Nitrogen (N) mobilization from mature leaves plays a key role in supplying amino acids to vegetative and reproductive sinks. However, it is unknown if the mobilized N is predominantly sourced by net N-export (a senescence-related process) or other source of N-export from leaves. We used a new approach to partition gross and net N-export from leaf blades at different developmental stages in Cleistogenes squarrosa (a perennial C4 grass). Net N-export was determined as net loss of leaf N with age, while gross N-export was quantified from isotopic mass balances obtained following 24 h-long 15N-labeling with nitrate on 10-12 developmentally distinct (mature and senescing) leaves of individual major tillers. Net N-export was apparent only in older leaves (leaf no. > 7, with leaves numbered basipetally from the tip of the tiller and leaf no. 2 the youngest fully-expanded leaf), while gross N-export was largely independent of leaf age category and was ∼8.4 times greater than the net N-export of a tiller. At whole-tiller level, N import compensated 88 ± 14 (SE) % of gross N-export of all mature blades leading to a net N-export of 0.51 ± 0.07 (SE) μg h-1 tiller-1. N-import was equivalent to 0.09 ± 0.01 (SE) d-1 of total leaf N, similar to reported rates of leaf protein turnover. Gross N-export from all mature blades of a tiller was ∼1.9-times the total demand of the immature tissues of the same (vegetative) tiller. Significant N-export is evident in all mature blades, and is not limited to senescence conditions, implying a much shorter mean residence time of leaf N than that calculated from net N-export. Gross N-export contributes not only to the N demand of the immature tissues of the same tiller but also to N supply of other sinks, such as newly formed tillers. N dynamics at tiller level is integrated with that of the remainder of the shoot, thus highlights the importance of integration of leaf-, tiller-, and plant-scale N dynamics.

中文翻译：

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营养性 C4 草叶的总氮和净氮输出

来自成熟叶片的氮 (N) 动员在向营养和生殖汇提供氨基酸方面起着关键作用。然而，尚不清楚动员的氮主要来自净氮输出（与衰老相关的过程）还是来自叶子的其他氮输出来源。我们使用了一种新方法来划分 Cleistogenes squarrosa（一种多年生 C4 草）不同发育阶段叶片的总氮和净氮输出。净 N 出口被确定为随着年龄的增长叶 N 的净损失，而总 N 出口通过在 10-12 片发育不同（成熟和衰老）的叶子上用硝酸盐进行 24 小时 15N 标记后获得的同位素质量平衡进行量化。个别主要分蘖。净氮出口仅在较老的叶子中明显（叶子编号> 7，叶子从分蘖尖端开始编号，叶子编号为 2 最年轻的完全展开的叶子），而总氮出口在很大程度上独立于叶龄类别，是分蘖净氮出口的 8.4 倍。在整个分蘖水平，氮进口补偿了所有成熟叶片氮出口总量的 88 ± 14 (SE) %，导致氮净出口为 0.51 ± 0.07 (SE) μg h-1 分蘖-1。N 输入相当于总叶 N 的 0.09 ± 0.01 (SE) d-1，类似于报告的叶蛋白质周转率。一个分蘖所有成熟叶片的总氮出口量是同一（营养）分蘖未成熟组织总需求量的 1.9 倍。显着的 N 出口在所有成熟叶片中都很明显，并且不限于衰老条件，这意味着叶片 N 的平均停留时间比根据净 N 出口计算的要短得多。总氮出口不仅有助于同一分蘖未成熟组织的氮需求，而且有助于其他汇的氮供应，例如新形成的分蘖。分蘖水平的氮动态与枝条其余部分的氮动态整合，因此突出了叶、分蘖和植物尺度氮动态整合的重要性。