Norway spruce;Picea abies;acclimation;daily course;photosynthesis;xanthophyll cycle;photoinhibition

The diurnal dynamics of photosynthetic activity, utilization of absorbed light energy within photosystem II (PSII) and xanthophyll cycle activity was studied in 4-year old seedlings of Norway spruce (Picea abies [L.] Karst.) under laboratory conditions. Plants were initially acclimated to low irradiance together with moderate air temperature (maximum PPFD at midday approx. 300 umol m-2 s-1, maximum temperature 25 °C). Then, the irradiance was increased (maximum PPFD approx. 1000 umol m-2 s-1), while the daily course of temperature remained unchanged. Finally, only the temperature over the entire day was increased (midday maximum 35 °C). The duration of all three acclimation regimes was 13 days. Acclimation to both elevated irradiance and elevated temperature resulted in a strong reduction of the net CO2 assimilation rate, particularly during noon hours. This reduction was associated with decreased stomatal conductance.

The diurnal dynamics of photosynthetic activity, utilization of absorbed light energy within photosystem II (PSII) and xanthophyll cycle activity was studied in 4-year old seedlings of Norway spruce (Picea abies [L.] Karst.) under laboratory conditions. Plants were initially acclimated to low irradiance together with moderate air temperature (maximum PPFD at midday approx. 300 umol m-2 s-1, maximum temperature 25 °C). Then, the irradiance was increased (maximum PPFD approx. 1000 umol m-2 s-1), while the daily course of temperature remained unchanged. Finally, only the temperature over the entire day was increased (midday maximum 35 °C). The duration of all three acclimation regimes was 13 days. Acclimation to both elevated irradiance and elevated temperature resulted in a strong reduction of the net CO2 assimilation rate, particularly during noon hours. This reduction was associated with decreased stomatal conductance.