Photosynthetic response to elevated CO2 in poplar (POP-EUROFACE) in relation to leaf nitrogen partitioning.

We examined the effect of elevated CO2 on photosynthesis and the allocation of leaf nitrogen to photosynthetic and non-photosynthetic pools in two poplar species (Populus alba, genotype 2AS11 and P. nigra, genotype can Pourtet) at the POP-EUROFACE site. In P. alba. the light-saturated photosynthetic rate at growth CO2 concentration (A(sat)) over two years was significantly higher (by 42%) in trees exposed to CO2 enrichment. In P. nigra, there was no significant difference, though on average A(sat) was 29% higher in the CO2 enrichment treatment. Stomatal conductance was significantly reduced by CO2 enrichment in both species: by 22% in P alba and by 18% in P nigra. Neither maximum carboxylation rate (V-max) nor maximum rate of electron transport (J(maxi) was reduced by CO2 enrichment. A change in the partitioning of leaf nitrogen between photosynthetic and non-photosynthetic pools was detected in P. nigra: partitioning into non-photosynthetic nitrogen increased by 15% in elevated CO2 conditions. No differences were detected between the sun and shade leaves of either studied species in terms of their photosynthetic responses to elevated CO,. The greater allocation of leaf nitrogen into the non-photosynthetic pool in elevated CO2 conditions displayed by P. nigra in comparison with P. alba may explain the non-significant photosynthetic stimulation in the former.