The C4 pathway

In this, the Hatch-Slack cycle, the C3 pathway is present but it is confined to cells deep in the body of the leaf. CO2 that diffuses into the leaves via the stomata meets mesophyll cells containing the enzyme phosphoenolpyruvate (PEP) carboxylase. This enzyme combines atmospheric CO2 with PEP to produce a four-carbon acid. This diffuses, and releases CO2 to the inner cells where it enters the traditional C3 pathway. PEP carboxylase has a much greater affinity than Rubisco for CO2. There are profound consequences.

First, C4 plants can absorb atmospheric CO2 much more effectively than C3 plants. As a result, C4 plants may lose much less water per unit of carbon fixed. Furthermore, the wasteful release of CO2 by photorespiration is almost wholly prevented and, as a consequence, the efficiency of the overall process of carbon fixation does not change with temperature. Finally, the concentration of Rubisco in the leaves is a third to a sixth of that in C3 plants, and the leaf nitrogen content is correspondingly lower. As a consequence of this, C4 plants are much less attractive to many herbivores and also achieve more photosynthesis per unit of nitrogen absorbed.

One might wonder how C4 plants, with such high water-use efficiency, have failed to dominate the vegetation of the world, but there are clear costs to set against the gains. The C4 system has a high light compensation point and is inefficient at low light intensities; C4 species are therefore ineffective as shade plants. Moreover, C4 plants have higher temperature optima for growth than C3 species: most C4 plants are found in arid regions or the tropics. In North America, C4 dicotyledonous species appear to be favored in sites of limited water supply (Figure 3.14) (Stowe & Teeri, 1978), whereas the abundance of C4 monocotyledonous species is strongly correlated with maximum daily temperatures during the growing season (Teeri & Stowe, 1976). But these correlations are not universal. More generally, where there are mixed populations of C3 and C4 plants, the proportion of C4 species tends to fall with elevation on mountain ranges, and in seasonal climates it is C4 species that tend to dominate the vegetation in the hot dry seasons and C3 species in the cooler wetter seasons. The few C4 species that extend into temperate regions (e.g. Spartina spp.) are found in marine or other saline environments where osmotic conditions may especially favor species with efficient water use.

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