Transpiration Lab: How Bush Bean Seedlings Lose Their Water

Hypothesis: If a plant is exposed to different conditions, then its rate of transpiration will be affected.

Materials:

• 2 week old bush bean seedling
• Petroleum jelly
• A light source with 100 watts (Flashlight)
• 0.1 mL pipet
• 10 mL pipet
• Weighing scale or balance
• Ring stand and clamps
• Scissors
• Fan
• Spray bottle
• Plastic bag
• 16 inch vinyl tubing
• Water
• Timer
• Something to record data with like a data table or a notebook
• Graph paper or a google spreadsheet to create a graph

Procedures:

1. Carefully place the non-tapered end of a 0.1 mL pipet into one end of a 16” piece of vinyl tubing. A small amount of petroleum jelly can be placed on the outside of the pipet at the non-tapered end to help insertion of the pipet. (Done By The Instructor)
2. Bend the tube into a U shape and secure onto a ring stand with clamp(s).The top of the tubing and the top of the pipet must be level in order to fill completely with water. (Done by Instructor)
3. Fill the tubing and 0.1 mL pipet completely with water from the tubing end, using a 10 mL pipet. No air bubbles should be present in the tubing or 0.1 mL pipet.
4. Insert a freshly cut stem from a 2 week old bush bean seedling into the end of the tubing and seal with petroleum jelly. Do not get petroleum jelly onto the cut end of the stem or the experiment will not work! No bubbles should be present in the tubing or 0.1 mL pipet (Figure 1).
5. Expose the stem to one of the following conditions assigned by your teacher:
• Room conditions
• 100 watt light source placed 1 meter from the stem
• Fan 1 meter from stem on low to create a breeze
• Mist of water from plant mister (cover the potometer immediately with a plastic bag after misting)
6. After a 10 minute equilibration period, read the water level in the pipet and record below as time 0. You may want to use a marker to draw a line at the water level for each time point. (Hint: 12 mm = 0.01 mL)
7. Obtain a reading once every 5 minutes for 30 minutes.
8. After 30 minutes, cut off the leaves of the bush bean seedling and blot off any excess water gently. Weigh the leaves.
• Weight of leaves 1.116 grams.
9. Estimate the total leaf surface area for the plant.
• Cut a 1 cm2 section out of a leaf
• Mass of the cut section = 0.015 grams.
• Multiply cut section mass by 10,000 to obtain the mass per square meter of the leaf = 150 g/m2
• Using the formula below obtain the total leaf surface area (m2):

Total Weight of Leaves (grams)      

                       Grams per m2                  = Total Leaf Surface Area (m2)

• Leaf Surface Area (m2) = 0.00744 m2
• Using the formula below calculate water loss in mL/m2 at each reading (Table 1) by dividing the reading by the calculated leaf surface area:

(Water Level at each T in mL) - (Water Level at Time 0 in mL)

                                             Total Leaf Surface Area in m2

11. Record the class averages in Table 3.
12. Plot the class average data of water loss (mL/m2 ) versus time (minutes). Using google spreadsheets or a piece of graph paper, plot water loss on the y-axis vs. time on the x-axis.

Table 1: Potometer Readings
Time (min)	0	5	10	15	20	25	30
Reading (mL)	0.025	0.030	0.030	0.040	0.045	0.047	0.050

Table 2: Individual Water Loss in mL/m2
Time (min)	0	5	10	15	20	25	30
Water Loss (mL)	0	0.030	0.030	0.040	0.045	0.047	0.050
Water Loss Per m2	0	4.032	4.032	5.4	6.05	6.32	6.72

Table 3: Class Average Water Loss in mL/m2
	Time (min)
Treatment	0	5	10	15	20	25	30
Room	0	0.92	1.88	2.980009	4.000009	4.7800178	5.660849
Light	0	1.52272	2.05125	2.8945	3.53325	4.12925	4.71275
Fan	0	3.12	4.772	6.772	8.172	9.618	10.902
Mist	0	1.0625	2.15	2.4	2.0025	2.515	2.7725

Graph Justification: As the title states, this graph displays the effect that different conditions have on a plant’s water loss. It seems that as the independent variable, time, increases, the dependent variable, water loss, increases as well. Each condition had a different effect on the plant’s water loss. For example the fan increased the water loss while the mist decreased the water loss. This graph is testing the hypothesis “If a plant is exposed to different conditions, then its rate of transpiration will be affected.”

