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Powerful microscope allows study of tree nanostructures

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Researchers from Virginia Tech; George-August University in Gottingen, Germany; and the Jackson Laboratory in Bar Harbor, Maine, have discovered how “check valves” in wood cells control sap flow and protect trees when they are injured. Using the new 4Pi microscope, scientists were able to see nanostructures inside of microscopic structures known as bordered pits within wood fiber cells.

Previously, wood had to be dried, coated with carbon and put under a high vacuum in order to be studied at the nanolevel. But, the 4Pi, combined with an optical imaging technique called confocal laser scanning microscopy, has made it possible to see the wood cells in a more natural state.

The team’s research is featured as the cover story in the September 2013 issue of the American Journal of Botany, in an article titled, “A new approach for the study of the chemical composition of bordered pit membranes: 4Pi and confocal laser scanning microscopy,” by Daniela Maschek, a student at George-August University; Barry Goodell, professor of sustainable biomaterials in Virginia Tech’s College of Natural resources and Environment; Jody Jellison, professor of plant pathology in the College of Agriculture and Life Sciences at Virginia Tech; Mark Lessard of the Jackson Laboratory; and Holger Militz, professor at George-August University.

“The wood fiber cell, which is 100 times longer than it is wide, can be a quarter-inch long, or longer,” said Goodell.

Along each cell are hundreds of bordered pits, which are shared between the walls of adjacent wood fiber cells. A tree’s fluid sap passes from one cell to the next through the bordered pits. Each pit contains a mesh of nano-sized cellulose fibers that radiates out from a thickened, solid, central region called the torus. Fluids ooze in through the mesh-like membrane, around the torus, and out the other side.

“When wood is injured, such as by an insect, or is being dried, like we do with kiln-dried lumber, the torus and part of the membrane will typically shift from the center of the pit to one side in order to seal the pit opening,” he said.

The researchers discovered how some of the nanostructures of the membrane move with the torus to seal off the pit and found the location of certain chemical components of the membrane, including crystalline cellulose and pectin.

“When we used specific chemical tags to dye the pectin red and the cellulose green, we saw for the first time that pectin surrounds the torus and forms ring-like structures at the margin of the torus,” Goodell said. “Scientists had never seen these before, nor knew that pectin formed these unique ring-like structures in the membrane. As part of the pit sealing process, pectin separates to form an outer fringe around the torus, while the bulk of the pectin gets pulled into the aperture to block it.”

In addition, the scientists discovered that the center of the torus is a hollow filled with a fluid or gel that gets squeezed out when the process to seal the pit begins.

According to Goodell: “When the tree is injured, sap starts to flow out and air flows in. This changes the partial pressure and, just like a check valve used in plumbing systems, the bordered pit closes. The movement and redistribution of the pectin is also in response to this partial pressure differential.

“When water flows within the cells, the pectin in specific locations in the cell wall swells and forms a gel. When it swells, it reduces the size of the pores in the bordered pit, thereby reducing water flow. It helps explain how trees seal off their cells so that they don’t ‘bleed to death’ or lose all their sap when they are injured. Without pectin, a tree cannot defend itself, and it can’t seal off damaged cells.”

The researchers speculate that the ring of pectin around the torus functions as a buffer to help relieve mechanical stress during the deformation of the pit membrane, “thus preventing the stiffer cellulose strands from detaching from the cell wall, or breaking in that area,” they say.

The full article and research findings can be read at

Written by cabinettrends

October 3, 2013 at 7:00 am

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