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Time- and reduction-dependent rise of photosystem II fluorescence during microseconds-long inductions in leaves

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Abstract

Lettuce (Lactuca sativa) and benth (Nicotiana benthamiana) leaves were illuminated with 720 nm background light to mix S-states and oxidize electron carriers. Green-filtered xenon flashes of different photon dose were applied and O2 evolution induced by a flash was measured. After light intensity gradient across the leaf was mathematically considered, the flash-induced PSII electron transport (= 4·O2 evolution) exponentially increased with the flash photon dose in any differential layer of the leaf optical density. This proved the absence of excitonic connectivity between PSII units. Time courses of flash light intensity and 680 nm chlorophyll fluorescence emission were recorded. While with connected PSII the sigmoidal fluorescence rise has been explained by quenching of excitation in closed PSII by its open neighbors, in the absence of connectivity the sigmoidicity indicates gradual rise of the fluorescence yield of an individual closed PSII during the induction. Two phases were discerned: the specific fluorescence yield immediately increased from Fo to 1.8Fo in a PSII, whose reaction center became closed; fluorescence yield of the closed PSII was keeping time-dependent rise from 1.8Fo to about 3Fo, approaching the flash fluorescence yield Ff = 0.6Fm during 40 μs. The time-dependent fluorescence rise was resolved from the quenching by 3Car triplets and related to protein conformational change. We suggest that QA reduction induces a conformational change, which by energetic or structural means closes the gate for excitation entrance into the central radical pair trap—efficiently when QB cannot accept the electron, but less efficiently when it can.

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Abbreviations

Chl:

Chlorophyll

DCMU:

3-(3,4-Dichlorophenyl)-1,1-dimethylurea

ETC:

Electron transport chain

ETR:

Electron transport rate

FI:

Fluorescence induction

F m :

Maximum fluorescence yield at the end of a saturation pulse

F f :

Fluorescence yield after a single-turnover flash

FR:

Far-red light

FRR:

Fast flash repetition rate method

LHCII:

Light-harvesting complex II

MTP:

Multiple-turnover pulse

PAM:

Pulse amplitude modulation

PFD, PAD:

Photon flux density, incident and absorbed

Pheo:

Pheophytin

PQ, PQH2 :

Plastoquinone and plastoquinol

PSI, PSII:

Photosystem I and photosystem II

P680:

Six-Chl complex in reaction center

Q A :

Primary quinone acceptor of PSII

Q B :

Secondary quinone acceptor of PSII

Specific yield:

Fluorescence yield of an individual PSII

STF:

Single-turnover flash

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Acknowledgements

Lettuce was a gift by R. Külasepp and E. Feldmann from Grüne Fee Tartu, benth plants were provided by Dr. Yuh-Shuh Wang, Tartu University. The LeCroy oscilloscope was made available by prof. A. Aabloo, Tartu University.

Funding

The project was financed by University of Tartu (basic funding), Institute of Technology, and by Estonian Academy of Science (A.L.).

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  1. Institute of Technology, University of Tartu, Nooruse st. 1, 50411, Tartu, Estonia

    Vello Oja & Agu Laisk

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Oja, V., Laisk, A. Time- and reduction-dependent rise of photosystem II fluorescence during microseconds-long inductions in leaves. Photosynth Res 145, 209–225 (2020). https://doi.org/10.1007/s11120-020-00783-2

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