Electron transport to oxygen mitigates against the photoinactivation of Photosystem II in vivo

The role of electron transport to Oz in mitigating against photoinactivation of Photosystem (PS) II was investigated in leaves of pea (Pisum sativum L.) grown in moderate light (250 mu mol m(-2) s(-1)). During short-term illumination, the electron flux at PS II and non-radiative dissipation of absorbed quanta, calculated from chlorophyll fluorescence quenching, increased with increasing O-2 concentration at each light regime tested. The photoinactivation of PS II in pea leaves was monitored by the oxygen yield per repetitive flash as a function of photon exposure (mel photons m(-2)). The number of functional PS II complexes decreased nonlinearly with increasing photon exposure, with greater photoinactivation of PS II at a lower O-2 concentration. The results suggest that electron transport to O-2, via the twin processes of oxygenase photorespiration and the Mehler reaction, mitigates against the photoinactivation of PS II in vivo, through both utilization of photons in electron transport and increased nonradiative dissipation of excitation. Photoprotection via electron transport to O-2 in vivo is a useful addition to the large extent of photoprotection mediated by carbon-assimilatory electron transport in 1.1 lo CO2 alone.