BIOL 304
Laboratory
Plant Physiology – Light and CO2 Effects on Photosynthetic Rate
Photosynthesis is the process by which plants convert sunlight into useable chemical energy. To exist in any environmental setting, plants must be able to convert solar energy in greater amounts than they require for maintenance in that setting. However, conditions vary from place to place in the environment, so there is no single best strategy to use to do this. Of those factors that affect photosynthetic rate in plants, light intensity, light wavelength (color), temperature and CO2 concentration have been found to be most closely linked to variations in rate.
Plants generally have physiological adaptations that reflect the conditions that prevail in their habitats. So, for example, plants from open environments (grasslands, early successional stages) tend to require higher light intensities for photosynthesis than plants from shady environments (forest floor habitats). Plants with C-4 photosynthesis are particularly adapted to open conditions, but are not well adapted to shade (which tend to be dominated by C-3 plants). In most habitats, CO2 concentrations tend not to vary as much as light. However, short-term changes can be measured in certain types of habitats (wetlands, for example) and there have been documented increases in CO2 measured over the past century that could have wide-ranging effects on photosynthesis in plants.
In this lab, we will do experiments to test the effects of environmental factors on photosynthesis using a computer simulation. The software we will use to do these experiments is capable of testing a number of different hypotheses. You will test one of these hypotheses for your exercise this week.
During your experiment you will measure changes in CO2 concentration over a plant leaf in a glass enclosure. Since photosynthesis uses CO2 and respiration produces CO2, the concentration of CO2 in the enclosure varies depending on the rates of each process. The net photosynthetic rate is equal to the difference between gross photosynthesis (the total amount of carbon fixed by the plant per unit time) and the respiration (the total amount of carbon lost metabolically during that same time). If gross photosynthesis exceeds respiration, there will be a net gain in carbon by the plant; if respiration exceeds photosynthesis, there will be a net loss of carbon in the plant. If photosynthesis equals respiration, there is no gain or loss of carbon by the plant because the amount of CO2 going into the plant equals the amount lost by respiration.
When environmental factors (such as light, for example) vary, photosynthesis
and/or respiration also vary. Generally, low light levels are
insufficient to maintain a positive carbon balance; but as light intensity
increases, photosynthesis also increases. When it exactly balances
respiration, the net carbon gain is zero (this is called the light compensation
point—the light intensity that permits photosynthesis to compensate for
respiratory needs). As light intensity increases further, photosynthesis
exceeds respiration and this continues until all of the photosynthetic capacity
of the plant is saturated (this is called the light saturation point—the
light intensity where increases in photosynthetic response to light levels
off). This is due to a complex interaction of both photosynthetic and
respiratory processes. The same sorts of responses are seen when factors
other than light (CO2, for instance) vary over the plant.
Hypothesis testing:
You will design and carry out an experiment to test one of the following hypotheses, collect and analyze the data and produce a graphical representation of your results.
1. Sun plants have higher light saturation levels than shade plants.
2. C-4 plants have higher light saturation levels than C-3 plants.
3. C-4 plants can tolerate higher temperatures than C-3 plants.
4. Increases in CO2 concentration generally increase photosynthesis in plants,
but there are limits to this effect and the effect is different in C-3 and C-4
plants.
5. Polyploid plants (those that have a chromosome
number that is a multiple of the base chromosome number; for example a tetraploid has a 4N chromosome number) frequently have
physiological responses that are very different from diploid plants (those with
a 2N chromosome number).
Procedure:
1. We will use the program “Leaf Lab” from Biology Labs
Online. Access this lab using the following address:
http//:biologylab.awlonline.com/ . When you get
to the welcome page, press the “Leaf Lab” button. You will
then need a username and password (instructor will provide) to begin. Do
not log on at the welcome page.
2. Depending on the hypothesis you chose, you will need to first choose an
appropriate plant leaf (“Choose Leaf” window) and measure it
(“Measure Area” window; all data will be reported on a unit area
basis, per meter squared).
3. Then you will collect data for that particular leaf. Imagine putting
the leaf in a glass cuvette, sending CO2 over it at a
known concentration, exposing it to known conditions of light and temperature,
and then (once the system stabilizes) collecting CO2 concentration data.
