Non-photochemical quenching

Photosynthetic_parameters_of_plants

Irradiance_and_carbon_assimilation_for_a_monoculture_of_Woloszynskia_halophila_at_different_pH

Non-photochemical quenching (NPQ) is a mechanism employed by plants, algae, and cyanobacteria to protect themselves from the adverse effects of excess light energy. This process is crucial for the regulation of light harvesting in the photosynthetic apparatus, particularly in the chloroplasts of plant cells.

Mechanism

NPQ involves the dissipation of excess absorbed light energy as heat, thereby preventing the formation of reactive oxygen species that can damage the photosystems. The process is primarily associated with the light-harvesting complex II (LHCII) in the thylakoid membrane of the chloroplasts.

The activation of NPQ is regulated by the proton gradient (ΔpH) across the thylakoid membrane, which is generated during the light reactions of photosynthesis. When the light intensity is high, the ΔpH increases, leading to the activation of the protein PsbS and the conversion of the pigment violaxanthin to zeaxanthin via the xanthophyll cycle. These changes facilitate the dissipation of excess energy as heat.

Types of NPQ

NPQ can be divided into several components based on their kinetics and underlying mechanisms:

• **qE (energy-dependent quenching)**: The fastest component, activated by the ΔpH and involving the xanthophyll cycle.
• **qT (state-transition quenching)**: Involves the redistribution of light-harvesting complexes between photosystem II (PSII) and photosystem I (PSI).
• **qI (photoinhibitory quenching)**: A slower component associated with the repair of damaged PSII reaction centers.

Importance

NPQ is essential for the protection of the photosynthetic machinery under fluctuating light conditions. It allows plants to adapt to changes in light intensity, thereby optimizing photosynthetic efficiency and preventing photodamage. This mechanism is particularly important in environments where light intensity can vary rapidly, such as in forests or aquatic ecosystems.

Research and Applications

Understanding NPQ is important for improving crop yields and developing stress-resistant plant varieties. Research in this area focuses on the genetic and molecular basis of NPQ, as well as its regulation and interaction with other photoprotective mechanisms.

Related Pages

• Photosynthesis
• Chloroplast
• Light-harvesting complex II
• Thylakoid membrane
• Xanthophyll cycle
• Photosystem II
• Photosystem I

See Also

• Photoprotection
• Reactive oxygen species
• Photoinhibition