Unravelling the different components of nonphotochemical quenching using a novel analytical pipeline

Photoprotection in plants includes processes collectively known as nonphotochemical quenching (NPQ), which quench excess excitation-energy in photosystem II. NPQ is triggered by acidification of the thylakoid lumen, which leads to PsbS-protein protonation and violaxanthin de-epoxidase activation, resulting in zeaxanthin accumulation. Despite extensive study, questions persist about the mechanisms of NPQ. We have set up a novel analytical pipeline to disentangle NPQ induction curves measured at many light intensities into a limited number of different kinetic components. To validate the method, we applied it to Chl-fluorescence measurements, which utilised the saturating-pulse methodology, on wild-type (wt) and zeaxanthin-lacking (npq1) Arabidopsis thaliana plants. NPQ induction curves in wt and npq1 can be explained by four components ( $\alpha$ , $\beta$ , $\gamma$ and $\delta$ ). The fastest two ( $\beta$ and $\gamma$ ) correlate with pH difference formed across the thylakoid membrane in wt and npq1. In wt, the slower component ( $\alpha$ ) appears to be due to the formation of zeaxanthin-related quenching whilst for npq1, this component is 'replaced' by a slower component ( $\delta$ ), which reflects a photoinhibition-like process that appears in the absence of zeaxanthin-induced quenching. Expanding this approach will allow the effects of mutations and other abiotic-stress factors to be directly probed by changes in these underlying components.