The obligate shade plant,
Tradescantia albiflora Kunth grown at 50 mol photons · m
–2 s
–1 and
Pisum sativum L. acclimated to two photon fluence rates, 50 and 300 mol · m
–2 · s
–1, were exposed to photoinhibitory light conditions of 1700 mol · m
–2 · s
–1 for 4 h at 22° C. Photosynthesis was assayed by measurement of CO
2-saturated O
2 evolution, and photosystem II (PSII) was assayed using modulated chlorophyll fluorescence and flash-yield determinations of functional reaction centres.
Tradescantia was most sensitive to photoinhibition, while pea grown at 300 mol · m
–2 · s
–1 was most resistant, with pea grown at 50 mol · m
–2 · s
–1 showing an intermediate sensitivity. A very good correlation was found between the decrease of functional PSII reaction centres and both the inhibition of photosynthesis and PSII photochemistry. Photoinhibition caused a decline in the maximum quantum yield for PSII electron transport as determined by the product of photochemical quenching (q
p) and the yield of open PSII reaction centres as given by the steady-state fluorescence ratio, F
vF
m, according to Genty et al. (1989, Biochim. Biophys. Acta
990, 81–92). The decrease in the quantum yield for PSII electron transport was fully accounted for by a decrease in F
vF
m, since q
p at a given photon fluence rate was similar for photoinhibited and noninhibited plants. Under lightsaturating conditions, the quantum yield of PSII electron transport was similar in photoinhibited and noninhibited plants. The data give support for the view that photoinhibition of the reaction centres of PSII represents a stable, long-term, down-regulation of photochemistry, which occurs in plants under sustained high-light conditions, and replaces part of the regulation usually exerted by the transthylakoid pH gradient. Furthermore, by investigating the susceptibility of differently lightacclimated sun and shade species to photoinhibition in relation to q
p, i.e. the fraction of open-to-closed PSII reaction centres, we also show that irrespective of light acclimation, plants become susceptible to photoinhibition when the majority of their PSII reaction centres are still open (i.e. primary quinone acceptor oxidized). Photoinhibition appears to be an unavoidable consequence of PSII function when light causes sustained closure of more than 40% of PSII reaction centres.Abbreviations F
o and F
o
minimal fluorescence when all PSII reaction centres are open in darkness and steady-state light, respectively
- F
m and F
m
maximal fluorescence when all PSII reaction centres are closed in darkand light-acclimated leaves, respectively
- F
v
variable fluorescence
- (F
m-F
o)
under steady-state light con-ditions
- F
s
steady-state fluorescence in light
- Q
A
the primary,stable quinone acceptor of PSII
- q
Ne
non-photochemical quench-ing of fluorescence due to high energy state
- (pH); q
Ni
non-photochemical quenching of fluorescence due to photoinhibition
- q
p
photochemical quenching of fluorescence
To whom correspondence should be addressedThis work was supported by the Swedish Natural Science Research Council (G.Ö.) and the award of a National Research Fellowship to J.M.A and W.S.C. We thank Dr. Paul Kriedemann, Division of Forestry and Forest Products, CSIRO, Canberra, Australia, for helpful discussions.
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