Original article

Effect of soil nitrogen supply on carbon assimilation by tree stems

¹ CNRS, Laboratoire Ecologie Systématique et Evolution, UMR8079, 91405 Orsay, France
² Univ Paris-Sud, 91405 Orsay, France
³ AgroParisTech, 75231 Paris, France

^* Corresponding author: daniel.berveiller@u-psud.fr

Received: 1 October 2009
Accepted: 19 January 2010

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 thylakoïds. 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.