Abstract
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 μmol m −2 s −1, T air range 15–25 °C), sunny days with moderate T air (HI, maximum PPFD 1000 μmol m −2 s −1, T air range 15–25 °C) and hot sunny days (HI-HT, maximum PPFD 1000 μmol m −2 s −1, T air range 20–35 °C). The plants were acclimated inside a growth chamber and each acclimation regime lasted for 13 d. Acclimation to HI conditions led to a strong depression of the net CO 2 assimilation rates ( A N), particularly during noon and afternoon periods. Exposure to the HI-HT regime led to a further decrease of A N even during the morning period. Insufficient stomatal conductance was found to be the main reason for depressed A N under HI and HI-HT conditions. Only slight changes of the maximum photosystem II (PSII) photochemical efficiency ( F V/ F M), in the range of 0.78–0.82, supported the resistance of the Norway spruce photosynthetic apparatus against PSII photoinhibition during acclimation to both HI and HI-HT conditions. The HI plants showed increased content of xanthophyll cycle pigments (VAZ) and enhanced efficiency of thermal energy dissipation within PSII ( D) that closely correlated with the increased DEPS. In contrast, acclimation to the HI-HT regime resulted in a slight reduction of VAZ content and significantly diminished D and DEPS values during the entire day in comparison with HI plants. These results indicate a minor role of the xanthophyll cycle-mediated thermal dissipation in PSII photoprotection under elevated temperatures. The different contributions of the thermal dissipation and non-assimilatory electron transport pathways in PSII photoprotection during acclimation of the Norway spruce photosynthetic apparatus to excess irradiance and heat stresses are discussed.
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