- 小陳Lv 61 decade agoFavorite Answer
The solution in the xylem of a plant stem is under negative pressure that results from the difference in the rate of water entry at the roots and water loss through the stomates of the leaves. In general the negative pressure potential of the xylem is equal to the water potential of the surrounding cells minus the osmotic potential that the solutes dissolved in the xylem sap creates. This solute concentration is typically low, and thus by estimating the negative pressure potential of the xylem, one can obtain an estimate of the water potential of the surrounding cells that differs from the true value by only the solute potential of the xylem, which is typically less than 5% of the water potential value. Procedurally, xylem pressure potentials can be measured by cutting a stem from a plant and placing it in a pressure bomb apparatus. When the stem is cut, the solution in the xylem retreats away from the cut because the atmospheric pressure pushes the solution up the cut xylem vessel. By placing the sample under pressure with the stem protruding into the atmosphere the amount of pressure required to just move the solution back to the cut surface of the xylem should equal (in magnitude, but be opposite in sign) to the pressure potential of xylem before the cut was made (less the solute potential of the xylem sap). This method of measuring water potential was initially developed at roughly the turn of the century by H.H. Dixon, and more was recently improved by P.F. Scholander in 1965. It is now a standard tool of plant physiologists for both field and laboratory measurements of plant water potential.
Using a razor blade obtain a freshly cut section of plant tissue onsisting of a section of stem and leaves not more than approximately 6 inches in length.
2009-03-08 14:53:17 補充：
Note that the section should be freshly cut, and under no circumstances should the section be re-cut to even the end. Thus, it is important to make a clean flat cut when the tissue is removed from its original source.
2009-03-08 14:54:00 補充：
Once the tissue has been obtained, insert the cut stem through the rubber stopper provided with the pressure bomb apparatus such that when the stopper is placed in the apparatus the cut end will protrude outside of the pressure bomb, and the leaves and remaining stem will be inside of the bomb.
2009-03-08 14:56:44 補充：
Place the metal retaining plate over the stopper and tighten down the 4 bolts that hold the plate in place taking care not to cross thread the bolts which should be easily turned by hand until they are snug. Then tighten them using the Allen wrench provided until they are firmly tightened.
2009-03-08 14:57:00 補充：
Carefully place the tissue inside the bomb, screw the lid onto the bomb. Double check to make sure that all bolts are securely tightened, and the lid is securely screwed in place.
2009-03-08 14:57:54 補充：
2009-03-08 15:00:52 補充：
2009-03-08 15:07:44 補充：