Photosynthetic oxygen exchange in C-4 grasses: the role of oxygen as electron acceptor

C-4 grasses of the NAD-ME type (Astrebla lappacea, Eleusine coracana, Eragrostis superba, Leptochloa dubia, Panicum coloratum, Panicum decompositum) and the NADP-ME type (Bothriochloa bladhii, Cenchrus ciliaris, Dichanthium sericeum, Panicum antidotale, Paspalum notatum, Pennisetum alopecuroides, Sorghum bicolor) were used to investigate the role of O-2 as an electron acceptor during C-4 photosynthesis. Mass spectrometric measurements of gross O-2 evolution and uptake were made concurrently with measurements of net CO2 uptake and chlorophyll fluorescence at different irradiances and leaf temperatures of 30 and 40degreesC. In all C-4 grasses gross O-2 uptake increased with increasing irradiance at very high CO2 partial pressures (pCO(2)) and was on average 18% of gross O-2 evolution. Gross O-2 uptake at high irradiance and high pCO(2) was on average 3.8 times greater than gross O-2 uptake in the dark. Furthermore, gross O-2 uptake in the light increased with O-2 concentration at both high CO2 and the compensation point, whereas gross O-2 uptake in the dark was insensitive to O-2 concentration. This suggests that a significant amount of O-2 uptake may be associated with the Mehler reaction, and that the Mehler reaction varies with irradiance and O-2 concentration. O-2 exchange characteristics at high pCO(2) were similar for NAD-ME and NADP-ME species. NAD-ME species had significantly greater O-2 uptake and evolution at the compensation point particularly at low irradiance compared to NADP-ME species, which could be related to different rates of photorespiratory O-2 uptake. There was a good correlation between electron transport rates estimated from chlorophyll fluorescence and gross O-2 evolution at high light and high pCO(2).