Photosynthetic oxygen exchange in C4 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 CO 2 uptake and chlorophyll fluorescence at different irradiances and leaf temperatures of 30 and 40 ∞ ∞ ∞ C. In all C 4 grasses gross O 2 uptake increased with increasing irradiance at very high CO 2 partial pressures ( p CO 2 ) and was on average 18% of gross O 2 evolution. Gross O 2 uptake at high irradiance and high p CO 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 CO 2 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 p CO 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 p CO 2 .