Transpiration
Transpiration Lab
Overview
In this laboratory you will apply what you learned about water potential from the Diffusion and Osmosis laboratory to the movement of water within plants. You will measure transpiration under different laboratory conditions and also study the organization of the plant stem as it relates to the movement of water.
Objectives
At the completion of this laboratory you should be able to:
• Describe how differences in water potential affect the transport of water from roots to stems to leaves.
• Relate transpiration to the overall process of water transport in plants.
• Discuss the importance of the properties of water, including hydrogen bonding, adhesion, and cohesion, to the transport of water in plants.
• Quantitatively demonstrate the effects of different environmental conditions on the rate of transpiration in plants.
Background
The amount of water needed daily by plants for 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 the water that is lost from aerial plant parts is not replaced by water transported up from plant roots, the plant will wilt and eventually die.
The transport of water up from the roots in the xylem tissue is governed by differences in water potential. These differences account for water movement, not only from cell to cell, but over long distances within the plant. In a root, minerals transported from the soil accumulate in the xylem vessels of the vascular tissues of the stele. This, in addition to the negative pressure (tension) in the xylem tissues, lowers the water potential of the xylem. Thus, water will move into the xylem by osmosis, forcing fluid up the xylem vessels. This upward movement results in root pressure, but this pressure can only move water a short distance up the xylem. Rather than being pushed up from below by root pressure, the water and dissolved minerals in the xylem (xylem sap) are pulled upward as a result of transpiration.
The stomatal openings of a leaf open into the air spaces that surround the mesophyll cells of the leaf. The moist air in these spaces has a higher water potential than the air outside the leaf and water tends to evaporate from the leaf surface moving from an area of higher water potential to an area of lower water potential. The moisture in the air spaces is replaced by water from the mesophyll cells, lowering their water potential (solute in the mesophyll cells becomes more concentrated when less water is present - recall that increasing solute concentration lowers water potential). Water will then move into the mesophyll cells by osmosis from surrounding cells with higher water potentials, including those of the xylem.
The gradient in water potential between the xylem and the air outside the leaf that is caused by transpiration results in the transpirational pull of water from the xylem into the leaf mesophyll cells and eventually into the leaf spaces. Cohesion of water molecules to one another due to hydrogen bond formation causes this pull to transmit throughout the column of water in the xylem, all the way from the leaves to the roots. Adhesion of water molecules to the walls of the xylem cells offsets the effects 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. Recall that increased pressure causes water potential to become more positive. Tension, however, because it is the opposite of pressure, causes water potential in the xylem to decrease. This decrease, transmitted through the column of fluid in the xylem all the way to the roots, causes water to move from the soil across the cortex of the root and into the xylem of the stele. A root structure called the Casparian Strip located in the root endodermis tissue prevents the backflow of water from the xylem tissues into the soil.
Since the opening of stomates, which allows transpiration to occur, is also required for the entry of CO2 used in photosynthesis, a balance must be maintained between the two processes. Plants accomplish this by regulating the opening and closing of stomates on the leaf surface.
Transpiration in Ornamental House Plants
Many environmental conditions, including those conditions that influence the opening and closing of stomates, will affect the rate of transpiration. Increases in temperature, light intensity, and the presence of dry air currents can increase transpiration rates. A humid environment usually slows the rate of transpiration. In this exercise you will measure the rate of transpiration by measuring the change in mass for the pansy.
Procedure
A. Preparation of the Plants:
1. Obtain a house plant. Record both the common and scientific name in the space below. Using a plastic bag and a rubber band, wrap the soil container to completely surround it so that only the stem of the plant is exposed. This will prevent water loss through evaporation from the soil. Mark your group’s name with a piece of masking tape and marker.
2. Take a baseline mass reading and record below.
Day 0 mass: Name of plant
B. Environmental Treatments:
Record which environmental condition your group has been assigned. Some options:
A) Control – room conditions of light and wind.
B) Light – place a floodlight 1 meter from the plants.
C) Dark – place a cardboard box over your plants.
D) Wind – place a fan on low speed 1 meter from the plants to create a gentle breeze.
Caution - be sure the breeze doesn’t hit other plants.
E) Humidity – encase the plants in a clear plastic bag or place them in a humidity chamber. Mist the leaves with water and leave the bottom of the bag open.
Your environmental treatment:
What is the importance of having a control in any experiment?
Which condition do you believe will result in the greatest water loss? Why?
Which condition do you believe will result in the lowest amount of water loss? Why?
C. Hypothesis, Results and Analysis:
Based on your new knowledge of transpiration, develop a hypothesis for what you believe will happen to your plant (with your specific environmental treatment). Write your formal hypothesis in the space below:
After 2 and 5 days, take new mass readings and record your data below. This data will be shared among all AP Biology groups and then analyzed using Microsoft Excel or similar program. The table on the next page will be used to record class data when available.
Day 2 Mass Day 5 Mass
CLASS DATA TABLE
|CONTROL CONDITION |DAY 0 |DAY 2 |DAY 5 |TOTAL % |
| |Mass (g) |Mass (g) |Mass (g) |Change in mass |
| | | | |(D5-D0) / (D0) |
| | | | | |
| | | | | |
| | | | | |
| | | | | |
|AVERAGE: | | | | |
|BRIGHT LIGHT CONDITION |DAY 0 |DAY 2 |DAY 5 |TOTAL % |
| |Mass (g) |Mass (g) |Mass (g) |Change in mass |
| | | | |(D5-D0) / (D0) |
| | | | | |
| | | | | |
| | | | | |
| | | | | |
|AVERAGE: | | | | |
|DARK CONDITION |DAY 0 Mass (g) |DAY 2 |DAY 5 Mass (g) |TOTAL % |
| | |Mass (g) | |Change in mass |
| | | | |(D5-D0) / (D0) |
| | | | | |
| | | | | |
| | | | | |
| | | | | |
|AVERAGE: | | | | |
|WIND CONDITION |DAY 0 |DAY 2 |DAY 5 |TOTAL % |
| |Mass (g) |Mass (g) |Mass (g) |Change in mass |
| | | | |(D5-D0) / (D0) |
| | | | | |
| | | | | |
| | | | | |
| | | | | |
|AVERAGE: | | | | |
|HUMID CONDITION |DAY 0 |DAY 2 |DAY 5 |TOTAL % |
| |Mass (g) |Mass (g) |Mass (g) |Change in mass |
| | | | |(D5-D0) / (D0) |
| | | | | |
| | | | | |
| | | | | |
| | | | | |
|AVERAGE: | | | | |
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Name _____________________ Period _____
Guttation
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