vendredi 26 décembre 2008

Why do Christmas trees survive?

The secret to the success of conifers—which include the planet's tallest and oldest trees--lies on their highly efficient valves that let water flow through simple conduits as easily as in the more complex angiosperm conduit system, researchers report in this week's Science.

In conifers, water flows upward through short, single-celled tracheids. In angiosperms, in contrast, the pipes consist of longer, multicellular conduits called vessels. Both systems have connecting pits—or valves—concentrated at the end-walls of the conduits. In conifers, water has to cross many more valves, suggesting that they may have higher flow resistance than angiosperms. However, the researchers found exactly the opposite. They showed that conifers apparently compensate for their structural handicaps because they have evolved pits that have much lower resistance than the average angiosperm—59 times lower, in fact.

"When you compare a conifer tracheid with an angiosperm vessel," explained study author John S. Sperry of the University of Utah, "when you put them head to head and compare the hydraulic resistivity for the same diameter, tracheids essentially have the same flow resistance as vessels, despite the fact that they are 10 times shorter, because the flow resistance through these fancy pits is so incredibly low."

Valves in conifers and angiosperms function in a slightly different way. The angiosperm valve is a simpler, evolutionarily older type of valve, which they retained from their ancestors. It has a cellulose pit membrane-- a thin mesh with very small pores—which functions by capillarity. "The pores in these valves are so small that the capillary forces set up as the air is drawn into the membranes are big enough that they keep the air from passing through," Sperry noted.

The conifer pit, in contrast, is a more recent acquisition. It also has a cellulose membrane, which has two distinct regions: a central torus, which is impermeable to water with very small pores, and a surrounding, very porous margo, which forms a loose web that holds to the torus. "In conifers, capillarity is not responsible for the ultimate sealing of the pit. The whole pit aspirates the torus, which blocks up the aperture," Sperry told The Scientist. "This explains why you can have such large pores in the margo."

Although the structural differences between the pits of angiosperms and conifers have been known for a long time, nobody had ever compared their hydraulic properties, until Jarmila Pittermann, at Sperry's lab, decided to take a look at the problem. When they studied the hydraulic resistance in the two major types of pits, the researchers found that the flow resistance in the conifer pit was, on average, 59 times lower per give area than in the angiosperm pit. "The torus-margo structure provides a huge advantage in terms of minimizing the flow resistance through these valves, and it does so without sacrificing safety from air entry" and the damaging phase-change from liquid to vapor, Sperry explained.

"For many years, I had taken for granted that the fairly porous torus-margo of conifers did for the conifers what the perforations did for angiosperms, but that was all speculative, it was never really experimentally proven," Pieter Baas, from the Nationaal Herbarium Nederland, at the Universiteit Leiden Branch in The Netherlands, told The Scientist.

N. Michele Holbrook of Harvard University found the work "beautiful." Sperry and his team "really demonstrated how these are two different solutions to a same basic problem," Holbrook added.

According to Sperry, the modern torus-margo type of pit membrane is a trade off, which allows conifers to compensate for the high-frequency of inwalls in their tracheid-based xylem. "Conifers have achieved similar, if not lower, hydraulic resistivity than angiosperms, and this is, in part, why conifers can compete so effectively with angiosperms despite the fact that their xylem is so much simpler," Sperry concluded.

© 1986-2008 The Scientist

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