Light Dependence of Photosynthesis Data Analysis Worksheet --------- C3 Plant ------- --------- C4 Plant --------- m m Photon Flux % O2/ Photosynthetic Rate % O2/ Photosynthetic Rate mol min (mol O2 /m2/s) min (mol O2 /m2/s) quanta/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 m Photosynthetic Rate Color of Filter % O2/min (mol O2 /m /s) 2 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?
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