Plant Physiology (Biology 327)  - Dr. Stephen G. Saupe;  College of St. Benedict/ St. John's University;  Biology Department; Collegeville, MN  56321; (320) 363 - 2782; (320) 363 - 3202, fax;    ssaupe@csbsju.edu

Stomatal Distribution and Density

Background Information:
Stomatal density, which refers to the number of stomata per unit area of the leaf, ranges in plants from approximately 1,400 to 40,000 stomata � cm-1 .  The number and distribution of the stomata plays an important role in determining the rate of gas exchange and water loss from a leaf.  For example, we might hypothesize that the more stomata, the greater the rate of transpiration.  Stomatal frequency is determined by counting the number of stomata in the microscope field of view (after we calculate the area of view).

Questions:
What is the density of stomata on the abaxial (bottom) and adaxial (top) surfaces of a broad bean leaf?  Are there fewer stomata on the adaxial side since this side is warmer because it is in direct sunlight?  What fraction of the leaf surface is covered by stomata?

Hypotheses:
Stomata will occur on both sides of the broad bean leaf.  We will find approximately 7500 and 6500 stomata � cm2 on the abaxial and adaxial surfaces, respectively, since these values have been reported for broad bean (Vicia faba) in the literature.  The literature also reports that approximately 1% of the broad bean leaf surface is open pore.

Protocol:

1. Decide which leaflet you will sample and where on an individual leaflet you will take your sample to insure uniformity of samples.  Be sure the leaflet is mature.

2. Remove a leaflet and determine its surface area using the Tracing Technique.  Record your data in Table 1.

3. Prepare an epidermal strip from the abaxial surface of the leaf and mount in a drop of water.

4. Count the number of stomata in 10 randomly selected fields at 400x.  Record your data in Table 2.  Record the magnification used.

5. Repeat with the adaxial surface.  Record your data in Table 2.

6. Measure the diameter of the microscope field at 400x with a stage micrometer.  Record your data in Table 3.

Data:

1. Complete Tables 1, 2, and 3.

2. Summarize your data by completing Table 4.

3. Calculate % of the leaf that is open space for each side of the leaf (Table 4).  To perform this calculation assume that an open stoma of Broad bean forms a rectangular pore (9 μm x 40 μm).  Write the equation that you used. (Hint:  1 mm2 = 106 μm2)

 Table 1:  Leaf area data for a Broad bean leaflet dimensions of paper standard (mm x mm) area of paper standard (mm2) weight of paper standard (g) weight of leaf tracing (g) area of one side of leaf (mm2) total area of leaf (mm2)
 Table 3.  Microscope area of field used to calculate Broad bean stomatal frequency magnification used diameter of field (mm) area of field (mm2)

 Table 2.  Stomatal frequency data for the adaxial and abaxial sides of a Broad bean leaflet Stomata field-1 1 2 3 4 5 6 7 8 9 10 Mean Abaxial Adaxial

 Table 4:  Data summary for Broad bean stomatal distribution Abaxial side Adaxial Side mean stomata mm-2 mean stomata cm-2 total stomata leaflet-1 % stomata % open surface stomatal ratio (abaxial / adaxial)

Conclusions: