PERSPECTIVES ON PHOTOINHIBITION AND PHOTORESPIRATION IN THE FIELD - QUINTESSENTIAL INEFFICIENCIES OF THE LIGHT AND DARK REACTIONS OF PHOTOSYNTHESIS

Taking the long-held view that photoinhibition embraces several proces ses leading to a reduction in the efficiency of light utilization in p hotosynthesis, and that photorespiration embraces several processes as sociated with O-2 uptake in the light, photoinhibition and photorespir ation now can be considered as inevitable, but essential inefficiencie s of photosynthesis which help preserve photosynthetic competence in b right light, Photorespiratory O-2 uptake via Rubisco, and O-2 uptake v ia the Mehler reaction, both promote non-assimilatory electron transpo rt, and stimulate photon utilization during CO2-limited photosynthesis in bright light, Although fluorescence studies show that the proporti on of total photon use via oxygenase photorespiration in air may decli ne to only about 10% in full sunlight, mass spectrometer studies show that O-2 uptake in Mehler reaction photorespiration in C-3 and CAM pla nts can still account for up to 50% of electron flow in saturating CO2 and light, The Mehlerascorbate peroxidase reaction has an additional role in sustaining membrane energization which promotes dynamic photoi nhibition and photon protection (rapidly reversible decrease in PSII e fficiency involving dissipation of the energy of excess photons in the antennae), Net CO2 and O-2 exchange studies evidently underestimate t he extent of total electron transport, and hence overestimate the exte nt of photon excess in bright light, leading to overestimates of the r ole of energy-dependent photon dissipation through dynamic photoinhibi tion, Nevertheless, in C-3 plants in air all of these processes help t o mitigate chronic photoinhibition and photon damage (slowly reversibl e decrease in PSII efficiency involving loss of reaction centre functi on), The possibility remains that residual electron transport to O-2 f rom intermediates in the vicinity of PSII may also lead to reactive O- 2 species that potentiate this photon damage.