qE

qE, or energy-dependent quenching, is the fastest component of non-photochemical quenching (NPQ) observed in photosynthetic organisms. It represents the rapid dissipation of excess absorbed light energy as heat within the light-harvesting antenna complexes of photosystem II (PSII), helping to protect the photosynthetic apparatus from light-induced damage when illumination is high.

The triggering of qE is linked to the proton gradient generated across the thylakoid membrane during photosynthesis. This lumen acidification lowers the pH in the thylakoid lumen, activating pH-sensitive elements such as the PsbS protein in many higher plants and green algae. Activation of these components induces conformational changes in antenna complexes, such as LHCII, that promote heat dissipation rather than energy transfer to reaction centers. The xanthophyll cycle also contributes to qE: violaxanthin is de-epoxidized to zeaxanthin under high light, and zeaxanthin facilitates energy dissipation in the antenna complexes.

In terms of kinetics, qE responds within seconds to minutes and is rapidly reversible when light intensity decreases or the proton gradient dissipates. It is the most dynamic and adjustable part of NPQ, whereas other components (such as qZ and qI) operate on longer timescales or reflect different protective processes, including pigment epoxidation dynamics or photoinhibition.

Organisms vary in the molecular players that mediate qE. In many green plants and some algae, PsbS and the xanthophyll cycle are central to qE. In other groups, such as certain diatoms, LHCSR proteins play a similar role. Overall, qE serves as a crucial photoprotective mechanism that balances light harvesting with the risk of photodamage under fluctuating light conditions.