The C3 pathway

In this, the Calvin-Benson cycle, CO2 is fixed into a three-carbon acid (phosphoglyceric acid) by the enzyme Rubisco, which is present in massive amounts in the leaves (25-30% of the total leaf nitrogen). This same enzyme can also act as an oxygenase, and this activity (photorespiration) can result in a wasteful release of CO2 - reducing by about one-third the net amounts of CO2 that are fixed. Photorespiration increases with temperature with the consequence that the overall efficiency of carbon fixation declines with increasing temperature.

The rate of photosynthesis of C3 plants increases with the intensity of radiation, but reaches a plateau. In many species, particularly shade species, this plateau occurs at radiation intensities far below that of full solar radiation (see Figure 3.6). Plants with C3 metabolism have low water-use efficiency compared with C4 and CAM plants (see below), mainly because in a C3 plant, CO2 diffuses rather slowly into the leaf and so allows time for a lot of water vapor to diffuse out of it.

Perhaps the most remarkable feature of C4 plants is that they do not seem to use their high water-use efficiency in faster shoot growth, but instead devote a greater fraction of the plant body to a well-developed root system. This is one of the hints that the rate of carbon assimilation is not the major limit to their growth, but that the shortage of water and/or nutrients matters more.

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