Effects of long-term exposure to elevated CO2 and N fertilization on the development of photosynthetic capacity and biomass accumulation in Quercus suber L.

The effects of long-term (4 year) CO2 enrichment (70 Pa versus 35 Pa) and nitrogen nutrition (8 mm versus 1 mM NO3-) on biomass accumulation and the development of photosynthetic capacity in leaves of cork oak (Quercus suber L., a Mediterranean evergreen tree) were studied. The evolution of photosynthetic parameters with leaf development was estimated by fitting the biochemical model of Farquhar et al. (Planta 149, 78-90, 1980) with modifications by Sharkey (Botanical Review 78, 71-75, 1985) to A-C-i response curves. CO2 enrichment had a small reduction effect on the development of the maximum CO2 fixation capacity by Rubisco (V-Cmax), and no effect over maximum electron transport capacity (J(max)), day-time respiration (R-d) and Triose-P utilization (TPU). However, there was a statistically significant effect of N fertilization and the interaction CO2 x N over the evolution of V-Cmax, J(max) and TPU. Relative stomatal limitation (estimated from A-C-i curves) was higher (+20%) for plants grown under ambient CO2 than for plants grown under elevated CO2. There was a significant effect of CO2 and N fertilization over total biomass accumulation as well as leaf area. Plants grown at elevated CO2 had 27% more biomass than plants grown at ambient CO2 when given high N. However, for plants grown under low N there was no significant effect of CO2 enrichment on biomass accumulation. Plants grown under low N also had significantly higher root: shoot ratios whereas there were no differences between CO2 treatments. The larger biomass accumulation of Q. suber under elevated CO2 is attributable to a higher availability of CO2 coupled to a larger leaf area, with no significant decrease in photosynthetic capacity under CO2 enrichment and elevated N fertilization. For low N fertilization, the effects of CO2 enrichment over leaf area and biomass accumulation are lost, suggesting that in native ecosystems with low N availability, the effects Of CO2 enrichment may be insignificant.