Mature leaves are the central organs involved in carbon (C) and nitrogen (N) metabolism to support the growth and development of citrus trees. These metabolisms may be greatly altered by fruiting and N supply, which can be verified by a typical field phenomenon in which leaf chlorosis appeared on leaves of fruiting branches but not on leaves of adjacent nonfruiting branches on citrus trees. To elucidate the underlying strategy of fruiting on C and N partitioning in citrus leaves, citrus trees were cultivated with low- and high-N supplies, and then leaves and phloem sap from fruiting and nonfruiting branches were analyzed. The results showed that fruiting drives N allocation but does not improve N assimilation while maintaining the nonstructural carbohydrate content of the leaves. Combined physiological analysis and nontargeted metabolomics showed that proline is a key metabolite of mature citrus leaves and phloem sap. Due to fruiting, the proline content in mature leaves and phloem sap decreased by 71∼73% and 17∼30%, respectively. Fruiting, followed by N supply, resulted in proline remobilization from mature leaves into phloem sap and enhanced the consumption of TCA cycle intermediates for N assimilation and energy generation. Catabolism of organic N, rather than N assimilation or C metabolism, is primarily accelerated in leaves of fruiting branches, becoming a major source of organic N (mainly proline) upon phloem transport to developing fruits. Furthermore, N supply strengthened N storage in leaves of both fruiting and nonfruiting branches and then facilitated proline turnover at higher levels, indicating the importance of timing for N management in orchards.

成熟的叶子是参与碳 (C) 和氮 (N) 代谢以支持柑橘树生长发育的中心器官。这些代谢可能会因结果和 N 供应而发生很大变化，这可以通过典型的田间现象得到证实，在这种现象中，柑橘树上结果枝的叶子上出现了叶片失绿，而相邻的非结果枝的叶子上却没有。为了阐明结实对柑橘叶片中 C 和 N 分配的潜在策略，用低氮和高氮供应栽培柑橘树，然后分析结实和非结实枝条的叶子和韧皮部汁液。结果表明，在保持叶片非结构性碳水化合物含量的同时，结果驱动 N 分配，但不会改善 N 同化。结合生理学分析和非靶向代谢组学表明，脯氨酸是成熟柑橘叶片和韧皮部汁液的关键代谢物。由于结实，成熟叶片和韧皮部汁液中的脯氨酸含量分别下降了71～73%和17～30%。结果，然后是 N 供应，导致脯氨酸从成熟叶片再活化到韧皮部汁液中，并增加了用于 N 同化和能量产生的 TCA 循环中间体的消耗。有机 N 的分解代谢，而不是 N 同化或 C 代谢，主要在果枝的叶子中加速，在韧皮部运输到发育中的果实时成为有机 N（主要是脯氨酸）的主要来源。此外，N 供应加强了结果和非结果分支叶片中的 N 储存，然后促进了较高水平的脯氨酸转换，