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 (Figure 1) 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 on a large scale, temperature can predict the interaction and mix of C₃ and C₄ plants. 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 (Figure 2) to describe the relationship of C₃ and C₄ plants to temperature and CO₂ concentrations.

Notice that the measure of photosynthesis rate is quantum yield (y-axis above), which can be considered as a surrogate for plant growth. C₃ plants show a strong decline in quantum yield with increasing temperature (left and middle panel above); whereas C₄ plants show no change in quantum yield with temperature (right panel above). 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 (i.e., includes both monocots and dicots).

SIMULATION

QUESTIONS

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°, 50°, and 60° North. 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), and 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), and 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?