title: Nitrogen sources for current-year shoot growth in 50-year-old sessile oak trees: an in situ (15)N labeling approach.
abstract: We used long-term in situ (15)N labeling of the soil to investigate the contribution of the two main nitrogen (N) sources (N uptake versus N reserves) to sun shoot growth from bud burst to full leaf expansion in 50-year-old sessile oaks. Recovery of (15)N by growing compartments (leaves, twigs and buds) and presence of (15)N in phloem sap were checked weekly. During the first 2 weeks following bud burst, remobilized N contributed ~90% of total N in growing leaves and twigs. Nitrogen uptake from the soil started concomitantly with N remobilization but contributed only slightly to bud burst. However, the fraction of total N due to N uptake increased markedly once bud burst had occurred, reaching 27% in fully expanded leaves and 18% in developed twigs. In phloem sap, the (15)N label appeared a few days after the beginning of labeling and increased until the end of bud burst, and then decreased at full leaf expansion in June. Of all the shoot compartments, leaves attracted most of the absorbed N, which accounted for 68% of new N in shoots, whereas twigs and new buds accounted for only 28 and 3%, respectively. New N allocated to leaves increased from unfolding to full expansion as total N concentration in the leaves decreased. Our results underline the crucial role played by stored N in rapid leaf growth and in the sustained growth of oak trees. Any factors that reduce N storage in autumn may therefore impair spring shoot growth.
authors.affiliation: Université Henri Poincaré, UMR 1137 Ecologie et Ecophysiologie Forestières IFR 110, F-54500 Vandoeuvre-les-Nancy, France.
authors.forename: Rana
authors.initials: R
authors.lastname: El Zein
authors.forename: Nathalie
authors.initials: N
authors.lastname: Bréda
authors.forename: Dominique
authors.initials: D
authors.lastname: Gérant
authors.forename: Bernd
authors.initials: B
authors.lastname: Zeller
authors.forename: P
authors.initials: P
authors.lastname: Maillard
mesh_terms: D000704:Analysis of Variance; D035264:Flowers / Q000254:growth & development / Q000378:metabolism; D005602:France; D007553:Isotope Labeling / Q000379:methods*; D016014:Linear Models; D009566:Nitrates / Q000378:metabolism; D009584:Nitrogen / Q000378:metabolism*; D009587:Nitrogen Isotopes; D052585:Phloem / Q000378:metabolism; D053147:Plant Exudates / Q000378:metabolism; D018520:Plant Shoots / Q000254:growth & development* / Q000378:metabolism*; D000644:Quaternary Ammonium Compounds / Q000378:metabolism; D029963:Quercus / Q000254:growth & development* / Q000378:metabolism*; D012987:Soil / Q000737:chemistry; D014867:Water / Q000378:metabolism
chemical_list: D009566:Nitrates; D009587:Nitrogen Isotopes; D053147:Plant Exudates; D000644:Quaternary Ammonium Compounds; D012987:Soil; D014867:Water; D009584:Nitrogen