Abstract
The seasonal variation in maximum photochemical efficiency of photosystem II (Fv/Fm) and the relationship between Fv/Fm 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 Fv/Fm-ratio gradually decreased during the autumn. There was between-year variation in the pattern of Fv/Fm 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 Fv/Fm within the temperature range –3 to 12°C. The light regime to which the needles were exposed during winter affected Fv/Fm, and moderately shaded shoots from the bottom of the canopy generally had a higher Fv/Fm-ratio than fully exposed shoots from the top of the canopy.
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