Figure 1.

The Redox State of the PQ Pool Is a Sensor of Environmental Change.

During steady state photosynthesis, the diffusion-dependent oxidation of PQH₂ is considered the rate-limiting step of PET. At photostasis (top), the rate of reduction of the PQ pool by electrons from PSII is balanced by its oxidation by PSI and the demands of downstream metabolism. Exposure of plants to high light (bottom left) results in PQH₂ accumulation, which is reflected by increased excitation pressure on PSII. High excitation pressure conditions can be also mimicked by other environmental conditions (bottom right) that limit the rate of the oxidation of the PQ pool by inhibiting downstream metabolism that consumes the electrons generated by PET.