NPQ

Non-photochemical quenching (NPQ) is a collection of photoprotective processes in photosynthetic organisms that dissipate excess absorbed light as heat instead of using it for photosynthesis. In plants, green algae, and cyanobacteria, NPQ reduces the risk of light-induced damage to photosystem II under fluctuating light and high irradiance. NPQ is rapidly reversible and part of short-term acclimation to light.

NPQ comprises several components. The fastest, qE (energy-dependent quenching), is triggered by the acidification of the thylakoid lumen under high light and is mediated by the PsbS protein and the xanthophyll cycle, which converts violaxanthin to zeaxanthin. Slower components include qT (state transitions) and qI (photoinhibition).

Measurement of NPQ is commonly performed by chlorophyll fluorescence using pulse-amplitude modulated techniques, expressed as NPQ = (F_m − F_m′)/F_m′, where F_m is the maximum fluorescence in darkness and F_m′ in light. NPQ rises within seconds to minutes after light exposure and relaxes when light decreases; its magnitude depends on light history, temperature, and nutrient status.

Distribution and variation: In higher plants and green algae, PsbS and the xanthophyll cycle are central to NPQ. In cyanobacteria, different mechanisms such as the orange carotenoid protein can mediate NPQ. NPQ enables rapid regulation of energy flow, balancing protection with photosynthetic efficiency and is influenced by environmental factors including light quality and intensity, temperature, and water status.

Applications: NPQ is a key parameter in plant physiology and crop science, informing models of photosynthesis and stress resilience. Understanding NPQ dynamics supports breeding or engineering plants with improved photoprotection and productivity under variable light conditions.