Analysis (Study Questions):

1. What is the total leaf surface area for the plant?
The total leaf surface area for the plant is 0.00744 m2. To calculate the surface area, one must weigh the leaves (1.116 grams) and a section that is  1 cm2 of a leaf (0.015 grams). Then it must be multiplied by 10,000 to obtain its mass per square meter (150 g/m2). This data must then be put into a formula which divides the weight of the leaves by its mass per meter square. The equation is shown below:

1.116 grams  
150 g/m2     = 0.00744 m2
2. Determine the average amount of water loss per milliliter per square meter for each treatment: room, light, fan, and mist.
After doing the experiment, each group recorded the data on a spreadsheet and the average for each condition was calculated. The average for water loss at room temperature was 5.660849 mL/m2. The average for water loss while there was a light was 4.71275 mL/m2. The average for water loss while the fan was on was 10.902 mL/m2. The average for water loss with mist was 2.7725 mL/m2.

3. Explain how each condition affects water loss.

The control of the experiment was the bush bean seedling that was in an environment at room temperature. Since it was a control, it did not have a great effect on its water loss. Light increases a plant’s water loss. This is because once the light hits the leaves, the stomata open and this allows water to be lost at a faster rate. The fan increased water loss because it removed the humid air surrounding the plant and made its environment drier. The plant was then able to release water into the atmosphere. The mist decreased water loss because it makes the air around the plant humid. When the air surrounding the plant is moist, the plant does not release water into the atmosphere.

4. How did these conditions affect the stem to leaf water gradient?

When the plant was exposed to different conditions, like the light or the fan, water traveled up the stem, creating a gradient, and into the leaves which released the water through the stomata. When the plant was exposed to the mist, the gradient decreased because the air surrounding the plant had a high water potential and therefore, water was not moving up the stem. When water is released from the plant, the water potential changes. This change creates a gradient because the water is moving to areas with different water potentials.

5. What is the importance of water potential in controlling the transport of water?

Water potential is important when it comes to transporting water because it influences the direction that the water goes in. If a plant is in an environment that has a low water potential (the atmosphere is dry), then the plant will release water into the atmosphere. This is because the plant had a high water potential and water moves from an area of high water potential to an area of low water potential.

6. Explain several mechanisms by which plants try to minimize water loss.

Plants have several mechanisms that allows them to minimize the amount of water they lose such as opening their stomata at night instead of during the day. This limits the amount of water that is being released into the atmosphere because the sun is not there to increase the rate of transpiration. Also, plants with smaller, thicker leaves have less stomata and a smaller surface area so they are prevented from releasing excess water. The plants can also store water in their stems which they can use during times when it is dry.

Conclusion:

After completing this lab, it was shown that placing the bush bean seedlings in different environments affected the amount of water the plant lost. At room temperature, the rate of transpiration was not affected as much but when it was exposed to the mist, light, and fan, the water loss changed. When the plant was exposed to the fan and light, the plant lost more water because it had a higher water potential than its surrounding environment. When the plant was sprayed with water (the mist), the rate of transpiration decreased because the plant did not release water into a humid atmosphere. This proved that my hypothesis “If a bush bean seedling is exposed to different conditions, then its rate of transpiration will be different in each environment” was correct. Some possible errors could have been that the calculations for water loss were done incorrectly. I noticed that some of the calculations that were inputted into the class spreadsheet were very different from the other calculations and this could have affected the outcome of the graph. Another error could have been that we made a mistake while reading the potometer or that we allowed an air bubble to appear in the tubing. If this experiment were to be repeated, a new condition could be experimented with such as temperature or the plant species could be changed.