Quantitative decomposition of non-photochemical quenching in Physcomitrium patens highlights synergistic roles of LhcSR and zeaxanthin

Non-photochemical quenching (NPQ) is a collective term for photoprotective processes that safely dissipate excess light energy as heat. The moss Physcomitrium patens is an interesting species for the study of NPQ as it contains PsbS (indispensable for NPQ in vascular plants), LhcSR (indispensable for NPQ in green algae) and a xanthophyll cycle, which interconverts violaxanthin (Vx) and zeaxanthin (Zx) and is also imperative for NPQ. Here, we aimed to disentangle the individual contributions of PsbS, LhcSR and Zx to NPQ. NPQ induction and relaxation were measured for wild-type P. patens and thirteen mutants with altered NPQ at a wide range of light intensities. We applied a multivariate data analysis pipeline to find distinct kinetic components underlying NPQ, together with their contributions to NPQ. A slowly-rising component provides most NPQ, especially at higher light intensities. Another component contains a transient NPQ peak with a fast rise, providing quick protection, and requires the presence of either PsbS or LhcSR. Both components are enhanced by the combined presence of Zx and LhcSR. While PsbS-related NPQ is less dependent on Zx, in contrast to the situation in vascular plants, Vx to Zx conversion enhances LhcSR-related NPQ at all light intensities and within the first minute of illumination. The influence of Zx is thus broader than previously recognized, especially through its synergistic interaction with LhcSR.