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Assignment 3
 

Assignment 3

Although plants are consuming CO2 during the dark reactions of photosynthesis, they are also producing CO2 as they produce ATP during cell respiration. Light compensation occurs when the rate of photosynthesis balances the rate of cell respiration such that the net rate of CO2 production is zero. In this experiment, you will learn to measure light compensation points in two different clones of goldenrod.

Perform an experiment using goldenrod sun and shade clones. Gas flow should be on medium. All other parameters can remain at their default values. For each leaf, measure photosynthetic rates for light intensities of 0, 20, 40, 60, 80, 100, and 120 mmol/m2/s. Plot the data for both leaves on the same graph. Enter the asymptote values from your previous plots of the goldenrod (assignment 5) and fit the intercept and slope, keeping the asymptotes constant. Note the point at which the curves cross the x-axis. This is called the light compensation point. Click and drag the mouse on the plot to determine the exact value for the light compensation point.
1 .       What was the value of the light compensation point for each of these clones? If you were simply measuring net CO2 concentration for a leaf at its light compensation point, you could interpret the data to mean that no photosynthesis was occurring in the leaf. Explain why this would not be a correct interpretation.  



2 .       Do the differences in light compensation points between sun and shade clones make sense given the conditions under which these clones would be growing in their natural ecosystems?  








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