Effects of lifelong [CO2] enrichment on carboxylation and light utilization of Quercus pubescens Willd. examined with gas exchange, biochemistry and optical techniques

Lifelong exposure to elevated concentrations of atmospheric CO2 may enhance carbon assimilation of trees with unlimited rooting volume and consequently may reduce requirements for photoprotective pigments. In early summer the effects of elevated [CO2] on carboxylation and light utilization of mature Quercus pubescens trees growing under chronic [CO2] enrichment at two CO2 springs and control sites in Italy were examined. Net photosynthesis was enhanced by 36 to 77%. There was no evidence of photosynthetic downregulation early in the growing season when sink demand presumably was greatest. Specifically, maximum assimilation at saturating [CO2], electron transport capacity, and Rubisco content, activity and carboxylation capacity were not significantly different in trees growing at the CO2 springs and their respective control sites. Foliar biochemical content, leaf reflectance index of chlorophyll pigments (NDVI), and photochemical efficiency of PSII (DeltaF/F-m') also were not significantly affected by [CO2] enrichment except that starch content and DeltaF/F-m' tended to be higher at one spring (42 and 15%, respectively). Contrary to expectation, prolonged elevation of [CO2] did not reduce xanthophyll cycle pigment pools or alter mid-day values of leaf reflectance index of xanthophyll cycle pigments (PRI), despite the enhancement of carbon assimilation. However, both these pigments and PRI were well correlated with electron transport capacity.