Merilo, E., Eensalu, E., Tulva, I., Räim, O., Calfapietra, C. and Kull, O. 2010. Photosynthetic Response to Elevated CO₂ in Poplar (POP-EUROFACE) in Relation to Leaf Nitrogen Partitioning.  Baltic Forestry 16(2): 162 – 171

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 Jean 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 Asat 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_cmax) nor maximum rate of electron transport (J_max) 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 CO₂. 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.

Key words: net CO₂ assimilation, nitrogen partitioning, non-photosynthetic nitrogen, photosynthetic down-regulation, Populus, stomatal conductance