Plants use solar energy, carbon dioxide, and water to create energy in the form of carbohydrates

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Introduction
Module 6 ENVIRONMENTAL CONTROLS OVER
PHOTOSYNTHESIS
How does photosynthesis change in proportion to
environmental conditions?
In this exercise we examined the influence of
light, carbon dioxide levels, plant species type
and habitat on the process of photosynthesis.
Plants use solar energy, carbon dioxide, and
water to create energy in the form of
carbohydratesprimarily glucose, a sugar made in
the chloroplasts of green leaves. Plants convert
this sugar into tissue which is a major source of
life and energy on our planet. For this reason
photosynthetic plants are called primary producer
species.
SEE-U 2001 Biosphere 2 Center, AZ Professor Tim
Kittel, TA Erika Geiger
Yuko Chitani Mei Ying Lai Lily Liew Asma Madad
Adam Nix Eli Pristoop J.C. Sylvan
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Many of the environmental conditions which
influence this process are variable around the
globe. Solar radiation levels, water resources,
the availability of the nitrogen compounds plants
need to build energy-converting enzymes, all
vary according to climate. In every biome plants
must adapt to these differences in order to
photosynthesize. And now human activities, like
the burning of fossil fuels, are raising CO2
levels world-widea change which could further
affect photosynthesis.

• In order to predict what the environmental
  impact of human activity on photosynthesis may be
  it is important to determine the relationship
  between different control variables in the
  processlight, available CO2, growing conditions,
  plant physiognomy. Using a device called a
  Infra-red gas analyzer (in this case, a LICOR
  6400 Portable Photosynthesis System), we
  attempted to isolate some of these environmental
  conditions within the partially controlled
  Rainforest Biome of Biosphere2.

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We concentrated our efforts on measuring and
analyzing the effects of light.

• How do plants respond to different light levels?
• Is there an immediate response (over 20 minutes
  for example), and are these responses different
  at different times of the year?
• Are the effects of increased light positive or
  negative?
• Do plants from different species respond
  differently?
• And do individual plants from the same species
  respond differently according to their habitat?
• Is a pothos growing in a lower, shaded layer of
  the rainforest more responsive to increased light
  levels than a pothos which has access to more
  light?
• And finally which is a more influential
  environmental condition with respect to
  photosynthesis, light levels or carbon dioxide
  levels?

• All of these questions (and their answers) can
  have a profound impact on our understanding of
  photosynthesis, of plant communities, and of
  their response to changing environmental
  conditions. The Biosphere 2 Center works as a
  beta-site for testing our hypotheses about
  photosynthesis and plant homeostasis in the wider
  global frame.

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METHODS
We divided this experiment into three components,
each designed to isolate photosynthetic activity
with respect to a control variable light,
available CO2 , and habitat, i.e. darker/
shadier areas vs. lighter/ edge areas of the
Rainforest Biome.

• With the assistance of Karen Vitkay, a
  researcher at the Biosphere2 Center, we used a
  LICOR Infra-red Gas Analyzer to monitor flux of
  carbon dioxide uptake by four leaf samples from
  different areas of the Rainforest Biome. The
  confined chamber on the unit, or cuvette, fits
  over an area of a leaf sample, or an entire leaf
  depending on sample size, and allows the
  researcher to control the amount of ambient CO2
  the sample is exposed to (measured in mico-mols
  m2/s), which can be adjusted through a process
  of carbon dioxide injection to above natural
  levels, and the amount of photosynthetic active
  radiation (PARi) available to the sample area of
  the leaf (measured in micro-mols of photons)
  which can also be adjusted from 0 to 1500.

Infra-red gas analyzer ( LI-COR 6400 Portable
Photosynthesis System)
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By isolating PARi and CO2 levels, we were able
to examine the direct effect of light on on
photosynthesis in four different samples
a.) a leaf from a Pothos vine (Epipremnum
pinnatum) in the lowest (and darkest) layer of
the biome, b.) a leaf of the same species
growing near the edge (or lightest) area of the
biome, c.) a leaf from a Banana plant (Musa sp.)
also growing in this well-lit area, and d.) a
leaf from an unknown species growing in the same
bright edge.

• In an independent test (called an ACi test) we
  also subjected this last sample to increased
  carbon dioxide levels, in order to see how the
  same plants photosynthetic process would respond
  by adjusting a different control value. We
  anticipated that photosynthesis would intensify
  in direct proportion to the amount made
  available to the leaf sample.

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Using the graphing functions of Microsoft Excel,
we compared some of our results with similar data
collected last year in the Biosphere by a
different group.
We evaluated ACi levels for the unknown species
sample against data about a Pothos vine
(Epipremnum pinnatum) growing in the same biome
in order to see if different species would react
differently to increased CO2 levels. We
compared our results with the PARi test for the
Pothos vines in the shade and in the light with
similar data collected from the same species last
year in order to determine whether there is any
significant seasonal difference in the plants
response to increased light levels.

• We evaluated our results from a PARi test on a
  Banana leaf growing near the edge of the biome
  with a last years test on the same species
  growing in the same area.
• We analyzed the different responses to PARi
  tests of three different plant species growing
  near the edge of the Rainforest Biome the Pothos
  vine, the Banana plant, and an unknown species.
  Data for this kind of test from last year were
  unavailable.

