Transpiration Lab


     Water is essential to plants in many ways. It first provides the major substance
for living, to keep cells from shriveling up and dying. The second major
function is to keep the plants rigidity. As plant cells become turgid, full of
water, the cells expand, filling the extent of their cell walls, which are kept
taught with turgor pressure. If the cells lose water, two problems occur. First,
the cells dehydrate, causing the organism to die. Second, turgor pressure is
lost as cells become flaccid, limp and unfilled, causing a loss of support for
the plants structure which makes it appear wilted. As aquatic plants evolved
into large complex land plants, an adaptation occurred in the center of plants
to allow full growth without the problem of water loss. A system of vascular
bundles extending from the tips of the furthest leaves to the deepest roots of
each plant developed, carrying water in xylem sap and sugar in phloem. While
phloem can transport sugar in any direction within the plant, xylem can only
move water up, from root to leaf. Once in the leaf, the water evaporates through
stomata—tiny gaps in the lower epidermis of each leaf, which are regulated by
guard cells—a process called transpiration The movement of water into and out
of the xylem involves water pressure factors in different sections of the plant.

As water slips into the roots through osmosis, a positive water pressure gently
pushes the water into the plants roots and supplies a jumpstart for the
water’s journey up the vascular bundle. However, it is not this pressure that
supplies a great force towards the upward movement of water; it is the
evaporation of water from the stomata that pulls water upward and out. When the
stomata are open to take in carbon dioxide for carbohydrate production, water
begins to evaporate and seep out of the tiny holes in each leaf. With a constant
pull of water outward, other water molecules are pulled up to replace it. The
pull is provided by the cohesive properties of water molecules as each leaving
molecule pulls on another molecule which is hydrogen bonded to it. The process
continues as a series of movements until all the water molecules in the xylem
sap are being pulled upward by their hydrogen bonds to the water molecules ahead
of them. Thus the slight negative pressure occurs. Different environmental
factors can have impacts on the intensity of water evaporation, and thus the
rate of plant transpiration. Just like water in an open environment, a dry
environment would increase the evaporation of water, and the rate of
transpiration. A hot or very bright environment would do the likewise.

Conversely, moist, dark, or cool environments would allow for a slower rate of
transpiration because water would not be as readily evaporative. When testing
the rate of transpiration for any given plant, I hypothesize that plants exposed
to copious quantities of light will transpire more rapidly than those in a
regular environment. Methods We selected a bean plant on which to test varied
environmental factors on transpiration. The different environments included
excessive sunlight—a floodlight one meter from the plant, wind/dry air—a
stationary fan approximately one meter away from the plant on low speed,
humid/rainy climate—leaves misted, then covered with a clear plastic bag (open
at the bottom for air exchange). Normal room conditions were also tested for the
control. One bean plant was used for each simulated environment. To set up the
experiment, four pieces of Tygon clear plastic tubing were cut to sixteen
inches. Inside each was placed the tip of a 0.1-mL pipette. Taking four ring
stands, one paired with each tube/pipette set, each end of the tubing was
clamped, so that the tubing made a "U" shape. Next the tubing was filled
with water so that no air bubbles were present and that water completely filled
the tubing and pipette. The four bean plants were each placed into the open end
of their respective tubing, then sealed with petroleum jelly around the sides
(to prevent accidental water evaporation). The plants were allowed to sit for
ten minutes before the initial reading was made, to allow for equilibration.

After recording levels of water for all plant environment simulations, readings
were made in ten minute increments until thirty minutes elapsed. After this, the
leaves were cut off of each plant to be weighed and measured. With these
figures, we found the total surface area of each plant, after which we could
calculate the rate of transpiration for each climate. Results To determine the
rate of transpiration for each tested bean plant, the cumulative water loss (in
milliliters) was divided by the leaf surface area of each plant (in meters
squared). This rate was figured for each time increment: initial, ten minutes,
twenty minutes, and thirty minutes. Table 1 shows these calculations for the
control, group a, floodlight, b, fan, c, and mist, d. The relationship among the
data is shown on Figure 1. The lines for test plants b and c both show high
rates for transpiration, while control plant a is at a moderate rate of
transpiration and test plant d has a relatively low rate of transpiration
compared to the other plants. Conclusion As Figure 1 shows, the plants tested in
dryer climates, b and c, showed higher rates of transpiration. This is due to
the greater potential for evaporation in their environments. The extra exposure
to light adds heat which dries up water vapor around the plant and inside the
leaves, as it leaves through the stomata. The water in the tube was then pulled
by the negative pressure created by the evaporation of water, increasing the
transpiration rate. With plant c, the fan dried water vapor around the plant and
in the leaves, causing the area to be dry, thus creating a negative pressure for
water in this plant as well. Plant d had a very low rate of transpiration
because its environment was very moist. Water was very unlikely to evaporate in
the misted enclosure, therefore causing the plant only to need to replace the
water which it used to maintain its turgor pressure. The environment for plant a
provided a normal room climate. Although evaporation was likely, it did not seem
to be a large factor in the plant’s functions. So, as water did escape from
the stomata of the plant’s leaves, the slow rate created enough negative
pressure to replace the water being lost to the air and being used by the plant,
which wasn’t very much. When this experiment was initially done in our
classroom, many faults occurred. Without prior experience handling plants and
petroleum jelly, the experiment is difficult. While it is a good idea to see the
experiment in order to understand it, the book provided the best data.