Mehler reaction plays a role in C₃ and C₄ photosynthesis under shade and low CO₂

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 O₂ exchange rate under varying irradiance and CO₂ concentration. In this study, mass spectrometric measurements of O₂ exchange were made using leaves of representative species of C₃ and C₄ grasses grown under natural light (control; PAR ~ 800 µmol quanta m^-2 s^-1) and shade (~ 300 µmol quanta m^-2 s^-1), and in representative species of gymnosperm, liverwort and fern grown under natural light. For all control grown plants measured at high CO₂, O₂ uptake rates were similar between the light and dark, and the ratio of Rubisco oxygenation to carboxylation (V_o/V_c) was low, which suggests little potential for Me, and that O₂ uptake was mainly due to photorespiration or mitochondrial respiration under these conditions. Low CO₂ stimulated O₂ uptake in the light, V_o/V_c and Me in all species. The C₃ species had similar V_o/V_c, but Me was highest in the grass and lowest in the fern. Among the C₄ grasses, shade increased O₂ uptake in the light, V_o/V_c and the assimilation quotient (AQ), particularly at low CO₂, whilst Me was only substantial at low CO₂ where it may contribute 20-50% of maximum electron flow under high light.