The carbon isotopic composition (δ13C) of foliage is often used as proxy for plant performance. However, the effect of N O 3 - vs. N H 4 + supply on δ13C of leaf metabolites and respired CO2 is largely unknown. We supplied tobacco plants with a gradient of N O 3 - to N H 4 + concentration ratios and determined gas exchange variables, concentrations and δ13C of tricarboxylic acid (TCA) cycle intermediates, δ13C of dark-respired CO2, and activities of key enzymes nitrate reductase, malic enzyme and phosphoenolpyruvate carboxylase. Net assimilation rate, dry biomass and concentrations of organic acids and starch decreased along the gradient. In contrast, respiration rates, concentrations of intercellular CO2, soluble sugars and amino acids increased. As N O 3 - decreased, activities of all measured enzymes decreased. δ13C of CO2 and organic acids closely co-varied and were more positive under N O 3 - supply, suggesting organic acids as potential substrates for respiration. Together with estimates of intra-molecular 13C enrichment in malate, we conclude that a change in the anaplerotic reaction of the TCA cycle possibly contributes to 13C enrichment in organic acids and respired CO2 under N O 3 - supply. Thus, the effect of N O 3 - vs. N H 4 + on δ13C is highly relevant, particularly if δ13C of leaf metabolites or respiration is used as proxy for plant performance.

中文翻译：

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硝酸盐和铵盐对植物代谢物的 δ13C 和烟叶呼吸 CO2 的影响不同

叶子的碳同位素组成 (δ13C) 通常用作植物性能的代表。然而，NO 3 - 与NH 4 + 供应对叶片代谢物的δ13C 和呼吸的CO2 的影响在很大程度上是未知的。我们为烟草植物提供了 NO 3 - 与 NH 4 + 浓度比的梯度，并确定了气体交换变量、三羧酸 (TCA) 循环中间体的浓度和 δ13C、暗呼吸 CO2 的 δ13C 以及关键酶硝酸还原酶的活性，苹果酸酶和磷酸烯醇式丙酮酸羧化酶。净同化率、干生物量以及有机酸和淀粉的浓度沿梯度下降。相反，呼吸速率、细胞间二氧化碳浓度、可溶性糖和氨基酸增加。随着NO 3 - 降低，所有测量的酶的活性都降低。CO2 和有机酸的 δ13C 密切共同变化并且在 NO 3 - 供应下更为正，表明有机酸是呼吸的潜在底物。连同对苹果酸盐中分子内 13C 富集的估计，我们得出结论，TCA 循环回补反应的变化可能有助于有机酸中 13C 富集，并在 NO 3 供应下呼吸 CO2。因此，NO 3 - 与NH 4 + 对δ13C 的影响是高度相关的，特别是如果叶代谢物或呼吸作用的δ13C 被用作植物性能的代表。我们得出结论，TCA 循环的回补反应的变化可能有助于 13C 富集有机酸和在 NO 3 供应下呼吸的 CO2。因此，NO 3 - 与NH 4 + 对δ13C 的影响是高度相关的，特别是如果叶代谢物或呼吸作用的δ13C 被用作植物性能的代表。我们得出结论，TCA 循环的回补反应的变化可能有助于 13C 富集有机酸和在 NO 3 供应下呼吸的 CO2。因此，NO 3 - 与NH 4 + 对δ13C 的影响是高度相关的，特别是如果叶代谢物或呼吸作用的δ13C 被用作植物性能的代表。