Photosynthetic oxygen exchange in C4 grasses: The role of oxygen as electron acceptor

Abstract

C4 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 C4 photosynthesis. Mass spectrometric measurements of gross O2 evolution and uptake were made concurrently with measurements of net CO2 uptake and chlorophyll fluorescence at different irradiances and leaf temperatures of 30 and 40°C. In all C4 grasses gross O2 uptake increased with increasing irradiance at very high CO2 partial pressures (pCO2) and was on average 18% of gross O2 evolution. Gross O2 uptake at high irradiance and high pCO2 was on average 3.8 times greater than gross O2 uptake in the dark. Furthermore, gross O2 uptake in the light increased with O 2 concentration at both high CO2 and the compensation point, whereas gross O2 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 O2 concentration. O 2 exchange characteristics at high pCO2 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 O2 uptake. There was a good correlation between electron transport rates estimated from chlorophyll fluorescence and gross O2 evolution at high light and high pCO2.