Submitted to: Photosynthetica
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 28, 2005
Publication Date: August 1, 2005
Citation: Bunce, J.A. 2005. What is the usual internal carbon dioxide concentration in C4 species under midday field conditions? Photosynthetica. 43:603-608.
Interpretive Summary: Global climatic changes are anticipated to increase the frequency of droughts. One way to increase the drought tolerance of crops would be to increase the efficiency with which crops use water, which is indicated by the carbon dioxide concentration inside their leaves. This research has shown that crops already noted for high water use efficiency, sorghum, corn and grain amaranth, often operate much less efficiently under humid conditions than they do when the air is drier. This indicates that there may be considerable scope for improving the water use efficiency and drought tolerance of these crops by breeding for lines which maintain high water use efficiency under all conditions. This work will be of interest to crop breeders attempting to increase the drought tolerance of crops.
The carbon dioxide concentrating system in C4 photosynthesis allows high rates of photosynthesis at low internal carbon dioxide concentrations, permitting higher rates of photosynthesis relative to stomatal conductance and higher water use efficiency than in C3 plants. A high rate of photosynthesis relative to stomatal conductance would be reflected in a low value of internal carbon dioxide. For an external concentration of 360 ppm (part per million), an internal concentration of 110 to 140 ppm is often given as typical for C4 species. Our field observations made near midday on three weedy C4 species and the C4 crop sorghum indicated mean internal carbon dioxide concentrations of 183 to 212 ppm. More extensive measurements in two other C4 crop species, grain amaranth and corn, indicated that internal concentrations averaged 212 and 196 ppm for these two species, respectively. Midday internal carbon dioxide concentrations decreased with increasing leaf to air water vapor pressure difference. It is concluded that midday internal carbon dioxide concentrations in C4 species under field conditions may often be considerably higher and more similar to those of C3 species than expected from measurements made on plants in controlled environments. Reducing stomatal conductance in C4 crops under humid conditions could potentially improve their water use efficiency without decreasing photosynthesis.