Photosynthetic acclimation to elevated CO2in poplar grown in glasshouse cabinets or in open top chambers depends on duration of exposure

The effects of elevated CO2 were studied on the photosynthetic gas exchange behaviour and leaf physiology of two contrasting poplar (Populus) hybrids grown and treated in open top chambers (OTCs in Antwerp, Belgium) and in closed glasshouse cabinets (GHCs in Sussex, UK). The CO2 concentrations used in the OTCs were ambient and ambient +350 μmol mol−1 while in the GHCs they were c. 360 μmol mol−1 versus 719 μmol mol−1. Measurements of photosynthetic gas exchange were made for euramerican and interamerican poplar hybrids in combination with measurements of dark respiration rate and Rubisco activity. Significant differences in the leaf anatomy and structure (leaf mass per area and chlorophyll content) were observed between the leaves grown in the OTCs and those grown in the GHCs. Elevated CO2 stimulated net photosynthesis in the poplar hybrids after 1 month in the GHCs and after 4 months in the OTCs, and there was no evidence of downward acclimation (or down-regulation) of photosynthesis when the plants in the two treatments were measured in their growth CO2 concentration. There was also no evidence of down-regulation of Rubisco activity and there were even examples of increases in Rubisco activity. Rubisco exerted a strong control over the light-saturated rate of photosynthesis, which was demonstrated by the close agreement between observed net photosynthetic rates and those that were predicted from Rubisco activities and Michaelis-Menten kinetics. After 17 months in elevated CO2 in the OTCs there was a significant loss of Rubisco activity for one of the hybrid clones, i.e. Beaupré, but not for clone Robusta. The effect of the CO2 measurement concentration (i.e. the short-term treatment effect) on net photosynthesis was always larger than the effect of the growth concentration in both the OTCs or GHCs (i.e. the longterm growth CO2 effect), with one exception. For the interamerican hybrid Beaupré dark respiration rates in the OTCs were not significantly affected by the elevated CO2 concentrations. The results suggest that for rapidly growing tree species, such as poplars, there is little evidence for downward acclimation of photosynthesis when plants are exposed to elevated CO2 for up to 4 months; longer term exposure reveals loss of Rubisco activity.