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 O 2 exchange rate under varying irradiance and CO 2 concentration. In this study, mass spectrometric measurements of O 2 exchange were made using leaves of representative species of C 3 and C 4 grasses grown under natural light (control; PAR ~ 800 Âµmol quanta m −2 s −1) and shade (~ 300 Âµmol 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 CO 2, O 2 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 O 2 uptake was mainly due to photorespiration or mitochondrial respiration under these conditions. Low CO 2 stimulated O 2 uptake in the light, V o/V c and Me in all species. The C 3 species had similar V o/V c, but Me was highest in the grass and lowest in the fern. Among the C 4 grasses, shade increased O 2 uptake in the light, V o/V c and the assimilation quotient (AQ), particularly at low CO 2, whilst Me was only substantial at low CO 2 where it may contribute 20-50% of maximum electron flow under high light.