Laboratory 9: Transpiration

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1 Laboratory 9: Transpiration YOU MUST KNOW The role of water potential and transpiration in the movement of water from roots to leaves. The effects of various environmental conditions on the rate of transpiration. How to identify xylem and phloem and relate their structure to their function. Overview of the Lab Transpiration is the major mechanism that drives the movement of water through a plant. In the first section of this laboratory, you investigate, facto r that influence the rate of transpiration. In the second section, you study plan anatomy as it relates to transport. Hints and Review I Review hydrogen bonding! (Topic 1, Chapter 3) In water, a hydrogen bond is weak bond between the hydrogen of one water molecule and the oxygen 0. another, and it accounts for the unique properties of water, including adhesion and cohesion. I Water enters a plant through the root hairs, passes through the tissues of the root into the xylem, and travels up through the xylem vessels into the leaves. I Transpiration is the evaporation of water from the leaves through the stomates. It is the major factor that pulls the water up through the plant. I When water enters the roots, hydrogen bonds link each water molecule to the next (cohesion) so the molecules of water are pulled up the thin xylem vessels like beads on a string. The water molecules also cling to the thin walls of the xylem cells (adhesion). The water moves up the plant, enters the leaves, moves into air spaces in the leaf, and then evaporates (transpires) through the stomata (singular, stoma). I Stomata are the pores in the epidermis of a leaf. There are hundreds of stomata in the epidermis of a leaf. Most are located in the lower epidermis. This reduces water loss because the lower surface receives less solar radiation than the upper surface. Each stoma allows the carbon dioxide necessary for photosynthesis to enter, while water evaporates through each one in transpiration.

~ Guard cells are cells surrounding each stoma. They help to regulate the rate of transpiration by opening and closing the stomata. To understand how they function, study the following figures.

2 ~ Guard cells are cells surrounding each stoma. They help to regulate the rate of transpiration by opening and closing the stomata. To understand how they function, study the following figures. As you look at the figures, keep in mind that an increase in solute concentration lowers the water potential of the solution, and that water moves from a region with higher water potential to a region of lower wa ter poten tial. Mesophyll ce ll s Xylem vessel Air space -.--, Water molecule Guard cell Stoma Xylem cell wall Adhesion.- """-:;1--Cohesion Figure 9.la Figure 9.lb I Notice that in Figure 9.2a the guard cells are turgid, or swollen, and the stomatal opening is large. This turgidity is caused by the accumulation of K+ (potassium ions) in the guard cells. As K+ levels increase in the guard cells, the water potential of the guard cells drops, and water enters the guard cells. I In Figure 9.2b, the guard cells have lost water, which causes the cells to become flaccid and the stomatal opening to close. This may occur when the plant has lost an excessive amount of water. In addition, it generally occurs daily as light levels drop and the use of CO 2 in photosynthesis decreases. Potassium ions '---1"" Turgid guard cell Vacuole filled with water ,.- Flacc id guard cell Stoma closed Stoma open Figure9.2a Figure 9.2b I A leaf needs carbon dioxide and water for photosynthesis. For carbon dioxide to enter, the stomata on the surface of the leaf must be open. Transpiration draws water from the roots into the leaf mesophyll. However, the plant must not lose so much water during transpiration that it wilts. The plant must strike a balance between conserving water and bringing in sufficient amounts of CO 2 for photosynthesis.

Air-tight seal Water-filled tube Calibrated pipette Potometer Figure 9.3 l One way to measure water loss from a plant is to use a potometer, a device measures the rate at which a plant draws up water.

3 Air-tight seal Water-filled tube Calibrated pipette Potometer Figure 9.3 l One way to measure water loss from a plant is to use a potometer, a device measures the rate at which a plant draws up water. Since the plant dray. water as it loses it by transpiration, you are able to measure the rate of tran ration. The basic elements of a potometer are I A plant cutting I A calibrated pipette to measure water loss I A length of clear plastic tubing I An air-tight seal between the plant and the water-filled tubing I Figure 9.4 shows a typical setup for this exercise_ The rate of water loss in ea condition is measured. I The control is the potometer in room conditions. I The potometer in bright light generally shows a higher rate of water loss, in''' cating more photosynthesis than the control. I The potometer in the fan shows increased water loss compared to the co" trol. The reason is that the air movement results in greater evaporation, 10\- ering water potential outside the plant surface, and more water is pulled fro the roots. I The plant cutting inside the plastic bag is in a situation of high humidit Because of high water potential in the area surrounding the leaves, the rate 0 transpiration will be low. I Go to Topic 8, Chapter 35, to review the plant tissue types. Refer to Figure 9. you read this section. You should be able to recognize and know the function o the following cell and tissue types: 1. Parenchyma is the most abundant cell type, and cells are relatively unspecialized. The mesophyll of leaves (where most photosynthesis occurs), and root cortex (starch storage) are parenchyma.

4 OAssemble 4 potometers. f) Place each potometer in a different environment: room conditions, mist, wind, and bright light. a. Control: b. Mist c. Wind d. Bright light room conditions e Measure water loss in each potameter every 3 minutes for 30 minutes. Figure 9.4 Phloem Xylem Epidermis Figure 9.5 Dieot stem 1 mm

2. Sclerenchyma cells make up fibers, have thick secondary cell walls, and often serve a support function. 3. Collenchyma cells have thickened cell walls and function in support. 4.

5 2. Sclerenchyma cells make up fibers, have thick secondary cell walls, and often serve a support function. 3. Collenchyma cells have thickened cell walls and function in support. 4. Xylem cells (tracheids and vessel elements) are dead at maturity. Their function is water transport. 5. Phloem (sieve tubes and companion cells) transports solutes. 6. Epidermal cells are outermost and serve a protective function.

Transpiration Lab. Introduction

Transpiration Lab. Introduction Transpiration Lab Name Introduction 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