Non-photochemical quenching

Non-photochemical quenching (NPQ) is a mechanism employed by plants and algae to protect themselves from the adverse effects of high light intensity.

To counter this problem, one photoprotective mechanism is so-called non-photochemical quenching (NPQ), which relies upon the conversion and dissipation of the excess excitation energy into heat.

The conformational changes are stimulated by a combination of transmembrane proton gradient, the photosystem II subunit S (PsBs) and the enzymatic conversion of the carotenoid violaxanthin to zeaxanthin (the xanthophyll cycle).

[4] This acidification also leads to the protonation of the PsBs subunit of PS II which catalyze the conversion of violaxanthin to zeaxanthin, and is involved in the alteration orientation of the photosystems at times of high light absorption to reduce the quantities of carbon dioxide created and start the non-photochemical quenching, along with the activation of enzyme violaxanthin de-epoxidase which eliminates an epoxide and forms an alkene on a six-member ring of violaxanthin giving rise to another carotenoid known as antheraxanthin.

Violaxanthin contains two epoxides each bonded to a six-member ring and when both are eliminated by de-epoxidase the carotenoid zeaxanthin is formed.

Carbon assimilation (red line) tends to saturate at high light intensities, while light absorption (blue line) increases linearly [ 2 ]
The relationship between irradiance and carbon assimilation for a monoculture of the plankton Woloszynskia halophila at different pH [ 3 ]