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 (70Pa versus 35 Pa) and nitrogen nutrition (8mM versus 1mM 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–Ci response curves. CO2 enrichment had a small reduction effect on the development of the maximum CO2 fixation capacity by Rubisco (VCmax), and no effect over maximum electron transport capacity (Jmax), day-time respiration(Rd) and Triose-P utilization (TPU). However, there was a statistically significant effect of N fertilization and the interaction CO2×N over the evolution of VCmax, Jmax and TPU. Relative stomatal limitation (estimated from A–Ci 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