The seasonal variation in maximum photochemical efficiency of photosystem II (F-v/F-m) and the relationship between F-v/F-m and climatic factors such as irradiance, frost-nights and daily mean temperature was studied in young Norway spruce trees for 4 years in northern Sweden. As a result of night frost, the (FvFm)-F-/-ratio gradually decreased during the autumn. There was between-year variation in the pattern of F-v/F-m in fully exposed shoots during autumn and spring, largely as an effect of differing temperature conditions. During spring, there was a strong apparent relationship between daily mean temperature and F-v/F-m within the temperature range -3 to 12 degrees C. The light regime to which the needles were exposed during winter affected F-v/F-m, and moderately shaded shoots from the bottom of the canopy generally had a higher F-v/F-m-ratio than fully exposed shoots from the top of the canopy.
