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Name: Period

Transpiration Objectives • To understand how water moves from roots to leaves in terms of the physical/chemical properties

of water and the forces provided by differences in water potential. • To test the effects of environmental variables on rates of transpiration using a controlled

experiment. 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 (the evaporation of water from the plant surface) and guttation (the loss of liquids from the ends of vascular tissues at the margins of leaves). If this water is not replaced, the plant will wilt and may die. The transport of water 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

(Figure 1)

Minerals actively transported into the root accumulate in the xylem, increasing solute concentration and therefore decreasing water potential. Water moves in by osmosis. As water enters the xylem, it forces fluid up the xylem due to hydrostatic root pressure. But the 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 tension. The “pull” on the water from transpiration results from the cohesion and adhesion of water molecules (Figure 1).

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The Details Transpiration begins with the evaporation of water through the stomata (stomates), 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 (moving from an area of high water potential to an area of lower water potential). The moisture in the air spaces is replaced by water from the adjacent mesophyll cells, lowering their water potential (since the cytoplasm becomes more concentrated). Water will then move into the mesophyll cells by osmosis from surrounding cells with high 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 occurs because of (1) the cohesion of water molecules to one another due to hydrogen bond formation, and (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 walls of the xylem inward. The tension also contributes to the lowering of 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. Plants lose as much as 90% of the water that enters the roots by the process of transpiration. Evaporation through the open stomates is clearly a major route of water loss in plants. 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 the stomata on the leaf surface. Many environmental conditions influence the opening and closing of stomates and also affect the rate of transpiration. Room temperature (control), humidity, air currents and light intensity, are the factors being investigated. Different plants also vary in the rate of transpiration and in the regulation of stomatal opening. In this lab, you will measure transpiration under various laboratory conditions using a potometer (see figure 2). Hypothesis State your hypotheses (on page 7) for each of the variables being tested.

Figure 2

A potometer (see figure 2) is a device that measures the rate at which a plant draws up water. Since the plant draws up water as it loses it by transpiration, you are able to measure the rate of transpiration. The basic elements of a potometer are: • A plant cutting • A calibrated pipette to

measure water loss • A length of clear plastic tubing • An air-tight seal between the

plant and the water-filled tubing

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An Overview of the Experimental Setup In order to measure some different rates of transpiration, follow the four steps outlined in the graphic below. For a review on assembling a potometer, see Figure 2.

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Leaf Surface Area Determination:

The rate of transpiration is measured as "𝑚𝑙𝑜𝑓𝐻2𝑂𝑙𝑜𝑠𝑡

𝑚𝑒𝑡𝑒𝑟2.𝑚𝑖𝑛𝑢𝑡𝑒

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Because water evaporates through the many stomata on the leaf surface, the rate of transpiration is directly related to the surface area. To arrive at the rate of transpiration, therefore, you must calculate the leaf surface area of each plant: Because most stomata are found in the lower epidermis, you will determine that surface area. o Lay the leaves to be measured on a 1-cm grid and trace their outlines. o Count the number of square centimeters. Estimate the area of the

partial squares. (Here's a simple method for this estimate: Count a partial square if it is at least half covered by the leaf; do not count partial squares that are less than half covered.

o Do not include the area of the stem (petiole) in your calculations.

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Calculations Equation for the conversion of surface area from centimeters2 to meter2.

"𝑥𝑐𝑚5

1 7 1𝑚5

10,000𝑐𝑚52 = 𝐴𝑟𝑒𝑎𝑖𝑛𝑚𝑒𝑡𝑒𝑟5

Example 1

"21𝑐𝑚5

1 7 1𝑚5

10,000𝑐𝑚52 = 0.0021𝑚5 = 2.1 × 10?@𝑚5

Equation for leaf water loss per minute

ABCDEFGHHIJKLCD

=MNOPQRNSTUVRWSWNQXNVNQRVRWYPWSUZRUWRUNSORUS[\QRVRW]

IJKLCDH=

QOQ]

IJK

Example 2

0.19𝑚𝑙0.0020𝑚5

3.0𝑚𝑖𝑛=

95𝑚𝑙𝑚5

3.0𝑚𝑖𝑛= 32

𝑚𝑙𝑚5

𝑚𝑖𝑛

Results Table 1: Leaf Surface Area

A B C D

Surface Area of leaf (counting the number of cm2)

Surface Area of leaf (counting the number of m2)

See example 1 above for calculation setup.

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Table 2: Volume (ml) According to the Potometer Readings for Leaf A, B, C and D

Volume of Water from Potometer (ml)

Time (minutes)

Room Temperature Leaf A (ml)

Plastic Bag Leaf B (ml)

Fan Leaf C (ml)

Bright Light Leaf D (ml)

3 0.19 0.13 0.25 0.30 6 0.39 0.26 0.51 0.58 9 0.49 0.39 0.76 0.99 12 0.71 0.53 1.20 1.42 15 0.99 0.64 1.37 1.68 18 1.11 0.70 1.89 1.98 21 1.28 0.98 2.30 2.25 24 1.48 1.11 2.61 2.51 27 1.71 1.23 2.80 2.82 30 1.92 1.34 3.00 3.01

Table 3: Water Loss in units of

MNOPQRNSTUVRWSWNQXNVNQRVRWYPWSUZRUWRUNSORUS[\QRVRW]

IJKLCDH=

QOQ]

IJK

Water Loss in Leaf A, B, C and D aQOQ]

IJKb

Time (minutes)

Room Temperature Leaf A (ml)

Plastic Bag Leaf B (ml)

Fan Leaf C (ml)

Bright Light Leaf D (ml)

3

6

9

12

15

18

21

24

27

30

Average

See example 2 on page 5 for calculation setup.

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1. What is your hypothesis for each of the variables that was tested (plastic bag, fan, bright light)?

a.

b.

c.

Analysis Questions: Make sure to refer to SPECIFIC numerical data to support your answers to the following questions. 2. Calculate the average amount of water loss per minute per square meter for each of the four

experiments and place in the table below (show an example of your calculation using one of your data points). Room Temperature Plastic Bag Fan Bright Light

3. Describe and explain any initial trends you notice in the averages you calculated. (i.e. Which

environmental condition resulted in the greatest water loss per minute per square meter? The least water loss? How do you know?)

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4. Explain the different rate of water loss in each experiment, taking into account how each condition

affects the gradient of water potential from inside the plant to outside the plant.

5. In our experiment, each plant had a seemingly unlimited supply of water to draw from. In real life,

however, when water is in short supply, plants may need to control transpiration by regulating stomatal opening and closing. If a plant were trying to conserve water, what would happen to its photosynthetic rate? Why?

6. Why did you need to calculate leaf surface area before analyzing your results? Hint: How might your

comparison of results be flawed if you didn’t take leaf surface area into account?

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7. Describe several adaptations that enable plants to reduce water loss from their leaves. Include both

structural and physiological adaptations.

Given the data table below, generate a graph showing the effects of light wavelength on transpiration of two leaves the same size, one in blue light and one in red light.

Time (min)

Volume of water transpiration in blue light (ml)

Volume of water transpiration in

red light (ml)

8. What is the slope of the line for plants in blue light and what is the slope of the line for the plant in red light?

9. What is the rate of transpiration in the blue light and

what is the rate of transpiration in the red light?

0.0 0.00 0.00

3.0 0.95 0.38

6.0 1.95 0.78

9.0 2.45 0.98

12.0 3.55 1.42

15.0 4.95 1.98

18.0 5.55 2.22

21.0 6.4 2.56

24.0 7.4 2.96

27.0 8.55 3.42

30.0 9.6 3.84

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