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 processes leading to a reduction in the efficiency of light utilization in photosynthesis, and that photorespiration embraces several processes associated with O-2 uptake in the light, photoinhibition and photorespiration now can be considered as inevitable, but essential inefficiencies of photosynthesis which help preserve photosynthetic competence in bright light, Photorespiratory O-2 uptake via Rubisco, and O-2 uptake via the Mehler reaction, both promote non-assimilatory electron transport, and stimulate photon utilization during CO2-limited photosynthesis in bright light, Although fluorescence studies show that the proportion of total photon use via oxygenase photorespiration in air may decline to only about 10% in full sunlight, mass spectrometer studies show that O-2 uptake in Mehler reaction photorespiration in C-3 and CAM plants 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 photoinhibition and photon protection (rapidly reversible decrease in PSII efficiency involving dissipation of the energy of excess photons in the antennae), Net CO2 and O-2 exchange studies evidently underestimate the extent of total electron transport, and hence overestimate the extent of photon excess in bright light, leading to overestimates of the role of energy-dependent photon dissipation through dynamic photoinhibition, Nevertheless, in C-3 plants in air all of these processes help to mitigate chronic photoinhibition and photon damage (slowly reversible decrease in PSII efficiency involving loss of reaction centre function), The possibility remains that residual electron transport to O-2 from intermediates in the vicinity of PSII may also lead to reactive O-2 species that potentiate this photon damage.