Time-resolved chlorophyll fluorescence of spruce needles after different light exposure

The impact of environmental light on the function of the photosystems was measured for a healthy and a declining spruce (Picea abies (L.) Karst) growing at an altitude of 840 m near Freudenstadt (Black Forest). Needles were collected from branches which had been exposed either to full sun light, or to reduced sun light (artificial shading by a wire mesh of 15-20% transmission), or which had been growing under natural shadow (north-eastern side of the tree). More than two annual time courses of chlorophyll concentration, subnanosecond fluorescence decay kinetics and delayed luminescence were measured since April 1993. A long-lived component of (prompt) chlorophyll fluorescence (lifetime about 3 ns as compared with 100-600 ps for intact photosystems) indicated an obstruction of energy transfer from the antenna molecules ro the reaction centres, whereas the integrals of delayed luminescence - measured within 0.5 s or 30 s after repetitive light pulses in the kHz range - appeared to be a measure of photosynthetic activity. All parameters showed a pronounced impact of high environmental light doses on the function of the photosystems - in particular in June-August - and a clear recovery after artificial shading. By measuring time-gated fluorescence spectra in the nanosecond range, the defect could be localized in Photosystem II of the needles from the damaged spruce.