Excitation spectra of chlorophyll fluorescence in spinach and barley chloroplasts at 4 K

Abstract

Excitation spectra of chlorophyll a fluorescence in chloroplasts from spinach and barley were measured at 4.2 K. The spectra showed about the same resolution as the corresponding absorption spectra. Excitation spectra for long-wave chlorophyll a emission (738 or 733 nm) indicate that the main absorption maximum of the photosystem (PS) I complex is at 680 nm, with minor bands at longer wavelengths. From the corresponding excitation spectra it was concluded that the emission bands at 686 and 695 nm both originate from the PS II complex. The main absorption bands of this complex were at 676 and 684 nm. The PS I and PS II excitation spectra both showed a contribution by the light-harvesting chlorophyll a b protein(s), but direct energy transfer from PS II to PS I was not observed at 4 K. Omission of Mg 2+ from the suspension favored energy transfer from the light-harvesting protein to PS I. Excitation spectra of a chlorophyll b-less mutant of barley showed an average efficiency of 50–60% for energy transfer from β-carotene to chlorophyll a in the PS I and in the PS II complexes.