. This form of energy production is supported by environmental pressure groups such as Greenpeace, while air turbines are meeting strong opposition from environmentalists for harming birdlife.

I  A British company has been working on the concept of the marine current turbine for years, and has installed their prototype off the coast of north Devon at Lynmouth. There are also dozens of other powerful tidal sites around British shores not least the famous channels of fast-moving water of Portland and St Catherine’s Point. But a Norwegian consortium of energy and engineering companies, Hammerfest Stroem, became the first to connect a marine turbine to an electricity grid in the autumn of 2003 in northern Norway’s Kval Sound. Around 30 houses in the town of Hammerfest are powered by the 300-kilowatt machine, which weighs 220 tonnes and stretches 30 metres from the base to the highest blade tip. Extensive trials of both the British and Norwegian designs are currently underway.