ELECTRON-TRANSPORT TO OXYGEN MITIGATES AGAINST THE PHOTOINACTIVATION OF PHOTOSYSTEM-II IN-VIVO

The role of electron transport to Oz in mitigating against photoinacti vation 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 fluorescen ce quenching, increased with increasing O-2 concentration at each ligh t regime tested. The photoinactivation of PS II in pea leaves was moni tored by the oxygen yield per repetitive flash as a function of photon exposure (mel photons m(-2)). The number of functional PS II complexe s decreased nonlinearly with increasing photon exposure, with greater photoinactivation of PS II at a lower O-2 concentration. The results s uggest that electron transport to O-2, via the twin processes of oxyge nase photorespiration and the Mehler reaction, mitigates against the p hotoinactivation of PS II in vivo, through both utilization of photons in electron transport and increased nonradiative dissipation of excit ation. Photoprotection via electron transport to O-2 in vivo is a usef ul addition to the large extent of photoprotection mediated by carbon- assimilatory electron transport in 1.1 lo CO2 alone.