Acclimation of Norway spruce photosynthetic apparatus to the combined effect of high irradiance and temperature.

Diurnal courses of photosynthetic gas exchange parameters, chlorophyll a fluorescence characteristics and the de-epoxidation state of the xanthophyll cycle pigments (DEPS) were measured during the gradual acclimation of 4-year-old Norway spruce seedlings to different photosynthetic photon flux density (PPFD) and air temperature (T(air)) regimes, simulating cloudy days with moderate T(air) (LI, maximum PPFD 300 micromol m(-2)s(-1), T(air) range 15-25 degrees C), sunny days with moderate T(air) (HI, maximum PPFD 1000 micromol m(-2)s(-1), T(air) range 15-25 degrees C) and hot sunny days (HI-HT, maximum PPFD 1000 micromol m(-2)s(-1), T(air) range 20-35 degrees C). The plants were acclimated inside a growth chamber and each acclimation regime lasted for 13d. Acclimation to HI conditions led to a strong depression of the net CO(2) assimilation rates (A(N)), particularly during noon and afternoon periods.