If the concentration in the cuvette goes down,
photosynthesis (which uses CO2) exceeds respiration; if it goes up, respiration
exceeds photosynthesis. The conditions can be altered using the slide
bars in the “Collect Data” window. When the conditions
stabilize, you can begin measuring data by pushing the “record” button.
4. To see what results you got in this experimental run, go to the
“Prepare Data” window. You can see how photosynthetic rate is
calculated and actually do the calculations (by highlighting the row of data
you want to compute and pushing the “compute” button. The
entire row can be deleted later if you don’t like it. For example,
if you see that CO2 exiting the cuvette is zero (THIS
IS SOMETHING TO AVOID), it means the leaf has probably used all available CO2
in the cuvette and cannot respond to increases in
light intensity. It would be a good idea in this case to increase the gas
flow through the system and collect data again. [Note: this is something you
should do routinely if you are analyzing corn leaves.]
5. Repeat this procedure with different light conditions
(vary light intensity, but keep everything else the same). Once
you have 3 or 4 sets of data, you can plot the results on a single graph.
In the “Plot Data” window, highlight the data you want to plot and
push the “Plot selected data” button. Notice that you can
plot either light intensity or CO2 concentration on the X-axis. Your choice
will depend on the hypothesis you chose to investigate. Photosynthetic
rate will always appear on the Y-axis.
6. The best line to fit the data must be obtained by you by moving the intercept,
slope, and asymptote. As you do this, the line changes shape and the
error around the line (shown as sum of squares) decreases. Try to
minimize this error as much as you can. This is the “best
fit” line for your data.
7. If you have a graph you like, label it and print it for your report.
8. Repeat this procedure for different leaves (sun vs. shade or C-3 vs. C-4,
for example).
Homework:
Write a short report (1-2 pages) that includes each of the following
sections:
· an introduction that provides background information and states your
hypothesis
· the experiments you did
· your results (include a copy of your raw data and plots of photosynthesis vs either light intensity or CO2 concentration)
· a short explanation of your results
Saving your work:
1. The easiest way to save your data (and graphs) is to print everything you
want during the lab exercise.
2. If you cannot complete the exercise during lab, save your work (as html
files) as a bookmark or a favorite place on your computer’s
browser. Or write down the html file name and look at it later from
another computer. These data files are stored as html files on the
Pearson server. They can be printed at any time from your computer.
3. As a last resort, you can send any text you saved to a file on your hard
drive or a diskette. These files can be opened by your computer’s
browser. Graphs must be saved separately as “gif” or
“bmp” files and opened later in a graphics program like Paint or
Photo Editor. Then they can be cut and pasted into your document.
You can right-click on an exported graph (html version) to save as a
“gif” file. For example the following graph was saved as a
“gif” file and then transferred into this Word document.
This is an example of the sort of graph you can export from the web site to
a Word document. The text portion of the html file must be saved
separately. Below is an example of saved text from the html file
containing the graph. It shows the statistics describing
the curve that was fitted to the raw data.
You can do the same thing with the raw data used to make the graph.
This is the raw data copied from the notebook, exported and saved as a text
file:
Lab Notes for LeafLab
----------------------------------------------------------------------
+---+----------------+------+------+--------+---------+-----+------+------+-----+
|Exp| Leaf | Flow |
Temp | CO2 in | CO2 out |Light|Filter| Area |
P |
+---+----------------+------+------+--------+---------+-----+------+------+-----+
| 1
Tomato
500 25 350.0
341.8 89 White 7.20 3.9 |
| 1
Tomato
500 25 350.0
305.9 356 White 7.20 20.9 |
| 1
Tomato
500 25 350.0
289.2 722 White 7.20 28.8 |
| 1
Tomato
500 25 350.0
282.2 1256 White 7.20 32.1 |
| 1
Tomato
500 25 350.0
279.9 1600 White 7.20 33.2 |
| 1
Tomato
500 25 350.0
278.2 1889 White 7.20 34.0 |
+---+----------------+------+------+--------+---------+-----+------+------+-----+