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Comparative Analysis of the effect of Light Intensity on the Transpiration Rate of Prunus domestica and Tropaeolum peregrinum.

Paper Type: Free Essay Subject: Biology
Wordcount: 2038 words Published: 1st Sep 2021

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Transpiration is the loss of water in plants, which occurs through the stomata or intercellular spaces, and in smaller amounts through the exposed walls of surface cells. The process facilitates the ascent of sap, which is the fluid consisting of water and dissolved substances from the roots to the leaves, (Columbia Encyclopedia, 2004).

According to Essenfeld and others (1996), “as water evaporates from the cells of a leaf or stem, replacement water is pulled from the xylem tissue. The evaporation of cells creates a negative pressure in the xylem, which pulls water upward.”

Transpiration rates are influenced by various environmental factors such as temperature, light, external humidity, air circulation and soil moisture, (Muller, 1979). The rates are higher on bright dry days and lowest at night or in drought conditions. Morphological factors such as leaf surfaces, cuticle layer and number of stomata also affect transpiration rates; where heavy cuticle layer and low stomata count warrant low transpiration rates, (The Columbia Encyclopedia, 2004).

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Muller (1979), claims that leaves that are exposed to light when the sun rises, have increased transpiration rates for two reasons. The first one is that the absorption of light by green leaves results in an increased temperature of the leaves, which eventually causes increased vapor pressure of water within the leaves and an increased water loss. The second reason, involves the direct influence of light on stomatal opening. Light increases the rate of water absorption and the resulting increased turgidity of the two guard cells, which form the boundary of each stoma, brings about the opening of the stomates, increasing transpiration rate.


I predict that the rate of transpiration is directly proportional to light intensity and leaf surface area. As light intensity increases, the rate of transpiration will also increase considerably. This is due to the positive influence of light on the rate of transpiration. Below is a prediction graph of the relationship of light intensity on transpiration rate.

Transpiration Rate

Light Intensity; leaf surface area

I also predict that the surface area of the leaves of Prunus domestica and Tropaeolum peregrine will affect the rate of transpiration. The leaves with higher surface area will have a higher transpiration rate.


The following are the variables to be considered in the experiment:

A. Independent Variable

The independent variable is the light intensity. I will vary light intensity by using a desk lamp in one treatment and placing the other setup in a shady part of the room for the other treatment.

B. Dependent Variable:

The dependent variable is the rate of transpiration which will I measure and express in mL/cm²/hr.

C. Control Variable:

The set-up and replicates will be placed inside the science laboratory where the temperature will be controlled. I will also control the humidity of the air by not using electric fans or air conditioners inside the laboratory.

Actual Method

For the Potometer:

  1. I will place the tip of a 0.1 ml pipette into a 16-inch clear plastic tubing.
  2. I will submerge the pipette with tubing in a tray of water. I will make sure that all air bubbles are eliminated.
  3. I will bend the tubing upward, into a “U” shape and clamp it on a ring stand to hold pipette with tubing.

For the plant set-up: Each experimental set-up will be composed of a cutting of Prunus domestica and Tropaeolum pereginum, with approximately the same branch diameter and number of leaves.

1. I will cut the plants underwater about 3cm up the stem. This will remove any blockages in the xylem from when the plant was cut previously. The xylem must not be crushed, so the plant will be cut at an angle with a sharp blade. The plant will be cut underwater to prevent any air bubbles getting into the xylem, as this may affect the final results.

2. I will insert each plant specimen to the potometer. I will make sure that the whole system is completely airtight. When the plant transpires, water will be pulled along the tubing. I will allow the apparatus to equilibrate for about 10 minutes.

