O2 exchange and chlorophyll fluorescent measurements in Australian native grasses: a comparison on NADP-ME and NAD-ME C4 subtypes

Full text: C4 grasses dominate the vegetation of Australia's tropical and arid grasslands. They have very diverse origins; however biochemically, they fall into three main types: NAD-ME, PCK and NADP-ME. The most notable differences among these subtypes are the contrasting leaf dry matter δ13C and geographic distribution according to rainfall. Our long-term aim is to understand the physiological significance of the biochemical diversification of C4 photosynthesis in grasses. Here we have investigated the O2 exchange and fluorescence characteristics in NADP-ME (C. ciliaris, B. bladhii, P. antidotale, P. alopecuroides) and NAD-ME (A. Iappacea, E. superba, L. dubia, P. coloratum) grasses. The plants were 3.5 to 5 weeks old and grown under well-watered conditions in a growth cabinet with 10/5/4/5 h of 0/400/700/400 μmol m-2 S-1 PAR. Mass spectrometric measurements of 16O2 and 18O2 exchange were made concurrently with measurements of chlorophyll fluorescence on leaf discs in a closed leaf chamber. There was a good correlation between electron transport rates estimated from chlorophyll fluorescence and 16O2 measurements (within 10%) at high light and high CO2 concentrations. At low light estimates from chlorophyll fluorescence were 15 to 20% greater than estimates obtained form 16O2 evolution. Maximal electron transport rates at saturating CO2 were similar between the two C4 subtypes at all irradiances. However, both chlorophyll fluorescence and measurements of 16O2 evolution, revealed that NAD-ME species had significantly greater electron transport rates at the CO2-compensation point than the NADP-ME species1'8O2 uptake at saturating CO2 increased with increasing irradiance and was approximately 20% of 16O2 evolution at all irradiances and for both C4 subtypes. It was approximately 3.5 times greater than dark 18O-2 uptake indicating a significant amount of O2 uptake was associated with the Mehler reaction. The observed differences between NAD and NADP ME grasses will be related to known differences in quantum yield and carbon isotope discrimination between these subtypes