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

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.