The amount of water needed daily by plants for the growth and maintenance of tissues is small in comparison to the amount that is lost through the process of transpiration and guttation. If this water is not replaced, the plant will wilt and may die. The transport up from the roots in the xylem is governed by differences in water potential ( the potential energy of water molecules). These differences account for water movement from cell to cell and over long distances in the plant. Gravity, pressure, and solute concentration all contribute to water potential and water always moves from an area of high water potential to an area of low water potential. The movement itself is facilitated by osmosis, root pressure, and adhesion and cohesion of water molecules.
The overall process: Minerals actively transported into the root accumulate in the xylem, increase solute concentration and decrease water potential. Water moves in by osmosis. As water enters the xylem, it forces fluid up the xylem due to hydrostatic root pressure. But this pressure can only move fluid a short distance. The most significant force moving the water and dissolved minerals in the xylem is upward pull as a result of transpiration, which creates a negative tension. The "pull" on the water from transpiration is increased as a result of cohesion and adhesion of water molecules.
The details: Transpiration begins with evaporation of water through the stomates (stomata), small openings in the leaf surface which open into air spaces that surround the mesophyll cells of the leaf. The moist air in these spaces has a higher water potential than the outside air, and water tends to evaporate from the leaf surface. The moisture in the air spaces is replaced by water from the adjacent mesophyll cells, lowering their water potential. Water will then move into the mesophyll cells by osmosis from surrounding cells with the higher water potentials including the xylem. As each water molecule moves into a mesophyll cell, it exerts a pull on the column of water molecules existing in the xylem all the way from the leaves to the roots. This transpirational pull is caused by (1) the cohesion of water molecules to one another due to hydrogen bond formation, (2) by adhesion of water molecules to the walls of the xylem cells which aids in offsetting the downward pull of gravity. The upward transpirational pull on the fluid in the xylem causes a tension (negative pressure) to form in the xylem, pulling the xylem walls inward. The tension also contributes to the lowering of the water potential in the xylem. This decrease in water potential, transmitted all the way from the leaf to the roots, causes water to move inward from the soil, across the cortex of the root, and into the xylem. Evaporation through the open stomates is a major route of water loss in the plant. However, the stomates must open to allow the entry of CO2 used in photosynthesis. Therefore, a balance must be maintained between the gain of CO2 and the loss of water by regulating the opening and closing of stomates on the leaf surface. Many environmental conditions influence the opening and closing of the stomates and also affect the rate of transpiration. Temperature, light intensity, air currents, and humidity are some of these factors. Different plants also vary in the rate of transpiration and in the regulation of stomatal opening.
Exercise 9A Transpiration
In this lab, you will measure transpiration under various laboratory conditions using a potometer. Four suggested plant species are Coleus, Oleander, Zebrina, and two week old bean seedlings.
0.1 mL pipette, plant cutting, ring stand, clamps, clear plastic tubing, petroleum jelly, fan, lamp, spray bottle, and plastic bag.
Each lab group will expose one plant to one treatment.
1. Place the tip of a 0.1 mL pipette into a 16 -inch piece of clear plastic tubing.
2. Submerge the tubing and the pipette in a shallow tray of water. Draw water through the tubing until all the air bubbles are eliminated.
3. Carefully cut your plant stem under water. This step is very important, because no air bubbles must be introduced into the xylem.
4. While your plant and tubing are submerged, insert the freshly cut stem into the open end of the tubing.
5. Bend the tubing upward into a "U" and use the clamp on a ring stand to hold both the pipette and the tubing.
6. If necessary use petroleum jelly to make an airtight seal surrounding the stem after it has been inserted into the tube. Do not put petroleum jelly on the end of the stem.
7. Let the potometer equilibrate for 10 minutes before recording the time zero reading.
8. Expose the plant in the tubing to one of the following treatments( you will be assigned a treatment by your teacher):
a). Room conditions.
b). Floodlight (over head projector light).
c). Fan ( place at least 1 meter from the plant, on low speed, creating a gentle breeze).
d). Mist ( mist leaves with water and cover with a transparent plastic bag; leave the bottom of the bag open).
9. Read the level of water in the pipette at the beginning of your experiment(time zero) and record your finding in Table 9.1.
10. Continue to record the water level in the pipette every 3 minutes for 30 minutes and record the data in Table 9.1.
Table 9.1: Potometer Readings