Time-resolved chlorophyll fluorescence of spruce needles under various light conditions.

Different branches of a declined spruce were exposed to full sunlight, reduced sunlight (using a fine wire mesh), and natural shadow. Subnanosecond decay kinetics and time-gated fluorescence spectra of individual needles were measured and compared with their chlorophyll concentration. Sunlight-exposed needles showed lower chlorophyll concentrations and higher intensitiesI 3 of a long-lived fluorescent component (decay time about 3 ns) than shadow needles. This seems to be due to a reduced energy transfer from the chlorophyll antenna molecules to the reaction centres of Photosystem II. After light reduction to 15-20% during one summer season the chlorophyll concentration increased, whereasI 3 decreased, thus proving some recovery of the photosynthetic apparatus.