..... (12/99 Urbana IL) Some exciting findings just out show how plants move fluids long distances in their phloem DOWNWARDS in plants. ..... As with most findings, these are built upon earlier findings. ..... In this case, upon the work of Ernst Munch in the 1930's.

..... The plants have a pump! ..When sucrose is made in the leaves, it is pumped into certain cells where it accumulates at up to 100 times the level found elsewhere in the plant. ..This produces an extremely hypertonic situation, so water, of course, diffuses into these cells causing a lot of pressure that pushes the sap through the phloem to the destination or "sink" tissues. .. Here is the overall scheme for the DOWNward movement:

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..... Conversely, there are several hypotheses as to how water rises UP in the xylem to the tops of tall trees, but each hypothesis does not - ahem - hold water, as it were (see transpiration). ..How this transpiration really happens can be the subject of your project.

GUIDED TOUR

1. Sunlight and carbon dioxide react in the chloroplast with the production of sucrose and ATP. ..Some of the sucrose (or glucose) is stored in vacuoles in the leaf cells, while other sucrose molecules diffuse through the plasmodesmata of the mesophyll cells.

2. When the sucrose reaches a phloem cell, both sucrose and a proton (H⁺) are pumped into the cell. ..The protons were the result of an ATP-driven reaction, as shown just above "the Pump."

3. Sucrose accumulates in this cell, and as it does so, the cell becomes evermore hypertonic. .. Water comes in and builds pressure, which pushes the sucrose/water through the phloem cell system. ..Being that the osmotic pressure is very significant, the "sap" can move a very significant distance. ..Whether or not intermediary pumps exist, this author does not yet know.

4. Finally the aqueous sucrose solution ("sap") reaches a terminal cell and the sucrose is pumped out, and the water is forced to be pumped out.

5. While some of the sucrose can then be split to its monomers and thence on to other hexoses, those hexoses and sucrose are pumped into a cell in the "sink tissue," where the sucrose is either used as an energy source for its own metabolism, or the sucrose is imported into cells that store the sucrose as starch in one of the cell's vacuoles. .. One might think of potatoes as an obvious example.

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   ..... The discovery group is led by Assoc. Prof. Daniel R. Bush, who is director of the interdepartmental program in Physiological and Molecular Plant Biology at Univ. of Illinois, Urbana, IL. ..Bush is also a scientist with the Photosynthesis Research Unit of the U.S. Dept. of Agriculture.

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