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.
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.
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.
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.