Thermal phase and excitonic connectivity in fluorescence induction

Area of Science:

• Photosynthesis research
• Plant physiology
• Biophysics

Background:

• Chlorophyll fluorescence induction (FI) provides insights into photosystem II (PSII) function.
• Understanding exciton behavior and electron transfer is crucial for plant photosynthesis research.

Purpose of the Study:

• To investigate the connectivity of PSII antennae using chlorophyll fluorescence induction.
• To characterize donor-side quenching mechanisms in PSII.

Main Methods:

• Recorded chlorophyll fluorescence induction curves in sunflower leaves under various pre-adaptation conditions (darkness, low light, DCMU inhibition).
• Analyzed FI curves against cumulative excitations quenched by PSII (nq), calculated from the complementary area above the FI curve.
• Applied a model assuming PSII antennae are connected in domains of four to fit the FI curves in DCMU-inhibited leaves.

Main Results:

• A sigmoidal FI curve was constructed by complementing recorded curves, fitting well with a model of PSII antennae in domains of four.
• This result, obtained by reducing QA with blocked QB, contrasts with previous findings using gradual QB site blockage.
• Donor side quenching increased with increasing photon flux density (PFD) and was attributed to electron transfer from excited P680* to oxidized tyrosine Z (TyrZ(ox)).

Conclusions:

• PSII antennae are likely organized in domains of four, influencing exciton dynamics.
• Donor side quenching, particularly at the J inflection, is primarily driven by photochemical quenching by TyrZ(ox) at high PFDs.
• This quenching mechanism is active while TyrZ remains oxidized and disappears as QA and PQ become reduced.