How does Transpiration affect Plants?

BIO 121


Water is essential to plants in many ways. It first provides the major substance for living, to keep cells from shriveling up and dying. Transpiration, the evaporation of water from plants, is the cause of loss of water. This mostly occurs during the time when the plants stomata is open for the passage of CO2 and oxygen during photosynthesis. 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. I selected a bean plant on which to test varied environmental factors on transpiration. The different environments that would be 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 could also tested for the control. One bean plant would be used for each simulated environment. To set up the experiment, four pieces of clear plastic tubing could be cut to sixteen inches. Inside each tube there would be a 0.1-mL pipette placed at the tip. Taking four ring stands, one paired with each tube/pipette set, each end of the tubing are clamped, so that the tubing made a “U” shape. Next the tubing is filled with water so that no air bubbles would be present and that water completely filled the tubing and pipette. The four bean plants are each placed into the open end of their respective tubing, and