Carbon physiology of Quercus pubescens Wild. growing at the Bossoleto CO2 spring in central Italy.

The steady increase in the atmospheric concentration of carbon dioxide has important implications for the future growth and productivity of natural and managed ecosystems and is of particular interest to determine the carbon sinks useful for maintaining carboxylation efficiency in plant response to elevated CO2 by eliminating the excess reduced carbon. The aim of this study was to evaluate the impact of exposure to naturally elevated CO2 concentrations on the leaf carbon economy of a Mediterranean oak, Quercus pubescens. Measurements of net photosynthesis, leaf conductance to water vapour, transpiration, isoprene emission, and chlorophyll a fluorescence parameters were made on one-year-old seedlings (transplanted ten months prior to the experiment) and indigenous trees growing within the vicinity of a CO2 spring and at an adjacent control site (4 km from the spring) on a clear day in August, 1993. Data on tree leaves were provided for comparison only. After measuring the leaves in situ, they were detached and allowed to dry for one hour, after which they were resampled for gas exchange and chlorophyll a fluorescence measurements. In addition, leaves from both seedlings and trees were sampled to enable specific leaf weight and tannin concentrations to be determined. Seedlings and the tree growing near the spring exhibited equal or slightly higher rates of photosynthesis, while leaf conductance was significantly lower, in comparison with plants growing at the control site. Instantaneous leaf water use efficiency was higher in plants growing near the spring than in control plants. In both seedlings and the tree growing at the spring site, rates of isoprene emission were about double those of control plants. In addition, both specific leaf weight and foliar tannin concentrations were significantly increased in the plants growing near the spring. Rapid desiccation of seedling leaves decreased all gas exchange and fluorescence parameters, and increased instantaneous water use efficiency; plants from the spring exhibited a 131% increase in water use efficiency, while plants from the control site increased water use efficiency by only 20%. This study indicates that many of the physiological responses to elevated CO2 might be similar in both seedlings and older trees. Integrated measures of carbon physiology including specific leaf weight and tannin concentration were affected by CO2 concentrations, and may have long-term significance for resistance to insect herbivory and litter decomposition rates. Higher rates of isoprene emission observed under elevated CO2 may provide a mechanism for the plants to utilize excess reducing power that they are incapable of using for growth.