O2-enhanced induction of photosynthesis in rice leaves: the Mehler-ascorbate peroxidase (MAP) pathway drives cyclic electron flow within PSII and cyclic electron flow around PSI

Abstract

Lowering the oxygen (O2) partial pressure from 21 kPa to 1 kPa delayed the light-dependent increase of the net carbon dioxide (CO2) assimilation rate in rice (Oryza sativa L. cv. Notohikari) leaves. Researching the underlying molecular mechanisms that act before the start of photosynthesis, we established the following facts. First, O2 at 21 kPa enhanced the quantum yield of PSII [Y(II)] and PSI [Y(I)]. More than 90% of Y(II) and Y(I) were not accounted for by O2-dependent electron flow in the Mehler-ascorbate peroxidase (MAP) pathway. Both yields increased further with the start of photosynthesis. Second, O2 enhanced photochemical quenching of chlorophyll (Chl) fluorescence (qL). qL also increased further with the rate of photosynthesis. Third, O2 enhanced the photo-oxidation of P700. Fourth, O2 suppressed the reduction of P700. Fifth, O2 enhanced non-photochemical quenching of Chl fluorescence (NPQ). These results showed that the MAP pathway triggered cyclic electron flow within PSII (CEF-II) and cyclic electron flow around PSI (CEF-I) by inducing ΔpH across thylakoid membranes and oxidizing the plastoquinone pool, before photosynthesis started. We propose that the photosynthetic electron transport system is controlled by the MAP pathway, which would explain the O2-dependent enhancement of the induction of photosynthesis. © 2012 Copyright Taylor and Francis Group, LLC.