Inhibition of photosynthesis in current-year needles of unfertilized and fertilized Norway spruce [Picea abies (L.) Karst.] during autumn and early winter

Inhibition of photosynthesis was followed during autumn and early winter in current-year sun and shade needles of unfertilized and fertilized Norway spruce Picea abies (L.) Karst.] by simultaneous measurements of photosynthetic O₂ evolution and chlorophyll a fluorescence at 20 °C. The CO₂-saturated rate of O₂ evolution was generally higher in sun needles of fertilized trees than in those of unfertilized trees over a wide range of incident photon flux densities (PFDs). Furthermore, the maximum photo-chemical efficiency of photosystem (PS) II, as indicated by the ratio of variable to maximum fluorescence (F_V/F_M) was generally higher for sun needles of fertilized trees. The depression of f_v/f_m during frost periods was more pronounced in sun needles than in shade needles, indicating that winter inhibition in Norway spruce is strongly light-dependent. However, the inhibition of the rate of O₂ evolution at high PFDs in needles of fertilized trees during early winter was partly independent of the light regime experienced by those needles in the field, which appeared to result in a pronounced decrease in the proportion of oxidized PS II reaction centres in shade needles. A nearly identical linear relationship between the quantum yield of PS II electron transport determined by chlorophyll fluorescence and the quantum yield of O₂ evolution (gross rate of O₂ evolution/PFD) was obtained for the investigated types of needles during autumn and early winter. Except for shade needles of fertilized trees, this appeared to be largely achieved by adjustments in thermal energy dissipation within PS II.