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

To determine the effect of growth under elevated COâ‚‚ partial pressures (pCOâ‚‚) on photosynthetic electron transport and photoprotective energy dissipation, we examined light-saturated net photosynthetic COâ‚‚ 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 pCOâ‚‚ (20 Pa above ambient) at the free-air COâ‚‚ 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 pCOâ‚‚ in comparison with needles of the same age cohort in ambient pCOâ‚‚. Thus, photosynthetic down-regulation at elevated pCOâ‚‚ was observed in the summer in year-old needles. In the winter, Asat was not significantly affected by growth pCOâ‚‚. Reductions in Asat, the capacity for photosynthetic Oâ‚‚ 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 pCOâ‚‚ had no significant effect on the capacity for photosynthetic Oâ‚‚ 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 pCOâ‚‚ on Calvin cycle activity.