What is the usual internal carbon dioxide concentration in C4 species under midday field conditions?

The carbon dioxide concentrating system in C-4 photosynthesis allows high net photosynthetic rates (P-N) at low internal carbon dioxide concentrations (C-i), permitting higher P-N relative to stomatal conductance (g(s)) than in C-3 plants. This relation Would be reflected in the ratio of C-i to external ambient (C-a) carbon dioxide concentration, which is often given as 0.3 or 0.4 for C-4 plants. For a C-a of 360 rho mol mol(-1) that Would mean a C-i about 110-140 mu mol mol(-1). Our field observations made near midday on three weedy C-4 Species, Amaranthus retroflexus, Echinochloa crus-galli, and Setaria faberi, and the C-4 crop Sorghum bicolor indicated mean values of C-i of 183-212 mu mol mol(-1) at C-a = 360 mu mol mol(-1). Measurements in two other C-4 crop species grown with three levels of N fertilizer indicated that while midday values of Ci at high photon flux were higher at limiting N, even at high nitrogen C-i averaged 212 and 196 mu mol mol(-1) for Amaranthus hypochondriacus and Zea mays, respectively. In these two crops midday C-i decreased with increasing leaf to air water vapor pressure difference. Averaged over all measurement days, the mean C-i across all C-4 species was 198 mu mol mol(-1), for a C-i/C-a ratio of 0.55. Prior measurements oil four herbaceous C-3 species using the same instrument indicated all average C-i/C-a ratio of 0.69. Hence midday C-i values in C-4 species under field conditions may often be considerably higher and more similar to those of C-3 species than expected from measurements made oil plants in controlled environments. Reducing gs in C-4 crops at low water vapor pressure differences Could potentially improve their water use efficiency without decreasing P-N.