Anti-Stokes fluorescence spectra of chloroplasts in Parachlorella kessleri and maize at room temperature as characterized by near-infrared continuous-wave laser fluorescence microscopy and absorption microscopy.

Microscopic autofluorescence spectral imaging of chloroplasts in maize mesophyll cells using near-infrared laser excitation has previously shown that a photosystem I spectral component exhibits an intensity similar to that of photosystem II at ~294 K when a continuous-wave laser at 800-820 nm is used. To establish the generality of this phenomenon, chloroplasts in Parachlorella kessleri cells (P. kessleri) were studied. A continuous-wave laser at 785 nm promoted photosystem-I-specific fluorescence in P. kessleri chloroplasts. The difference in chlorophyll fluorescence peak wavelengths between P. kessleri and maize correlated well with those observed at cryogenic temperatures. To further clarify the nature of anti-Stokes fluorescence, we studied chloroplasts in acetone-treated P. kessleri cells (in a medium containing 15% (v/v) acetone) by microscopic fluorescence and absorption spectra on a cell-by-cell basis. A continuous-wave laser at 785 nm led to significant fluorescence from acetone-treated cells, which was attributed to aggregation of chlorophylls. Anti-Stokes fluorescence spectral imaging thus seems to be effective for detection of lowest-energy trap states that are only weakly fluorescent.