Carbon isotope discrimination as a diagnostic tool for C4 photosynthesis in C3-C4 intermediate species

Combined gas exchange and carbon isotope discrimination measurement and modeling is used to detect and estimate the contribution of the C-4 cycle in the C-3-C-4 intermediate species.The presence and activity of the C-4 cycle in C-3-C-4 intermediate species have proven difficult to analyze, especially when such activity is low. This study proposes a strategy to detect C-4 activity and estimate its contribution to overall photosynthesis in intermediate plants, by using tunable diode laser absorption spectroscopy (TDLAS) coupled to gas exchange systems to simultaneously measure the CO2 responses of CO2 assimilation (A) and carbon isotope discrimination (Delta) under low O-2 partial pressure. Mathematical models of C-3-C-4 photosynthesis and Delta are then fitted concurrently to both responses using the same set of constants. This strategy was applied to the intermediate species Flaveria floridana and F. brownii, and to F. pringlei and F. bidentis as C-3 and C-4 controls, respectively. Our results support the presence of a functional C-4 cycle in F. floridana, that can fix 12-21% of carbon. In F. brownii, 75-100% of carbon is fixed via the C-4 cycle, and the contribution of mesophyll Rubisco to overall carbon assimilation increases with CO2 partial pressure in both intermediate plants. Combined gas exchange and Delta measurement and modeling is a powerful diagnostic tool for C-4 photosynthesis.