Making the most of trees' pulling power

摘要

Synthetic trees could one day extract and purify the traces of water held even in arid earth, and help make high-quality wines. That's 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't 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 "negative pressure".rnTo understand how this works, imagine the column of water in a tree between the roots and leaves.rnThe 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.rnNow Stroock has managed tornreproduce 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. "The problem has always been creating pores of the right size," Stroock says. "Hydrogel has the right-sized nanopores naturally."