Do Mature Leaves Provide a Net Source of Nitrogen Supporting Shoot Growth in Rhododendron ferrugineum?

• Nitrogen transfers in mature (c. 40-yr-old) Rhododendron ferrugineum plants from mature leaves to growing shoots (Sh0) were estimated in the field at the branch scale. • Before bud break, the N pool of either 1-yr-old (L1) or 2-yr-old (L2) leaves of selected branches was labelled with 15 NH4 Cl. The N dynamics were then investigated on three occasions during shoot growth. Mineral N uptake by roots was estimated by supplying seedlings with 15 NH4+ or 15 NO3- (glasshouse) and mature plants with 15 NH4+ (field). • Approximately 60% of the 15 N supplied was recovered in the labelled branches (40% in L1 leaves and 19% in Sh0) 78 d after L1 labelling. A similar pattern was observed for L2 labelling. Only traces of 15 N were detected in 1- and 2-yr-old-stems. Although changes in the isotopic contents showed the occurrence of N transfer from mature leaves to the shoot, total L1 and L2 leaf N content was not modified. Simultaneously, exogenous 15 N uptake by the roots was very low. • We conclude that, despite the internal N cycling, mature leaves do not constitute a net source of nitrogen supporting shoot growth. The latter depends mainly upon endogenous nitrogen stored in woody stems and roots.