title: Vertical leaf nitrogen distribution in relation to nitrogen status in grassland plants.
abstract: Vertical gradients of leaf nitrogen (N) per unit leaf area (NLA) are viewed as plastic responses that optimize N utilization with respect to carbon assimilation. However, it has been shown that plant species, sowing density and N availability affect the steepness of the NLA gradient relative to the photon flux density (PFD) gradient. This paper tests the hypothesis that such variation is related to the N status of the plant. The N status was analysed using the concept of the critical N concentration (Ncrit) in which shoot N per unit dry mass (NSM) decreases with shoot mass, and a negative deviation of actual NSM from Ncrit indicates N shortage in the plant. The hypothesis was tested with contrasting grassland species Medicago sativa, Dactylis glomerata and Taraxacum officinale by varying PFD and N availability, plant density and hierarchical positions of individuals within stands. Combinations of all treatments showed a general negative correlation between the N allocation coefficient (i.e. the slope of the NLA-PFD relationship) and NSM for all three species. Thus, NLA, relative to PFD, gradients became steeper with increasing shoot mass and increasing N shortage in the plant. These data are consistent with the view that internal N availability is an important factor in modifying the NLA gradient.
authors.affiliation: Lehrstuhl für Grünlandlehre, Technische Universität München, D-85350 Freising, Germany. loetscher@wzw.tum.de
authors.forename: Markus
authors.initials: M
authors.lastname: Lötscher
authors.forename: Katharina
authors.initials: K
authors.lastname: Stroh
authors.forename: Hans
authors.initials: H
authors.lastname: Schnyder
mesh_terms: D018533:Biomass; D002244:Carbon / Q000378:metabolism; D031745:Dactylis / Q000737:chemistry / Q000502:physiology; D008027:Light; D008129:Lolium / Q000737:chemistry / Q000502:physiology; D000455:Medicago sativa / Q000737:chemistry / Q000502:physiology; D009584:Nitrogen / Q000032:analysis / Q000502:physiology*; D018515:Plant Leaves / Q000737:chemistry / Q000502:physiology*; D018520:Plant Shoots / Q000737:chemistry / Q000502:physiology; D006109:Poaceae / Q000737:chemistry / Q000502:physiology*
chemical_list: D002244:Carbon; D009584:Nitrogen