Uncoupling nitrogen requirements for spring growth from root uptake in a young evergreen shrub (Rhododendron ferrugineum)

• Internal cycling of nitrogen (N) was investigated in a subalpine field population of the evergreen shrub Rhododendron ferrugineum during spring growth. • The foliar nitrogen of 5-yr-old-plants was directly labeled with ¹⁵ N and subsequently traced to all plant compartments. In addition, ¹⁵ N-ammonium uptake was estimated in glasshouse experiments. • Before shoot growth, redistribution of ¹⁵ N occurred in the plant without net N transfer. During spring development, the decreases in both leaf ¹⁵ N and total N were almost identical in terms of percentage, and most of the ¹⁵ N withdrawn from the leaf compartments was recovered in the growing shoots. Net changes in the N contents of the various leaf and woody compartments indicate that internal remobilization (especially from 1-yr-old leaves) could have met most of the N needs of new shoot growth. Simultaneously, the rate of mineral N uptake was very low. • Thus, leaves in young plants provide N for new shoots (by contrast with old individuals) and allow, with woody tissues, almost complete uncoupling of N requirement for spring growth from root uptake.