Lötscher Markus, Stroh Katharina, Schnyder Hans
Lehrstuhl für Grünlandlehre, Technische Universität München, D-85350 Freising, Germany.
Ann Bot. 2003 Nov;92(5):679-88. doi: 10.1093/aob/mcg188. Epub 2003 Sep 10.
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.
单位叶面积的叶片氮含量（NLA）的垂直梯度被视为一种可塑性响应，它能在碳同化方面优化氮的利用。然而，已有研究表明，植物物种、播种密度和氮有效性会影响NLA梯度相对于光量子通量密度（PFD）梯度的陡峭程度。本文检验了这样一种假设，即这种变化与植物的氮状况有关。利用临界氮浓度（Ncrit）的概念分析氮状况，其中单位干质量的地上部氮含量（NSM）随地上部质量的增加而降低，实际NSM相对于Ncrit的负偏差表明植物存在氮短缺。通过改变PFD和氮有效性、植物密度以及林分内个体的层次位置，用对比性的草地物种紫花苜蓿、鸭茅和蒲公英对该假设进行了检验。所有处理的组合表明，对于所有这三个物种，氮分配系数（即NLA-PFD关系的斜率）与NSM之间普遍呈负相关。因此，相对于PFD而言，随着地上部质量的增加以及植物氮短缺的加剧，NLA梯度变得更陡。这些数据与以下观点一致，即内部氮有效性是改变NLA梯度的一个重要因素。
