SYNTHETIC trees could one day extract and purify the traces of water held even in arid earth, and help make high-quality wines. That鈥檚 the claim of a team who have reproduced the way plants pump water to heights of many tens of metres.
For years, it was thought that capillary action couldn鈥檛 raise water more than about 10 metres, and this posed a puzzle: how do trees and other tall plants pump water from their roots to their topmost leaves? The favoured answer among plant biologists is that evaporation across the leaf membrane puts water inside the tree under 鈥渘egative pressure鈥�.
To understand how this works, imagine the column of water in a tree between the roots and leaves. The water molecules in this column are held together by hydrogen bonds, like links in a chain. The pressure of this water can be reduced so far that it actually becomes negative. That places the water column under tension, as if the chain were being pulled taut, explains Abraham Stroock at Cornell University in Ithaca, New York. Loss of water by evaporation from the leaves at the top of the chain then pulls the water up the tree.
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Now Stroock has managed to reproduce this process in the lab. He says the big challenge was to find a membrane that works in the same way as a leaf, allowing water, but not air, to pass across it. He and Tobias Wheeler, also at Cornell, found that the hydrogel material used in soft contact lenses turns out to have just the right properties. 鈥淭he problem has always been creating pores of the right size,鈥� Stroock says. 鈥淗ydrogel has the right-sized nanopores naturally.鈥�
Stroock and Wheeler used the material to make synthetic trees just a few centimetres tall and tested their ability to pull water out of a reservoir. He found that they could produce negative pressures of more than 10 atmospheres 鈥� enough to pump water to a height of 100 metres. That鈥檚 about the height of the world鈥檚 tallest trees.
鈥淚t鈥檚 an outstanding piece of work,鈥� says Missy Holbrook, a plant biologist at Harvard University. Because the hydrogel, unlike a tree, is transparent, 鈥渇or the first time, we can see visual evidence of the processes involved鈥�, Holbrook says.
The devices also make it straightforward to measure negative pressure in plants, something that could prove invaluable to horticulturalists. For example, too little negative pressure causes grapes to ripen too quickly, but too much can kill them. 鈥淭his gives vine growers a way to monitor and control this factor,鈥� Stroock says.
Stroock also points out that large trees pump gallons of water a day out of soil. He suggests that scaled-up synthetic trees could extract drinking water from deep down in this way.
