Bio122 Fall 2015
Lecture 3: Photosynthesis Pathways – Activity #3
Name _______________________________________________
Hands-On Problem 5.1: Some Like It Hot: Comparison of C3 and C4 Pathways
(This exercise is based on Ehleringer, J. R., Cerling, T. E., and Helliker, B. R. 1997. C4 photosynthesis,
atmospheric CO2, and climate. Oecologia 112: 285–299.)
INTRODUCTION
As you learned in Chapter 5, high rates of photosynthesis that occur at high temperatures can result in
increased O2 concentrations relative to CO2. Under these conditions, typical plants with C3 metabolism
experience high rates of photorespiration. Photorespiration is costly and results in the loss of energy that
could otherwise be used for plant maintenance or growth. Apparently this has been a strong selective force
on plant metabolism, because an alternative pathway, C4 metabolism, that avoids photorespiration, has
evolved multiple times. Most C4 plants are grasses (monocots) and they include some of the most important
food crops such as corn and sugarcane.
Although many factors interact to determine which species of plants coexist in an ecological community,
photosynthetic efficiency at various temperatures can be an important large-scale predictor of plant
distributions. In the paper by Ehleringer et al., (1997), they suggest that at a large scale temperature can
predict the interaction and mix of C3 and C4 plants (Figure 2). In addition, atmospheric concentration of CO2
can influence the likelihood of photorespiration and thus the relative efficiency of C3 and C4 plants. If
concentrations of CO2 are high, photorespiration by C3 plants will be negligible even at high temperatures.
Ehleringer et al., (1997) provided the following panel of plots to describe the relationship of C3 and C4 plants
to temperature and CO2 concentrations.
Figure 2
Notice that the measure of photosynthesis rate is quantum yield, which can be considered as a surrogate for
plant growth. C3 plants show a strong decline in quantum yield with increasing temperature; whereas, C4
plants show no change in quantum yield with temperature. This difference sets up a trade-off between the
two types of metabolism at various temperatures. We will use these relationships along with a simulation of
temperature at various latitudes in the plains region (grassland biome) of the central United States and
Canada to determine what plant communities should look like at high or low latitudes. You will simulate the
mix of C3 and C4 plants that should exist at various latitudes for different CO2 levels. You will consider how
the distribution of C3 and C4 plants should change from preindustrial CO2 levels (180 ppmV) to potential
future CO2 levels (500 ppmV). We are simulating a mixed grass-forb prairie.
SIMULATION
http://sites.sinauer.com/ecology3e/problem05.html
Notice in the simulation window you can choose the CO2 level corresponding to three time periods
(preindustrial to future). You can also choose the latitude at which to make the comparison (values can range
from 25° to 60° North latitude). C3 plants are shown in green and C4 plants are in brown. Bars show quantum
yield (our measure of growth) of each type of plant. We assume that plants that have lower relative growth
will not coexist over long periods of time with plants that have higher relative growth. If one type of plant
shows substantially higher growth rates over the year, then we would predict a grassland dominated by that
plant type. If growth rates are relatively even over the course of the year, then we predict coexistence and a
resulting mixed grassland. For our purposes, a difference of less than 10% (of the smaller value) is predicted
to result in a mixed grassland.
Question 1
Start at preindustrial CO2 levels and run the first simulation at 30°N. Note how photosynthesis changes over
the course of the year, and the annual quantum yield. Now rerun the simulation for 40° to 60° North at ten
degree intervals. Plot annual yield by latitude for both photosynthetic types. Which photosynthetic type will
dominate? At what range of latitudes do you predict C4 dominance, mixed prairie (both C3 and C4), C3
dominance?
Question 2
Now change the CO2 level to current (350 ppmV) and run the same range of latitudes noting the relative
growth rate of both photosynthetic types at each latitude. Plot annual yield by latitude for both
photosynthetic types. Which photosynthetic type will dominate? At what range of latitudes do you predict C4
dominance, mixed prairie (both C3 and C4), C3 dominance?
Run the same series of simulations for a future CO2 level (500 ppmV). Run the same range of latitudes noting
the relative growth rate of both photosynthetic types at each latitude. Plot annual yield by latitude for both
photosynthetic types. Which photosynthetic type will dominate? At what range of latitudes do you predict C4
dominance, mixed prairie (both C3 and C4), C3 dominance?
Question 3
Considering all three of your graphs, how is the mix of C3 and C4 plants likely to change with future changes
in CO2 levels? Explain why. Note that we are only considering changes in CO2 and not corresponding
temperature change. How might higher temperatures affect the relative growth of these two photosynthetic
types?