PHOTOSYNTHETIC ACCLIMATION TO ELEVATED CO2 IN 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 exc hange behaviour and leaf physiology of two contrasting poplar (Populus ) hybrids grown and treated in open top chambers (OTCs in Antwerp, Bel gium) and in closed glasshouse cabinets (GHCs in Sussex, UK). The CO2 concentrations used in the OTCs were ambient and ambient +350 mu mol m ol(-1), while in the GHCs they were c, 360 mu mol mol(-1) versus 719 m u mol mol(-1). Measurements of photosynthetic gas exchange were made f or euramerican and interamerican poplar hybrids in combination with me asurements 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 OTC s and those grown in the GHCs. Elevated CO2 stimulated net photosynthe sis 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 do wnregulation) of photosynthesis when the plants in the two treatments were measured in their growth CO2 concentration. There was also no evi dence of downregulation of Rubisco activity and there were even exampl es of increases in Rubisco activity. Rubisco exerted a strong control over the light-saturated rate of photosynthesis, which was demonstrate d by the close agreement between observed net photosynthetic rates and those that were predicted from Rubisco activities and Michaelis-Mente n kinetics. After 17 months in elevated CO2 in the OTCs there was a si gnificant loss of Rubisco activity for one of the hybrid clones, i.e. Beaupre, but not for clone Robusta, The effect of the CO2 measurement concentration (i.e. the short-term treatment effect) on net photosynth esis was always larger than the effect of the growth concentration in both the OTCs or GHCs (i.e. the long-term growth CO2 effect), with one exception, For the interamerican hybrid Beaupre dark respiration rate s in the OTCs were not significantly affected by the elevated CO2 conc entrations. 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 mo nths; longer term exposure reveals loss of Rubisco activity.