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To determine the effect of growth under elevated CO 2  partial pressures ( p CO 2 ) on photosynthetic electron transport and photoprotective energy dissipation, we examined light-saturated net photosynthetic CO 2  assimilation ( A  sat ), the capacity for photosynthetic O 2  evolution, chlorophyll fluorescence emission and the pigment composition of upper-canopy loblolly pine needles in the eighth year of exposure to elevated  p CO 2  (20 Pa above ambient) at the free-air CO 2  enrichment facility in the Duke Forest. During the summer growing season,  A  sat  was 50% higher in current-year needles and 24% higher in year-old needles in elevated  p CO 2  in comparison with needles of the same age cohort in ambient  p CO 2 . Thus, photosynthetic down-regulation at elevated  p CO 2  was observed in the summer in year-old needles. In the winter,  A  sat  was not significantly affected by growth  p CO 2 . Reductions in  A  sat , the capacity for photosynthetic O 2  evolution and photosystem II (PSII) efficiency in the light-acclimated and fully-oxidized states were observed in the winter when compared to summer. Growth at elevated  p CO 2  had no significant effect on the capacity for photosynthetic O 2  evolution, PSII efficiencies in the light-acclimated and fully-oxidized states, chlorophyll content or the size and conversion state of the xanthophyll cycle, regardless of season or needle age cohort. Therefore, we observed no evidence that photosynthetic electron transport or photoprotective energy dissipation responded to compensate for the effects of elevated  p CO 2  on Calvin cycle activity.

tree physiology

Seasonal response of photosynthetic electron transport and energy dissipation in the eighth year of exposure to elevated atmospheric CO 2  (FACE) in  Pinus taeda  (loblolly pine)