Time- and reduction-dependent rise of photosystem II fluorescence during microseconds-long inductions in leaves
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ORIGINAL ARTICLE
Time‑ and reduction‑dependent rise of photosystem II fluorescence during microseconds‑long inductions in leaves Vello Oja1 · Agu Laisk1 Received: 12 June 2020 / Accepted: 2 September 2020 © Springer Nature B.V. 2020
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 O 2 evolution induced by a flash was measured. After light intensity gradient across the leaf was mathematically considered, the flashinduced 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 F o 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. Keywords Photosynthesis · Leaves · Chlorophyll fluorescence · Microsecond induction Abbreviations Chl Chlorophyll DCMU 3-(3,4-Dichlorophenyl)-1,1-dimethylurea ETC Electron transport chain ETR Electron transport rate FI Fluorescence induction Fm Maximum fluorescence yield at the end of a saturation pulse Ff 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 * Agu Laisk [email protected] 1
Institute of Technology, University of Tartu, Nooruse st. 1, 50411 Tartu, Estonia
Pheo Pheophytin PQ, PQH2 Plastoquinone and plastoquinol PSI, PSII Photosystem I and photosystem II P680 Six-Chl complex in reaction center QA Primary quinone acceptor of PSII QB Secondary quinone acceptor of PSII Specific yield Fluorescence yield of an individual PSII STF Single-turnover flash
Introduction Analysis of chlorophyll fluorescence induction (FI) curves is a simple technique widely used for investigation of initial events of electron
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