C4 plants suppress photorespiration by concentrating CO2 in bundle-sheath cells.

Photorespiration adds another wrinkle to the challenge of acquiring CO2 from the air. Recall from Chapter 8 that either CO2 or O2 can be a substrate for rubisco, the key enzyme in the Calvin cycle. When CO2 is the substrate, the Calvin cycle produces carbohydrates through photosynthesis. When O2 is the substrate, there is a net loss of energy and a release of CO2, the process called photorespiration. (Photorespiration is similar to aerobic respiration only in the sense that it uses O2 and releases CO2. However, plants do not gain energy; they lose it.)

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Photorespiration presents a significant challenge for land plants for two reasons. The first is that air contains approximately 21% O2 but only 0.04% CO2. Although rubisco reacts more readily with CO2 than with O2, the sheer abundance of O2 means that rubisco uses O2 as a substrate some of the time. The second reason is that air provides much less of a thermal buffer than does water, such that organisms on land experience higher and more variable temperatures than do organisms that live in water. Temperature has a major effect on photorespiration because the selectivity of rubisco for CO2 over O2 is reduced as temperatures increase. At moderate leaf temperatures, O2 is the substrate instead of CO2 as often as 1 time out of 4. At higher temperatures, O2 is even more likely to be the substrate for rubisco.

Some plants have evolved a way to reduce the energy and carbon losses associated with photorespiration. These are the C4 plants, which suppress photorespiration by increasing the concentration of CO2 in the immediate vicinity of rubisco. C4 plants take their name from the fact that they, like CAM plants, use PEP carboxylase to produce 4-carbon organic acids that subsequently supply the Calvin cycle with CO2. The Calvin cycle produces 3-carbon compounds (Chapter 8), and plants that do not use 4-carbon organic acids to supply the Calvin cycle with CO2 are thus referred to as C3 plants.

Both CAM and C4 plants produce 4-carbon organic acids as the entry point for photosynthesis. However, in CAM plants, CO2 capture and the Calvin cycle take place at different times; in C4 plants, they take place in different cells.

C4 plants initially capture CO2 in mesophyll cells by means of PEP carboxylase, which combines a dissolved form of CO2 (bicarbonate ion, HCO3–) with the 3-carbon compound PEP. This produces 4-carbon organic acids that diffuse through plasmodesmata into the bundle sheath (Fig. 29.7), a cylinder of cells that surrounds each vein. Once inside bundle-sheath cells, the 4-carbon compounds are decarboxylated, releasing CO2 that is then incorporated into carbohydrates through the Calvin cycle (Chapter 8). The C4 cycle is completed as the 3-carbon molecules generated during decarboxylation diffuse back to the chloroplasts in the mesophyll cells, where ATP is used to re-form PEP.

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FIG. 29.7 C4 photosynthesis. C4 plants suppress photorespiration by concentrating CO2 in bundle-sheath cells.

The significance of the C4 cycle is that it operates much faster than the Calvin cycle because of the very low catalytic rate of rubisco (Chapter 8). As a result, the concentration of CO2 within bundle-sheath cells builds up, reaching levels as much as five times higher than in the air surrounding the leaf. The high concentration of CO2 in bundle-sheath cells makes it unlikely that rubisco will use O2 as a substrate. Thus, the C4 cycle functions like a “fuel-injection” system that increases the efficiency of the Calvin cycle in bundle sheath cells.

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C4 plants have high rates of photosynthesis because they do not suffer the losses in energy and reduced carbon associated with photorespiration (Fig. 29.8). At the same time, C4 plants lose less water because they can restrict diffusion through their stomata to a greater extent than a C3 plant while still maintaining high concentrations of CO2 in bundle-sheath cells. However, C4 photosynthesis has a greater energy requirement than conventional (C3) photosynthesis, as ATP must be used to regenerate PEP in the C4 cycle. Thus, C4 photosynthesis confers an advantage in hot, sunny environments where rates of photorespiration and transpiration would otherwise be high. C4 photosynthesis has evolved as many as 20 times, but is most common among tropical grasses and plants of open habitats with warm temperatures. C4 plants include a number of important crops, including maize (corn), sugarcane, and sorghum, as well as some of the most noxious agricultural weeds.

HOW DO WE KNOW?

FIG. 29.8

How do we know that C4 photosynthesis suppresses photorespiration?

BACKGROUND Studies using radioactively labeled CO2 showed that some species initially incorporate CO2 into 4-carbon compounds instead of the 3-carbon compounds that are the first products in the Calvin cycle. These C4 plants also have high rates of photosynthesis. Is this a new, more efficient photosynthetic pathway? Or do C4 plants have high rates of photosynthesis because they are able to avoid the carbon and energy losses associated with photorespiration?

HYPOTHESIS C4 plants do not exhibit photorespiration.

METHOD “Air tests,” as these experiments were first called, compared rates of photosynthesis in normal air (21% O2) and in an experimental gas mixture in which the concentration of O2 is only 1%. When the concentration of O2 is low, rubisco has a low probability of using O2 (instead of CO2) as a substrate, and thus photorespiration does not occur.

RESULTS

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FIG. 29.8

CONCLUSION Photosynthesis in C4 plants is not affected by differences in O4 concentration, indicating that significant photorespiration is not occurring in these plants. In contrast, the photosynthetic rate of the C3 plants increased in the low O4 environment, indicating that photorespiration depresses rates of photosynthesis in 21% O4.

FOLLOW-UP WORK The higher photosynthetic efficiency of C4 photosynthesis has prompted efforts, so far unsuccessful, to incorporate this pathway into C3 crops such as rice.

SOURCE Bjorkman, O., and J. Berry. 1973. “High-Efficiency Photosynthesis.” Scientific American 229:80–93.

Quick Check 2 How does the formation of 4-carbon organic acids increase the efficiency of water use in both CAM and C4 plants?

Quick Check 2 Answer

CAM plants open their stomata at night when rates of evaporation are low and close them during the day to conserve water. At night, CO2 is combined with 3-carbon PEP to form 4-carbon organic acids that are stored in the vacuole; during the day, these 4-carbon organic acids are converted back to PEP and CO2, providing CAM plants with a source of CO2 for photosynthesis, even though their stomata are closed. The production of 4-carbon organic acids by C4 plants results in an increase in the concentration of CO2 in the bundle-sheath cells. The high concentration of CO2 in bundle-sheath cells makes it unlikely that rubisco will use O2 as a substrate, preventing the losses in energy and reduced carbon associated with photorespiration.