Interactive effects of nitrogen and phosphorus on the acclimation potential of foliage photosynthetic properties of cork oak, Quercus suber, to elevated atmospheric CO2 concentrations

Leaf gas-exchange and chemical composition were investigated in seedlings o f Quercus suber L. grown for 21 months either at elevated (700 mu mol mol(- 1)) or normal (350 mu mol mol-l) ambient atmospheric CO2 concentrations, [C O2], in a sandy nutrient-poor soil with either 'high' N (0.3 mol N m(-3) in the irrigation solution) or with 'low' N (0.05 mol N m(-3)) and with a con stant suboptimal concentration of the other macro- and micronutrients. Alth ough elevated [CO2] yielded the greatest total plant biomass in 'high' nitr ogen treatment, it resulted in lower leaf nutrient concentrations in all ca ses, independent of the nutrient addition regime, and in greater nonstructu ral carbohydrate concentrations. By contrast, nitrogen treatment did not af fect foliar N concentrations, but resulted in lower phosphorus concentratio ns, suggesting that under lower N, P use-efficiency in foliar biomass produ ction was lower. Phosphorus deficiency was evident in all treatments, as ph otosynthesis became CO2 insensitive at intercellular CO2 concentrations lar ger than approximate to 300 mu mol mol-l, and net assimilation rates measur ed at an ambient [CO2] of 350 mu mol mol(-1) or at 700 mu mol mol(-1) were not significantly different. Moreover, there was a positive correlation of foliar P with maximum Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygen ase) carboxylase activity (V-cmax), which potentially limits photosynthesis at low [CO2], and the capacities of photosynthetic electron transport (J(m ax)) and phosphate utilization (P-max), which are potentially limiting at h igh [CO2]. None of these potential limits was correlated with foliar nitrog en concentration, indicating that photosynthetic N use-efficiency was direc tly dependent on foliar P availability. Though the tendencies were towards lower capacities of potential limitations of photosynthesis in high [CO2] g rown specimens, the effects were statistically insignificant, because of (i ) large within-treatment variability related to foliar P, and (ii) small de creases in P/N ratio with increasing [CO2], resulting in balanced changes i n other foliar compounds potentially limiting carbon acquisition. The resul ts of the current study indicate that under P-deficiency, the down-regulati on of excess biochemical capacities proceeds in a similar manner in leaves grown under normal and elevated [CO2], and also that foliar P/N ratios for optimum photosynthesis are likely to increase with increasing growth CO2 co ncentrations.