Cb. Osmond et Sc. Grace, PERSPECTIVES ON PHOTOINHIBITION AND PHOTORESPIRATION IN THE FIELD - QUINTESSENTIAL INEFFICIENCIES OF THE LIGHT AND DARK REACTIONS OF PHOTOSYNTHESIS, Journal of Experimental Botany, 46, 1995, pp. 1351-1362
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.