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Results

• Both sunny and shaded Epipremnum pinnata were
  exposed to the same incremental changes in light
  intensity. The photosynthetic rate of the plant
  located in a more sunny area experienced a higher
  rate of photosynthesis (see Fig. 1).
• A comparison between 2000 and 2001 PARi tests for
  shaded Epipremnum pinnata was affected by the ACi
  test that was performed on the same sample
  immediately before the 2000 PARi test (see Fig.
  2).
• Rate of photosynthesis decreased as we lowered
  the CO2 level and increased as we increased light
  intensity (see Fig. 3).
• A comparison of ACi curves for 2000 Epipremnum
  pinnata and 2001 unknown specie indicates
  different rates of photosynthesis for different
  species (see Fig. 4).
• A comparison of Banana, Epipremnum pinnata, and
  sp. Unknown PARi curves indicated a significantly
  lower rate of photosynthesis for Banana (see Fig.
  5).
• A comparison of 2000 and 2001 Banana PARi curves
  showed a lower rate of photosynthesis in the 2001
  Banana. The two curves show different responses
  to the same changes in light intensity (see Fig.
  6).
• General trends
• Positive correlation between PARi and
  photosynthesis.
• Positive correlation between CO2 and
  photosynthesis.
• Peak of photosynthesis between 0-200 PARi.

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Fig. 1 PARi Test for Sunny Shaded Epipremnum
pinnata, 2001
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Fig. 2 PARi Test for Shaded Epipremnum pinnata,
2000 2001
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Fig. 3 PARi and ACi Curves for sp. Unknown, 2001
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Fig. 4 ACi Curve for Unknown and Epipremnum
pinnata, 2000 2001
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Fig. 5 PARi Test for Epipremnum pinnata, Banana
Unknown, 2001
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Fig. 6 PARi Test for Banana Leaf, 2000 2001
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Table 1 2000 and 2001 Banana PARi Data
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Discussion

• For the PARi tests of a shaded Epipremnum
  pinnata and a sunny Epipremnum pinnata in figure
  1, the sunny Epipremnum pinnata photosynthesized
  at a higher rate throughout the trial. However,
  it achieved its peak photosynthetic rate at 150
  mmol/m2s, then its rate decreased with
  increasing light intensity. The shaded
  Epipremnum pinnata never reached as high a
  photosynthetic rate as the sunny plant, but its
  photosynthetic rate increased steadily with light
  intensity. We think this is a result of the
  sunny Epipremnum pinnatas consistent exposure to
  high levels of light. Because it is frequently
  exposed to light, it has developed photosynthetic
  structures to take advantage of the light. The
  shaded Epipremnum pinnata does not have the same
  photosynthetic structures because it would be a
  waste of energy to develop them if it is not
  exposed to the same light levels.

• Some other possible explanations for a
  difference in photosynthetic rate between the two
  Epipremnum pinnata are difference in temperature
  and humidity levels. These are slightly
  different and could have contributed to the
  difference in the photosynthetic rates, but the
  fact that they were grown in different areas is
  probably more responsible for the differing
  rates.
• Our hypothesis as to why the sunny Epipremnum
  pinnata shows a decrease in its photosynthetic
  rate is photoinhibition. In this process at
  extremely high light intensity plants down
  regulate their photosynthetic rates and use
  different pigments to absorb light.
• In figure 2, the PARi curves of last years
  shaded Epipremnum pinnata and this years shaded
  Epipremnum pinnata are compared. The
  photosynthetic rate of this years Epipremnum
  pinnata is dramatically higher throughout the
  curve. Last years pothos was releasing rather
  than assimilating CO2. The reasoning behind this
  is that the Epipremnum pinnata used for the PARi
  test last year had just been through an ACi curve
  and did not have the carbon dioxide available to
  take advantage of the high light intensity levels
  and perform photosynthesis
• Figure 3 shows that the level of Carbon dioxide
  has a greater effect on the photosynthetic rate
  than the light intensity.
• In figure 4 the Epipremnum pinnata from last
  year seems to have a higher rate of response than
  the unknown species, however, the Epipremnum
  pinnata from last year was only tested up to a
  CO2 level of 684 ppm.
• In figure 5 the PARi curves of the Epipremnum
  pinnata and unknown specie were relatively
  similar and significantly higher than the PARi
  curve of the bananna. Based on figure 6 there
  was a significant difference in the behavior of
  the banana leaves from last years and this
  years data. Last years banana leaf had more
  moisture available to it than this years (See
  Fig. 1). This years Epipremnum pinnata had a
  similar curve at the beginning of the test but
  quickly overheated and did not have enough
  moisture to maintain optimum temperature for
  photosynthesis.
• Some possible sources error include LICOR
  malfunction and human error in data collection.

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Conclusion

• How does photosynthesis (psn) change in
  proportion to environmental conditions?
• 1. Epipremnum pinnata that grows in a light
  environment shows a higher rate of photosynthesis
  for a given light intensity than Epipremnum
  pinnata grown in a darker environment.
• At very high light intensity levels Epipremnum
  pinnata exhibits photinhibition.
• In the unknown species CO2 has a greater effect
  than light intensity on phtosynthesis.
• 4. This years Epipremnum pinnata had a
  similar curve at the beginning of the test to
  last years but quickly overheated and did not
  have enough moisture to maintain optimum
  temperature for photosynthesis.
• 5. Available moisture is an extremely
  important factor in photosynthesis.

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References Acknowledgements
Danoff-Burg, James A. Flow of Matter and Energy
Module 6 Producers The Basis of Ecosystems.
CERC, Columbia University
Thanks to Karen Vitkay for showing us around and
helping us with this exercise.