Spectroscopic studies on origination of the peak at 730 nm in delayed fluorescence of chloroplasts.

The origination of the peak at 730 nm in the delayed fluorescence (DF) spectrum of chloroplasts was studied using various optical analysis methods. The DF spectrum showed that the main emission peak was at about 685 nm, with a small shoulder at 730 nm when the chloroplast concentration was < 7.8 microg/mL. The intensity of the peak at 685 nm decreased, while the intensity of the peak at 730 nm increased, when the chloroplast concentrations were increased from 7.8 to 31.2 microg/mL. With the concentration increasing, the peak at 730 nm became dominant while the peak at 685 nm finally disappeared. The DF decay kinetic curves showed that the intensity of the peak at 730 nm decayed as the same speed as the intensity of the peak at 685 nm during the entire relaxation process (0.5-30.5 s). With the excitation wavelength at 685 nm, the emission intensity was stronger in the excitation spectrum at 730 nm. The absorption spectrum demonstrated that the ratio A(685):A(730) remained almost constant when the chloroplast concentration increased. The results suggest that the peak at 730 nm appearing in DF is mainly contributed by the fluorescence of photosystem I (PSI), generated by the re-absorption of 685 nm band DF.