Effect of mineral nutrient content on oxygen exchange and chlo

(approximately 600--1000 μmol m s) and O2 uptake in darkness were measured in needles of control, irrigated and irrigated-fertilized trees of Norway spruce (Picea abies (L.) Karst.). Measurements were made at 20 °C and at high CO2 concentrations. The results suggest that, at given times of the year, a major part of the variation in gross photosynthesis of current-year and one-year-old needles across treatments is associated with differences in needle N content. Furthermore, the rate of O2 uptake measured after 5 or 10 min in darkness was positively correlated with both the preceding rate of gross O2 evolution and the N content in fully expanded current-year needles. Measurements of chlorophyll a fluorescence, taken simultaneously with measurements of O2 evolution in currentyear sun needles, showed that Stern-Volmer quenching of minimum fluorescence and the ratio of variable to maximum fluorescence in the darkand light-adapted state were strongly correlated with the gross rate of O2 evolution. This suggests that the increased rate of gross photosynthesis in needles of irrigated-fertilized trees was associated with adjustments in the thermal energy dissipation within photosystem II.