PURPOSE:  To investigate the factors that influence transpiration rates in winter conifers.

MATERIALS (per group):  Potometer (ring stand, 2 clamps, 0.01 ml pipette, 1 ml pipette, 2 rubber stoppers, clear plastic tubing, two-way connectors), conifer twigs, marking pen, clock with a second hand, metric ruler, 200 watt lamp, fan.

INTRODUCTION:  Transpiration is the loss of water from plant tissues by evaporation.  Several leaf structures are important in controlling the rate of transpiration.  The outer layer of cells, the epidermis, is coated with a layer of wax-like material called cutin.  This layer, called the cuticle, retards water loss from the surface of the leaf.  However, the cuticle is not completely impermeable; about 5-10% of the water loss in a summer-active plant is by cuticular transpiration.  Most transpiration takes place through tiny openings called stomates located on the surfaces of leaves.  If you look closely at the needles of a conifer such as pine or hemlock, you will see rows of stomates appearing as white lines.

Water molecules move through the stomates when there is a diffusion gradient, so that the water concentration is greater inside the leaf than in the surrounding air.  Stomates are surrounded by pairs of specialized cells called guard cells which control the size of the stomatal openings.  When the stomates are closed, transpiration is sharply decreased.

Any factor that affects the rate of diffusion will affect water loss by transpiration.  For example, in summer-active plants an increase in air temperature will increase the rate of diffusion by increasing molecular motion.  Exposure to sunlight will increase the temperature and increase the chemical activity within the leaf, resulting in the opening of the stomates.  A breeze will prevent water vapor from accumulating around the leaf, thus increasing the diffusion gradient.

The effect of each of these factors on transpiration rates can be demonstrated on summer-active plants with the use of a potometer.  Consider the following data obtained from a summer-active conifer.

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In this activity, you will determine the transpiration rates of several conifer species in winter for comparison with the data from a summer-active conifer.

PREDICTION:

1. Do you think that a winter conifer loses water at a higher, lower, or the same rate as the same species during the summer?  How do you think wind and light affect transpiration in winter?  Explain your ideas.

PROCEDURE:

A. Set up a potometer using a winter conifer twig acclimated to room temperature.  Each group may test a different species, such as pine, spruce, hemlock, or fir.  Your teacher will explain the construction and use of a potometer.  The water column from the pipette through the rubber stopper to the twig must be unbroken with no air bubbles.  To accomplish this, the apparatus must be assembled under water in a deep pan or sink.

B. Mark the initial water level on the pipette with a marking pen and begin timing the rate of transpiration under standard room conditions.  After exactly two minutes, measure water loss to the nearest tenth of a millimeter (0.1 mm) along the length of the pipette with your ruler and record.  Continue to measure and record the water loss at two minute intervals for a total of ten minutes.

C. Expose the twig to a fan at medium speed.  Record the rate of transpiration (mm/2 min) for a total of 10 minutes.

D.  Remove the fan and expose the twig to a lamp at a distance of 25 centimeters.  Record the transpiration rate for a total of 10 minutes.

E.  Calculate the actual water loss in each two minute interval as the difference (__ on data sheet) between successive readings.  Then calculate the average rate of water loss under each of the three test conditions and record on the data sheet.

F.  Construct a graph with transpiration rate (mm/2 min) on the vertical axis and time in minutes on the horizontal axis.  Plot each of the 15 measurements you recorded and connect the points with a dotted line.  Draw vertical lines to mark off the three test conditions and label them as standard, fan, and lamp.  On the same graph, plot the average water loss under those three test conditions and connect the points with a solid line.  Share your graph with the class.

CONCLUSIONS:

2. Compare the average water loss of your winter conifer under standard room conditions to the data for a summer conifer.  Did your winter conifer lose water at a higher, lower, or the same rate as the summer conifer?  Was your prediction correct?

3. Using your data, describe the change in transpiration rate when the winter conifer twig was exposed to the fan.

4. Using your data, describe the change in transpiration rate when the winter conifer twig was exposed to the lamp.

5. Compare the average water losses for your winter conifer exposed to the fan and lamp to the data for a summer conifer.  Did the fan and the lamp have the same effect on transpiration rates for both conifers?

DISCUSSION:

6. Based on class data, compare transpiration rates among the different winter conifer species.  How can you account for the differences, if any?

7. How do you account for the difference in transpiration rates between conifers tested under standard room conditions in the winter and in the summer?

8. How do you account for the change in transpiration rate when the winter conifer twig was exposed to the fan?

9. How do you account for the change in transpiration rate when the winter conifer twig was exposed to the lamp?

10. What do you suppose would happen if the twig was exposed to both the fan and the lamp?  Why?

11. Deciduous trees, such as beech and maple, overwinter in the leafless condition.  How would the data differ if a deciduous twig was used in the potometer?

12. Both deciduous trees and conifer trees have evolved successful adaptive strategies for reducing transpiration during the winter.  What are these strategies and how do they work?

Name______________________________________

TRANSPIRATION DATA SHEET

Winter Conifer Species:_________________________

Initial Water Level Reading: _____________ ______