Additive Effects of Multiple Photoprotective Mechanisms Drive Efficient Photosynthesis Under Variable Light Conditions.

To cope with changing external conditions, plants undergo dynamic acclimation processes that adjust their photosynthetic machinery, optimising energy use while minimising damage to photosystems (PS). Key photoprotective mechanisms include non-photochemical quenching (NPQ), which dissipates excess excitation energy, and alternative electron transport (AET) pathways, which prevent over-reduction of the photosynthetic electron transport chain. This study provides a comprehensive analysis of how various photoprotective mechanisms contribute to long-term acclimation to high and fluctuating light in Physcomitrium patens, a moss that exhibits well-conserved photoprotective responses that can provide valuable insights into the adaptation of these mechanisms during evolution. Our results demonstrate that modulation of photoprotection at the level of both PSII and PSI is critical for maintaining photosynthetic efficiency and enabling acclimation to variable light conditions. P. patens mutants deficient in NPQ or AET, when exposed to high or fluctuating light all displayed growth defects, reduced photosynthetic efficiency and unbalanced PSI and PSII activity compared to WT plants. These findings indicate that photosynthetic response to varying light conditions depends on the complementary action of multiple protective strategies, rather than a single dominant photoprotective mechanism.