Photosynthetic energy storage of Photosystems I and II in the spectral range of photosynthetically active radiation in intact sugar maple leaves.

The relative activity of Photosystems (PS) I and II in the spectral range between 400 and 720 nm was studied by measuring photosynthetic energy storage (ES) of an intact sugar maple leaf using photoacoustic spectroscopy. ES, determined with a modulated (80 Hz) monochromatic light beam in the presence of saturating intensity of background non-modulated white light, indicated the total energy stored by both photosystems (EST). Using background far-red light, ES of PS I (ESPS I) was quantified. ESPS II was derived from EST-ESPS I. EST dependence on intensity and wavelength of modulated light was studied at 470, 560, 640 and 680 nm. EST was maximum in red light and minimum in blue light. It decreased with an increase in modulated light intensity. The ratio ESPS II/ESPS I, measured at 640 nm, remained nearly constant with an increase in modulated light intensity. The relative quantum yield of EST spectrum showed two peaks around 610 and 660 nm, and declined sharply after 680 nm, revealing a clear red drop. ESPS I spectrum presented peaks around 610 and 670 nm, and a minimum between 440 and 470 nm. ESPS I was observed beyond 700 nm up to 720 nm, indicating the energy stored by cyclic electron transport. ESPS II spectrum showed broad peaks, around 460, 490, 600 and 660 nm, and a shoulder between 530 and 560 nm. ESPS II was always higher than ESPS I between 400 and 690 nm and reached zero around 700 nm.