Photoinhibition of photosystem II : inactivation, protein damage and turnover

Carotenoids and photoprotection in plants : a role for the xanthophyll zeaxanthin

Inhibition of photosynthesis by excess excitation energy is initiated in the reaction center of photosystem II The primary site of photoinhibition in the reaction center (components of primary charge separation or secondary electron acceptor QB) is still disputed Photoinhibition is characterized by quenching of variable chlorophyll flurescence (Fv), resulting from increased thermal dissipation of excitation energy Varying responses of initial fluorescence (F0), however, seem to indicate involvement of different mechanisms As far as photoinhibition is reversible within minutes to hours, it can be viewed as a controlled protective mechanism that serves to dissipate excessive energy, Supposedly, another dissipative mechanism, distinguished by its faster kinetics (response within seconds), is related to the energy-dependent fluorescence quenching

Photoinhibition of photosynthesis. An evaluation of damaging and protective mechanisms

Chlorophyll a fluorescence is a highly sensitive, non-destructive, and reliable tool for measuring, rather quickly, photosynthetic efficiency, particularly of Photosystem II (PSII), the water-plastoquinone oxidoreductase. We briefly review here the connection between the fast (up to 2 s) chlorophyll fluorescence rise and PSII, as well as the empirical use of the fluorescence rise kinetics in understanding photosynthetic reactions, particularly of PSII. When dark-adapted photosynthetic samples are exposed to light, a fluorescence induction is observed, known as the Kautsky effect, after Hans Kautsky, the discoverer of the phenomenon showing the existence of variable fluorescence. The chlorophyll fluorescence intensity rises from a minimum level (the O level), in less than 1 s, to a maximum level (the P-level) via two intermediate steps labeled J and I. This is followed by a decline to a lower semi-steady state level, the S level, which is reached in about one minute. We provide here an educational review on how this phenomenon has been exploited through analysis of the fast OJIP fluorescence transient, by discussing basic assumptions, derivation of equations, as well as application to PSII-related questions.

On the relation between the Kautsky effect (chlorophyll a fluorescence induction) and Photosystem II: basics and applications of the OJIP fluorescence transient.

Dynamics of photosynthetic membrane composition and function

Photoinhibition of photosynthesis is manifested at the level of the leaf as a loss of CO2 fixation and at the level of the chloroplast thylakoid membrane as a loss of photosystem II electron-transport capacity. At the photosystem II level, photoinhibition is manifested by a lowered chlorophyll a variable fluorescence yield, by a lowered amplitude of the light-induced absorbance change at 320 nm (ΔA320) and 540-minus-550 nm (ΔA540–550), attributed to inhibition of the photoreduction of the primary plastoquinone QA molecule. A correlation of the kinetics of variable fluorescence yield loss with the inhibition of QA photoreduction suggested that photoinhibited reaction centers are incapable of generating a stable charge separation but are highly efficient in the trapping and non-photochemical dissipation of absorbed light. The direct effect of photoinhibition on primary photochemical parameters of photosystem II suggested a permanent reaction center modification the nature of which remains to be determined.

Robyn E. Cleland

Papers

Mechanism of photoinhibition: photochemical reaction center inactivation in system II of chloroplasts