'Gold standard' measurements of photosynthesis of sorghum, maize, setaria, and rice

Rice is currently the most important food crop globally because it feeds roughly half of the planet's population and approximately three-quarters of a billion of the worlds' poorest specifically depend on rice for their survival. The authors are currently part of a large consortium attempting to transfer C4 photosynthesis into rice to improve photosynthesis, and hence, the yield of rice. As part of the project 'gold standard' measurements of photosynthesis need to be carried out to provide a solid baseline of crop parameters useful for rice improvement. In this study, comparative 'gold standard' gas exchange measurements were established on sorghum, maize, setaria and rice in order to produce a comparison in terms of biochemical components. Plants utilizing the C4 pathway (e.g. sorghum, maize, and seteria) are photosynthetically efficient and have a competitive advantage over C3 plants (rice). C3 and C4 plants are clearly differentiated by their maximum assimilation rate (Amax) and CO2 compensation point. C4 plants are known to have a CO2 compensation point closed to zero, which indicates the plants ability to use CO2 more efficiently that provides for higher photosynthesis, growth and yield. A higher CO2 compensation point (50 ppm) is characteristic of C3 plants. Results of gas exchange measurement showed that rice had significantly lower assimilation rate (33.87 umol m-2 s-1) than sorghum (49.38), maize (50.55) and setaria (40.67). The average CO2 compensation point on the other had of sorghum, maize, setaria, and rice was 0.57, 0.12, 3.80, and 55 ppm, respectively. These results are highly reproducible and thus, can be used to compare rice with other wild relatives, transgenics, and mutants to screen for gain-of-function mutants in C3 species and loss-of-function mutants in C4 species (intermediate of C4 features).