Oula Ghannoum, University of Western Sydney, Australia
Approximately 85% of all terrestrial plant species perform C3 photosynthesis, while about 3% fix atmospheric CO2 via the C4 photosynthetic pathway. About 10% of plants carry out crassulation acid metabolism (CAM) and are usually found in highly xeric sites (deserts, epiphytic habitats). C4 plants predominate in open and arid habitats, and also include several important food crops such as maize and sugarcane. This section also covers other, less common photosynthetic modes, such as single-cell C4, C3-C4 intermediate and SAM photosynthesis.
A decline in atmospheric CO2 concentration during past millennia has likely provided the initial impetus for the evolution of C4 photosynthesis. High temperature and low water availability may have constituted additional evolutionary pressures. The key feature of C4 photosynthesis is the operation of a CO2 concentrating mechanism which elevates CO2 concentration around Rubisco sites. Hence, C4 plants have a competitive advantage over C3 plants at high temperature and under strong light because of a reduction in photorespiration and an increase in absolute rates of CO2 fixation at current ambient CO2. Such increase in photosynthetic efficiency results in faster carbon gain and commonly higher growth rates, particularly in subtropical and tropical environments. Consequently, and in response to the looming food security crisis, a global research effort led by IRRI (International Rice Research Institute) is underway to bioengineer C4 photosynthetic traits into major C3 crops, such as rice, in order to boost their photosynthesis, and thus, improve yield and resource use efficiency.
In response to CO2 limitation, not only C3-C4 intermediate, but also CAM and SAM variants have evolved with metabolic concentrating devices which enhance Rubisco performance (Sections 2.2.8 and 2.2.9).