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 (ΔF/F_m′) also were not significantly affected by [CO₂] enrichment except that starch content and ΔF/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.