Mehler reaction plays a role in C and C photosynthesis under shade and low CO.

Alternative electron fluxes such as the cyclic electron flux (CEF) around photosystem I (PSI) and Mehler reaction (Me) are essential for efficient photosynthesis because they generate additional ATP and protect both photosystems against photoinhibition. The capacity for Me can be estimated by measuring O exchange rate under varying irradiance and CO concentration. In this study, mass spectrometric measurements of O exchange were made using leaves of representative species of C and C grasses grown under natural light (control; PAR ~ 800 µmol quanta m s) and shade (~ 300 µmol quanta m s), and in representative species of gymnosperm, liverwort and fern grown under natural light. For all control grown plants measured at high CO, O uptake rates were similar between the light and dark, and the ratio of Rubisco oxygenation to carboxylation (V/V) was low, which suggests little potential for Me, and that O uptake was mainly due to photorespiration or mitochondrial respiration under these conditions. Low CO stimulated O uptake in the light, V/V and Me in all species. The C species had similar V/V, but Me was highest in the grass and lowest in the fern. Among the C grasses, shade increased O uptake in the light, V/V and the assimilation quotient (AQ), particularly at low CO, whilst Me was only substantial at low CO where it may contribute 20-50% of maximum electron flow under high light.