After root uptake, nitrate is effluxed back to the medium, assimilated locally, or translocated to shoots. Rooted black cottonwood (Populus trichocarpa) scions were supplied with a NO₃⁻‐based (0.5 mM) nutrient medium of known isotopic composition (δ¹⁵N), and xylem sap was collected by pressure bombing. To establish a sampling protocol, sap was collected from lower and upper stem sections at 0.1–0.2 MPa above the balancing pressure, and after increasing the pressure by a further 0.5 MPa. Xylem sap from upper stem sections was partially diluted at higher pressure. Further analysis was restricted to sap obtained from intact shoots at low pressure. Total‐, NO₃⁻‐N and, by difference, organic‐N concentrations ranged from 6.1–11.0, 1.2–2.4, and 4.6–9.4 mM, while discrimination relative to the nutrient medium was −6.3 to 0.5‰, −23.3 to −11.5‰ and − 1.3 to 4.9‰, respectively. There was diurnal variation in δ¹⁵N of total‐ and organic‐N, but not NO₃⁻. The difference in δ¹⁵N between xylem NO₃⁻ and organic‐N suggests that discrimination by nitrate reductase is near 25.1 ± 1.6‰. When this value was used in an isotope mass balance model, the predicted xylem sap NO₃⁻‐N to total‐N ratio closely matched direct measurement.

中文翻译：

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接近稳态水培条件下木质部汁液中同位素组成及总氮和硝酸盐浓度

根吸收后，硝酸盐外排回培养基，被本地吸收，或转移到芽上。将生根的黑三角叶（毛果杨）接穗与NO分别供给₃ ^-为基础的（0.5毫摩尔）已知同位素组成的营养培养基（δ ¹⁵ N），和木质部汁液通过压力轰炸收集。为了建立采样方案，在高于平衡压力0.1–0.2 MPa的压力下，从上部和下部茎部收集汁液，然后再增加0.5 MPa的压力。来自上部茎部分的木质部汁液在较高压力下被部分稀释。进一步的分析仅限于从完整芽在低压下获得的汁液。Total-，NO ₃ ^-‐N和有机N的差异范围为6.1–11.0、1.2–2.4和4.6–9.4 mM，而相对于营养培养基的辨别力为-6.3至0.5‰，-23.3至-11.5‰和-1.3分别为4.9‰。有在δ日变化¹⁵的total-和有机-N N，而不是NO ₃ ^- 。在δ之差¹⁵木质部NO之间的N个₃ ^-和有机-N表明由硝酸还原酶歧视是接近25.1±1.6‰。在同位素质量平衡模型中使用此值时，预测的木质部汁液NO ₃ ^-- N与总N的比率与直接测量值非常匹配。