Introduction

1. 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 carbohydrates—primarily 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

2. 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-wide—a 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 process—light, 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.

3. 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.

4. 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. METHODS 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 m^2/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)

5. 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 plant’s 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.

6. 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 year’s 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.

7. 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.

8. Fig. 1: PARi Test for Sunny & Shaded Epipremnum pinnata, 2001

9. Fig. 2: PARi Test for Shaded Epipremnum pinnata, 2000 & 2001

10. Fig. 3: PARi and ACi Curves for sp. Unknown, 2001

11. Fig. 4: ACi Curve for Unknown and Epipremnum pinnata, 2000 & 2001

12. Fig. 5: PARi Test for Epipremnum pinnata, Banana & Unknown, 2001

13. Fig. 6: PARi Test for Banana Leaf, 2000 & 2001

14. Table 1: 2000 and 2001 Banana PARi Data

15. 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/m^2s, 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 pinnata’s 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 year’s shaded Epipremnum pinnata and this year’s shaded Epipremnum pinnata are compared. The photosynthetic rate of this year’s Epipremnum pinnata is dramatically higher throughout the curve. Last year’s 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 year’s and this year’s data. Last year’s banana leaf had more moisture available to it than this years (See Fig. 1). This year’s 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.

16. 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 year’s Epipremnum pinnata had a similar curve at the beginning of the test to last year’s 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.

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