State transitions in Physcomitrium patens studied with time-resolved fluorescence.

Photosynthetic organisms adapt their light-harvesting apparatus continuously to the ever-changing light environment. The model moss Physcomitrium patens shares acclimation responses with both the vascular plant Arabidopsis thaliana and the green alga Chlamydomonas reinhardtii. In vivo time-resolved fluorescence spectroscopy has previously helped to clarify the mechanisms of acclimation in A. thaliana and C. reinhardtii. However, it has not yet been applied to study acclimation responses in P. patens. In this work, we studied state transitions in P. patens with in vivo time-resolved fluorescence spectroscopy. We found one short (35–45 ps) and one longer (165–190 ps) in vivo photosystem I (PSI)-related fluorescence lifetime. We suggest that the short fluorescence lifetime is associated with PSI with a small, plant-like antenna, whereas the second component arises from PSI with a larger antenna size. The PSI antenna size is affected by state transitions, a dynamic process that balances the energy capture between PSI and photosystem II (PSII). We found that state transitions in P. patens act most strongly at an exceptionally low light intensity of 3 µmol m− 2 s− 1 actinic light. The in vivo time-resolved fluorescence spectra suggest that state transitions at this actinic light intensity mainly affect PSII and PSI-large, despite PSI-large being excited far less than PSI-small. Our findings indicate that state transitions in P. patens rebalance the excitation-energy distribution to a similar level as vascular plants, but at different light intensities. Our results suggest that state 1 to state 2 transitions primarily increase the abundance of PSI-large.