Photoinhibition or photoprotection of photosynthesis? Update on the (newly termed) sustained quenching component qH

Non-photochemical quenching (NPQ) of chlorophyll fluorescence is a valuable feature for the study of photosynthetic organisms' light utilization and dissipation. However, all too often NPQ is simply equated with the harmless dissipation of excess absorbed light energy as heat. This is not always the case as some processes cause NPQ without thermal dissipation. Photoinhibitory quenching, qI, is sustained NPQ that continuously depresses the commonly used fluorescence parameter “quantum yield of photosystem II (PSII)”, or Fv/Fm, and is often viewed as a result of PSII core inactivation due to D1 damage. Inactivated PSII cores might have a photoprotective role but that is not the topic of the present review. Instead, this review focuses on a sustained photoprotective antenna quenching component, which we have termed qH, and summarizes the recently uncovered molecular players of this sustained form of NPQ.