Plant Physiology Spring 2006
Lab 9
Effects of water and nutrients on stomatal conductance, photosynthesis and water-use efficiency in plants
Part A.
Relationship between relative leaf water content, photosynthesis, transpiration and stomatal conductance.
Introduction
As leaves transpire, leaf water potential declines if the water that is lost by the leaf is not replenished at the same
rate from the roots. Transpiration results in a tension that is generated on the water column in the xylem in the
leaves, stems and roots. The tension means that the xylem has a negative pressure potential (ψp). For most of the
time, the xylem tissue of plants has a negligible solute potential. This means that the water potential of the xylem
must equal the negative pressure potential in the xylem. It is assumed that the water potential of the xylem is in
equilibrium (the same) as the water potential of the surrounding living leaf cells. As water potential of leaf cells
decline, the turgor pressure of leaf cells must decline as water is lost from those cells to the atmosphere via the
stomata. A certain water potential is reached that will trigger various signals including ABA to causes K+ ions to
move out of the guard cells. Water will moves out of the guard cells via osmosis and the turgor pressure of the
guard cells will decrease until the cells are flaccid. At this point, the stomata should close. It makes perfect sense
then that the rates of photosynthesis, transpiration and stomatal conductance should all be impacted as the leaf
water potential declines.
Procedure
Start by stating your working hypothesis about the relationship between relative water content, leaf transpiration,
photosynthesis and stomatal conductance?
As a group, obtain 6 leaves by cutting them at the petioles or at the base of the leaf blade with a scissors. Place
five of them in a growth chamber. Bring the one leaf to the lab. Immediately, determine the rate of
photosynthesis, transpiration and stomatal conductance using the IRGA. Record your data in Table 1 below. .
Immediately after taking this measurement, measure the leaf area of your leaf blade. Then punch two leaf discs
from the leaf blade and measure its fresh mass to the nearest 0.1mg (analytical balance in Biochem lab, Record
data in Table 2). Weigh the remaining leaf blade minus the two leaf discs to obtain the fresh mass of the leaf
blade. Place the remaining leaf blade in a jelly jar cap in the oven for 48 hours. Obtain the dry mass of the leaf
blade to determine the water content of your leaf (Record data in Table 1). Float each leaf disc in distilled water
in a labeled Petri dish for 24 hours. The Petri dishes will be placed under the fluorescent lights on the book shelf
in the Swarthout lab. After 24 hours, blot the leaf discs dry and weigh the saturated fresh mass of leaf disc.
Discard the distilled water and allow the leaf disc to dry in a jelly jar cap in the oven. Obtain the dry mass of the
leaf disc after 48 hours of drying. Record all your data in Table 2. Immediately, move onto the next leaf in the
growth chamber and repeat the process.
Table 1.
Plant
Time
ID
After
cutting
Table 2
Plant ID
Photo Trans
Time after
cutting
Cond
Fresh mass
Leaf disc 1
Leaf area
Fresh mass
Leaf disc 2
Fresh
Leaf
mass
Sat. mass
Leaf disc 1
Dry leaf
mass
Sat. mass
Leaf disc 2
Dry mass
Leaf disc 1
Dry mass
Leaf disc 2
Data analyses
Water content = fresh mass – dry mass
Relative water content = fresh mass – dry mass/ saturated mass – dry mass
1. Plot relative water content versus rates of stomatal conductance, photosynthesis and transpiration in your
leaves measured at the various times. Are there any correlations among the variables?
2. Using the maximum transpiration rate, the mean leaf blade area and water content of your first leaf to calculate
how long it should have taken for your leaf to turnover all of its water. Show all you calculations in a Box.
3. On separate graphs, plot the relative water content of your leaf and stomatal conductance versus time to see at
what time and relative water content your leaf closed its stomata.
