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 of Qucrcus suber L. grown for 21 months either at elevated (700 umol mol-1) or normal (350 umol mol-1) ambient atmospheric CO2 concentrations, [CO2], in a sandy nutrientpoor 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 constant suboptimal concentration of the other macro- and micronutrients. Although elevated [CO2] yielded the greatest total plant biomass in 'high' nitrogen treatment, it resulted in lower leaf nutrient concentrations in all cases, independent of the nutrient addition regime, and in greater nonstructural carbohydrate concentrations. By contrast, nitrogen treatment did not affect foliar N concentrations, but resulted in lower phosphorus concentrations, suggesting that under lower N, P use-efficiency in foliar biomass production was lower. Phosphorus deficiency was evident in all treatments, as photosynthesis became CO2 insensitive at intercellular CO2 concentrations larger than 300 umol mol-1, and net assimilation rates measured at an ambient [CO2] of 350 umol mol-1 or at 700 umol mol-1 were not significantly different. Moreover, there was a positive correlation of foliar P with maximum Rubisco (Ribulose-l,5-bisphosphate carboxylase/oxygenase) carboxylase activity (Vcmax), which potentially limits photosynthesis at low [CO2], and the capacities of photosynthetic electron transport (Vmax) and phosphate utilization (Pmax)/ which are potentially limiting at high [CO2]. None of these potential limits was correlated with foliar nitrogen concentration, indicating that photosynthetic N useefficiency was directly dependent on foliar P availability. Though the tendencies were towards lower capacities of potential limitations of photosynthesis in high [CO2] grown specimens, the effects were statistically insignificant, because of (i) large within-treatment variability related to foliar P, and (ii) small decreases in P/N ratio with increasing [CCy, resulting in balanced changes in other foliar compounds potentially limiting carbon acquisition. The results of the current study indicate that under P-deficiency, the down-regulation 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 concentrations. © 1999 Blackwell Science Ltd.
Niinemets, U; Tenhunen, JD; Canta, NR; Craves, MM; Faria, T; Pereira, JS; Reynolds, F
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