Effect of soil nitrogen supply on carbon assimilation by tree stems

Abstract

• Nitrogen (N) is one of the most important resources for plants, generally enhancing leaf photosynthesis because a large part of it is allocated to Rubisco and thylakoids. This is well known in leaves where photosynthesis (i.e. gas exchange, Rubisco activity, chlorophyll content) is positively correlated to leaf N content. • In order to test this hypothesis in stems, N concentration, CO2 exchange and also Rubisco and PEP carboxylase activities were measured in summer on current-year stems of young European beeches (Fagus sylvatica L.) growing on soils of different N content. • The CO2 refixation rate of stems increased from 58.5% to 74.3% when stem N concentration increased from 5.7 to 10.1 mg g−1 DW. A hyperbolic relationship was obtained between stem gross photosynthesis and N concentration, with an x-intercept of 0.3 mmol N g−1 DW. Stem PEP carboxylase activity was higher in stems than in leaves and increased with stem N concentration whereas Rubisco activity did not change between treatments in both tissues. • In spite of a low nitrogen investment in stem photosynthesis (low PNUE), these results suggest that (1) stems invest more N in CO2 refixation when more N is widely available, (2) stem photosynthesis is able to operate at low N concentration and (3) stem PEP carboxylase is involved in stem carbon refixation, but also simultaneously supplies carbon skeletons for N assimilation.