  1. I will label it “Treatment 1”.
  2. I will repeat steps 1-3 and label it “Treatment 2”.
  3. I will repeat steps 1-3 and label it “Control”.
  4. For each treatment, I will repeat steps 1-3 three times and label them as replicate 1, replicate2, and replicate 3, respectively.
  5. For Treatment 1, a desk lamp will be placed 24 inches from the plants. Treatment 1 will be labeled as “Desk Lamp Treatment”. The three replicates of this treatment will be set-up similarly.
  6. For Treatment 2, the set up with the two species of plants will be placed in a shady area of the room, where no direct light will be reaching it. Treatment 2 will be labeled as “No Light Treatment”. The three replicates of this treatment will be set-up similarly.
  7. For the control treatment, the set up with the two species of plants will be placed in an ambient part of the room where no direct light source can reach it and at the same time, it shall not be placed on a shady portion of the room. The three replicates of this treatment shall be set up similarly.
  8. I will observe the set up of each treatment with respective replicates for 8 hours. I will measure the amount of water in the pipette at the beginning of the experiment and label the time as “0”. I will record the measurement.
  9. I will record the water level in the pipette of each treatment and replicates every hour, for 8 hours.
  10. After the experiment, I will cut the leaves off the plant to calculate the leaf surface area.
  11. I will use the “Leaf Trace Method” to calculate the leaf surface area.

Leaf Trace Method:

  1. I will arrange the leaves on the grid of a graphing paper.
  2. I will trace the edge pattern directly onto the grid.
  3. I will count all of the grids in the graphing paper that are completely within the tracing and at the same time, estimate the number of grids that lie partially within the leaf tracing. In the graphing paper, a square of four (4) blocks is equivalent to 1 cm².
  4. I will calculate the total surface area by dividing the total number of blocks covered, by 4.
  5. I will get the average leaf surface area by dividing the total surface area by the number of leaves.

Apparatus List

For each set-up in the experiment, the following will be needed:

  • Cuttings of Prunus domestica and Tropaeolum peregrinum
  • 0.1 mL pipette for the potometer
  • ring stand to hold the pipette with tubing
  • clamps to hold pipette with tubing
  • clear plastic tubing for the potometer
  • desk lamp for the lamp treatment
  • stop clock for measuring time

Safety Procedures:

  1. I will not be using any hazardous substances, but I must be careful not to spill any water on the workbench.
  2. The sharp blade must be used with care, as it is very sharp and fingers can be cut easily. When they are not in use, the blades must be kept inside their box so that other people will not hurt themselves if they are left lying around.
  3. I will not break any branches of the plants that I will not be using for the experiment. This means that I will not be disturbing any organisms unnecessarily that live on the plant.
  4. The apparatus must be positioned steadily on the surface. It is quite bulky, and I must be careful not to knock it over and spill the water.

Treatment of Results:

  1. I will measure the individual water loss at each reading and divide it by the calculated leaf surface (average) and tabulate it in a manner similar to the table below:

Table1: Water Loss in mL/ cm²/hr.

Time Intervals












Water loss (mL)


Water Loss per cm²


B. I will record all the averages of the data for each treatment and tabulate it in the same manner below:

Table 2: Average cumulative Water Loss in ml/cm²/hr

Time (hour)











Treatment 1 (Desk Lamp)


Treatment 2 (No Light)


Treatment 3 (Control)


C. For each treatment and replicate, I will graph the average of the data for each time interval.

D. To compare the mean Treatment 1 (Desk Lamp Treatment) and Treatment 2 (No Light Treatment); based on light intensity; I will treat it statistically by using One-way Analysis of Variance (One way ANOVA). In this way, I can determine if there is a significant difference between the mean scores of Treatment 1 (Desk Lamp Treatment) and Treatment 2 (No Light treatment).


Berg. L. R., 1997. Introductory Botany – Plants, People and the Environment. First Edition. Brooks Cole. Massachusetts.

Columbia Encyclopedia, Sixth Edition, 2004. Columbia University Press, New York.

Daily Record. Gardening: City Oasis is Little Miracle, 2004. Gale Group. Glasgow.

Essenfeld, B., Gontang, C., Moore, R., 1996. Biology. Second Edition. Addison-Wesley. New York.

Hill, A. 1952. Economic botany: A Textbook of Useful Plants and Plant Products. McGraw-Hill. New York.

Muller, W., 1979. Botany: A Functional Approach. Fourth Edition. Macmillan Publishing Co. New York.

The Biology Place: LaBench Activity, 2006. http://www.phschool.com/science/biology_place/labbench/lab9/design.html. [Accessed: April 9, 2006].

Wikipedia: The Free Encyclopedia. Prunus domestica. http://en.wikipedia.org/wiki/Plum. and http://en.wikipedia.org/wiki/Tropaeolum_peregrinum. [Accessed: April 10, 2006].


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