2.2.1  Evolution of the C4 mode

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Approximately 85% of all terrestrial plant species are C3 plants, while about 10% are CAM plants and are usually found in highly xeric sites (deserts, epiphytic habitats). The remainder are C4 plants, and these become more dominant with aridity. Naively, we could conclude that evolution of C4 photosynthesis was a response to selection by high temperature and low water availability. More likely, a decline in atmospheric CO2 concentration during past millennia provided an initial impetus.

One hundred million years ago (Mid-Cretaceous), atmospheric CO2 was between 1500 and 3000 µL L–1, or four to ten times postindustrial levels. Given such a large concentration of CO2 at that time, photorespiration of C3 plants was inhibited (Section 2.3) so that photosynthetic efficiency was higher than it is now. In addition, maximum photosynthetic rates were double twentieth century values, and the energy cost of photosynthesis would have been around three ATP and two NADPH per molecule of CO2 fixed. However, as atmospheric CO2 concentrations declined to approximately 250–300 µL L–1, photosynthetic rates were halved, photo-respiration increased substantially, photosynthetic efficiency declined and the energetic costs of photosynthesis increased to approximately five ATP and 3.2 NADPH per CO2 molecule fixed. Such events would have generated a strong selection pressure for genetic variants with increased carboxylation efficiency and increased photosynthetic rates.

Angiosperms have a higher relative specificity of Rubisco for CO2 than ferns and mosses (see examples of other less evolutionarily advanced species in Figure 2.2). Such differences imply minor evolution in this highly conserved molecule of Rubisco and there is little variation between species of vascular plants. Consequently, alteration of Rubisco in response to a changing atmospheric CO2 concentration has not been an option.

By contrast, evolution of a new photosynthetic pathway (C4) has occurred independently and on several occasions in diverse taxa over 50 to 60 million years as CO2 levels declined. The oldest identifiable fossils with pronounced bundle sheath layers are seven million years old, although necessary metabolic pathways could have evolved earlier, prior to this adaptation in anatomy. C4 plants are known to differ from C3 plants in their discrimination against atmospheric 13CO2, and shifts in the stable carbon-isotope signature of soil carbonate layers that reflect emergence of C4 plants have been dated at 7.5 million years bp. By inference, C4 photosynthesis evolved in response to a significant decline in atmospheric CO2 concen-tration, from 1500–3000 µL L–1 to about 300 µL L–1. By evolving a CO2-concentrating mechanism, C4 plants presented their Rubisco with an elevated partial pressure of CO2 despite lower atmospheric CO2. As a consequence, photorespiration was inhibited, maximum photosynthetic rates increased and energetic costs reduced.