Photoprotection in a desert moss: dynamic excitation quenching during the hydration cycle of the Syntrichia caninervis.

Desiccation-tolerant mosses survive extreme water loss by activating efficient photoprotective mechanisms that prevent damage to the photosynthetic apparatus. The most resistant moss Syntrichia caninervis (S. caninervis) represents an important model for studying the molecular basis of dehydration resilience; however, the membrane and protein level processes, which enable rapid recovery of photosynthetic activity upon rehydration, remain insufficiently understood. In this work, we investigate the fluorescence (FL) dynamics of the photosynthetic apparatus of S. caninervis under three hydration states (hydrated, dehydrated and desiccated) in whole leaves using time-resolved FL spectroscopy. Time-resolved measurements with ~ 10 ps resolution were performed to resolve excitation dynamics in both photosystems I (PSI) and II (PSII). The complex FL datasets were analysed using the spectral decomposition approach. Dehydrated and desiccated samples exhibit substantially reduced FL intensity compared to hydrated moss in steady-state experiments, indicating efficient quenching of chlorophyll excitation upon drying. The time-resolved FL measurements revealed that excitation transfer rates and the nature of quenched states strongly depend on hydration level in S. caninervis. In the hydrated state, FL dynamics are consistent with functional photosystems in plants. Upon dehydration, non-photochemical quenching is activated in both PSII and PSI, with PSII quenching persisting even at low temperatures. We therefore conclude, that S. caninervis employs few hydration-dependent photoprotective regimes, including distinct quenching mechanisms at both photosystems. Our results suggest the presence of ultrafast quenching processes under severe dehydration, which are progressively relieved during rehydration and replaced by alternative mechanisms supporting the recovery of photosynthetic activity.