Non-photochemical quenching (NPQ) in photoprotection: insights into NPQ levels required to avoid photoinactivation and photoinhibition.

Plant photosynthesis is highly responsive to fluctuations in environmental cues. To achieve optimal photosynthetic performance, plants must accurately regulate light absorption, maintaining a dynamic balance between energy supply and consumption in the field. Understanding the potential damage and imbalances caused by excessive light during photosynthesis necessitates a comprehensive insight into the protective role of non-photochemical quenching (NPQ). This rapid photoprotective mechanism dissipates excess excitation energy as heat and is ubiquitous throughout the plant kingdom. Previous reviews have primarily focused on the regulation of NPQ amplitude, often overlooking its efficiency in photoprotection. This review outlines the significance, components, and mechanisms of NPQ, presenting fundamental equations that quantitatively describe both NPQ amplitude and its protective functions. I highlight the methodological approaches to quantify the NPQ levels necessary to prevent photoinactivation and photoinhibition, respectively. I conclude by identifying key open questions regarding NPQ and suggesting directions for future research.