Light Dependence of Photosynthesis
Data Analysis Worksheet
--------- C3 Plant -------
Photon Flux
mol
quanta/m2/s)
m
% O2/
min
--------- C4 Plant ---------
Photosynthetic Rate
(mol O2 /m2/s)
m
% O2/
min
Photosynthetic Rate
(mol O2 /m2/s)
100%
80%
60%
40%
20%
10%
5%
2.5%
When you have calculated rates of photosynthesis at each light intensity used in your
experiment, present your data for both the C3 and C4 plant as a graph with
photosynthetic rate mol O2/m2/sec) plotted on the y axis and light intensity mol
quanta/m2/s) on the x axis. This will produce a photosynthetic light response curve.
An example of such a curve for a generalized leaf is shown in the graph below. Be
aware that the light intensities you will be using are not the same as "incident
illumination" give in the following graph, but they are proportional to it.
From QUBIT SYSTEMS Inc Laboratory Manual
The photosynthetic light response curve of a particular plant is influenced by many
factors, and a study of the components of the curve can tell us a great deal about the
physiology and ecophysiology of the plant. The following are important aspects of such
a curve:
The Light Compensation Point. Extrapolate the linear portion of your light response
curve to intercept the x axis at the point where photosynthetic rate is zero. The light
intensity at this point is called the light compensation point.
1) What does this point represent in terms of the physiology of the plant?
2) Compare the light compensation points in the C3 and C4 plants and suggest reasons
for any of the differences that you see.
The Rate of Dark Respiration. If the linear part of the light response curve is
extrapolated to intercept the y axis at zero light intensity, the negative rate of
photosynthesis at this point gives an estimate of "dark" respiration rate.
3) What metabolic processes are included in "dark" respiration?
4) Do the rates of dark respiration differ in C3 and C4 plants? If so, suggest reasons
for this difference.
5) Criticize this method for estimating dark respiration and suggest a way in which it
may be measured directly.
Photochemical Efficiency. Photochemical efficiency may be defined as the increase
in photosynthetic rate achieved per unit increase in light absorbed by the leaf.
6) What part of the curve would you examine to determine photochemical efficiency?
7) Compare the photochemical efficiencies of C3 and C4 plants and explain any
differences that you observe. C4 species are usually restricted to high light
environments. Do your data indicate any reason for this?
8) In your experiment, you did not measure light absorbance by the leaf, but only the
amount of light transmitted through the leaf. Describe how you would change the
design of the experiment to make more accurate measurements of photochemical
efficiency.
The Light Saturation Point of Photosynthesis. The light intensity beyond which the
light response curve plateaus is called the light saturation point of photosynthesis. At
this point increases in light intensity do not cause increases in photosynthetic rate, so
other factors apart from the supply of light must be limiting the photosynthetic process.
9) What might these factors be?
10) Did you measure the light saturation point in your experiment? If not, why do you
think the light saturation point was not reached?
If the light saturation point was reached, do you think that CO2 supply was the major
factor limiting photosynthesis at this point? How would you test this? Remember
that your breath contains approximately 100 times the concentration of CO 2 in the
atmosphere.
11) Your experiment involved a C3 dicotyledonous plant and a C4 monocotyledonous
plant. Do you think this experiment allows you to make generalized conclusions about
the photosynthetic characteristics of C3 and C4 species? How would you improve the
experiment to make comparisons more informative?
12) Remove a leaf from the C3 and C4 plants and hold them up to the light. What do
you notice about the veins of each leaf? How does this anatomy correlate with the
mechanisms of carbon fixation in each species?
Effect of Light Quality
Color of Filter
m
% O2/min
Photosynthetic Rate
(mol O2 /m2/s)
White
Red
Blue
Green
1)
How did photosynthetic rate differ in each light color? Relate your results
to the absorption spectra of photosynthetic pigments.
2)
What characteristics regarding the amount of light transmitted by the filters
would be important in order to make valid comparisons of the effect of various
wavelengths of light?