4. Compare the data for the three species measured by the different individuals in the class.
Why would different species behave differently with respect to when they will close their stomata? What
physiological factors could influence the above relationships? Discuss the answers to this in your discussion
section of your report.
Part B
The use of a pressure-volume curve and thermocouple psychrometer to determine the bulk tissue elastic
modulus in different plant species.
The pressure chamber and thermocouple psychrometer techniques are the main methods that can be used to
measure osmotic and water potentials of living plant cells. We can determine the relationship between water
potential of the leaf and its relative water content by constructing a pressure volume curve. If we plot the inverse
of water potential against the relative water content, we will see a steep drop in water potential as the leaf loses
water. The drop becomes more gradual as the leaf becomes flaccid. If we extend the gradual portion of the curve
to the y-axis, we will estimate the osmotic potential of the tissue at full hydration at the y-intercept. Once we have
the osmotic potential of the tissue at full hydration, we can estimate the turgor pressure of the leaf tissue at the
respective water potentials using equation 1.
ψw = ψs + ψp (equation 1)
We can then plot the turgor pressure versus relative water content. The slope of the linear portion of the curve
represents the bulk modulus of elasticity for that leaf tissue (equation 2)
ε = ψp/RWC
A higher bulk modulus of elasticity means that the cell walls are less elastic and the cell will experience a greater
drop in water potential for a given unit loss of water.
Procedure
Obtain three leaf twigs from a species (English ivy, Tall fescue and Gerandium) that have been hydrating in
solution for the past three hours.
Obtain the fresh mass of the hydrated twigs or leaf blades.
Determine the initial value of ψw of the first two leaves using the pressure chamber. On the third leaf, determine
the osmotic potential of the leaf tissue using the thermocouple psychrometer. Your instructor will show you how
to use this instrument.
Allow the first two leaves to dry on the lab bench for 10 minutes.
Determine fresh mass of the leaves.
Measure the ψw using the pressure chamber. Blot the water that is bubbling at the cut surface.
Determine the fresh mass of the leaves again.
Use the mean mass of before and after reading water potential to determine the relative water content of the leaf
when the water potential reading was taken.
Allow the leaves to dry on the lab bench for 10 minutes again.
Repeat the procedure at least 6 times.
Place the leaf in the oven for 48 hours. Determine the dry mass of each leaf. You need the dry mass of the leaf to
calculate relative water content at each sampling period.
Record you data in Table 3.
Plant ID
Fresh mass before
Water potential
Fresh mass after
Dry mass
Construct the pressure volume curve using your data (Figure 1). Determine the osmotic potential from the figure.
Calculate the turgor pressure at each water potential that you measured (show this in a Table). Plot the turgor
pressure against the relative water content (Figure 2). Calculate the bulk modulus of elasticity using the first 4
points of the linear curve in Figure 2.
Part C.
The effect of mineral nutrient deficiency on the rates of photosynthesis, stomatal conductance and
transpiration
We will simply measure the rates of photosynthesis on your mineral nutrition plants.
State your hypothesis about how the mineral nutrient deficiency should impact photosynthesis. Provide a logical
rationale for your working hypothesis.
Procedure
You will measure the rate of photosynthesis of all 8 corn plants. Each group member should measure at least two
plants. Record your rates of photosynthesis, stomatal conductance and transpiration in Table 4. Perform a t-test
on your data to see if the mineral nutrient deficiency impacted rates of photosynthesis, stomatal conductance and
transpiration significantly in your experimental plants.
Table 4.
Plant ID
Photosynthesis
Transpiration
Stomatal
Conductance
Construct graphs showing the means photosynthetic, transpiration and stomatal conductance rates in your two
treatments. Using the t-tests, interpret whether you have any significant treatment effects. Discuss your results in
the context of why that particular mineral nutrient deficiency should impact rates of photosynthesis.
You are required to cite three peer-reviewed journal articles that relates to parts A,B and C of today’s lab
exercise. The lab report is due on 6 April 2006.