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Figure 14.9 Maize (a subtropical C4 species) has a high optimum temperature (~30 °C) for CO2 assimilation, while barley (a temperate C3 species) has a lower, but less distinct optimum (~15 °C). Doubling CO2 has little effect on CO2 assimilation rate by maize (not shown here), but greatly increases the absolute rate and optimum temperature for barley by suppressing photorespiration. This O2-dependent loss of carbon increases with temperature and is largely responsible for the low optimum temperature of photosynthesis in many C3 species (Based on Labate et al. 1990)