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 F v /F m -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°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.
Summary. Dark respiration and photosynthetic carbon dioxide refixation in purple and green Picea abies cones were investigated from budbreak to cone maturity. The rate of dark respiration per unit dry weight and CO2 refixation capacity decreased during cone maturation. At the beginning of the growing season, photosynthetic CO 2 refixation could reduce the amount of CO 2 released by respiration in green and purple cones by 50% and 40%, respectively. The seasonal performance of the components of the cone carbon balance was calculated using information on the seasonal course of respiration, refixation capacity and the light response curves of cone photosynthesis, as well as the actual light and temperature regime in the field. The daily gain of CO 2 refixation reached 28%-34% of respiration in green and 22%-26% in purple cones during the first month of their growth, but decreased later in the season. Over the entire growth period refixation reduced carbon costs of cone production in both cone colour polymorphs by 16%-17%.
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