Seasonal response of photosynthetic electron transport and energy dissipation in the eighth year of exposure to elevated atmospheric CO₂(FACE) in Pinus taeda (loblolly pine)

To determine the effect of growth under elevated CO₂ partial pressures (pCO₂) on photosynthetic electron transport and photoprotective energy dissipation, we examined light-saturated net photosynthetic CO₂ assimilation (Asat), the capacity for photosynthetic O2 evolution, chlorophyll fluorescence emission and the pigment composition of upper-canopy loblolly pine needles in the eighth year of exposure to elevated pCO₂ (20 Pa above ambient) at the free-air CO₂ enrichment facility in the Duke Forest. During the summer growing season, Asat was 50% higher in current-year needles and 24% higher in year-old needles in elevated pCO₂ in comparison with needles of the same age cohort in ambient pCO₂. Thus, photosynthetic down-regulation at elevated pCO₂ was observed in the summer in year-old needles. In the winter, Asat was not significantly affected by growth pCO₂. Reductions in Asat, the capacity for photosynthetic O₂ 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 pCO₂ had no significant effect on the capacity for photosynthetic O₂ 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 pCO₂ on Calvin cycle activity.