RECOVERY OF PHOTOSYNTHESIS IN 1-YEAR-OLD NEEDLES OF UNFERTILIZED AND FERTILIZED NORWAY SPRUCE (PICEA-ABIES (L) KARST) DURING SPRING

Photosynthetic O-2 evolution and chlorophyll a fluorescence were measured in 1-year-old needles of unfertilized and fertilized trees of Norway spruce (Picea abies (L.) Karst.) during recovery of photosynthesis from winter inhibition in northern Sweden. Measurements were made under laboratory conditions at 20 degrees C. In general, the CO2-saturated rate of O-2 evolution was higher in needles of fertilized trees than in needles of unfertilized trees over a wide range of incident photon flux densities. Furthermore, the maximum photochemical efficiency of photosystem (PS) II, as indicated by the ratio of variable to maximum fluorescence (F-V/F-M) was higher in needles of fertilized trees than in needles of unfertilized trees. The largest differences in F-V/F-M between the two treatments occurred before the main recovery of photosynthesis from winter inhibition in late May. The rate of O-2 evolution was higher in needles of north-facing branches than in needles of south-facing branches in the middle of May. Simultaneous measurements of O-2 exchange and chlorophyll fluorescence indicated that differences in the rate of O-2 evolution between the two treatments were paralleled by differences in the rate of PS II electron transport determined by chlorophyll fluorescence. We suggest that, during recovery of photosynthesis from winter inhibition, the balance between carbon assimilation and PS II electron transport was maintained largely by adjustments in the nonphotochemical dissipation of excitation energy within PS II.