Evergreen citrus trees exhibit distinct seasonal nitrogen remobilization patterns between mature leaves and bark.

Seasonal nitrogen (N) storage and remobilization are critical for tree growth. Deciduous trees primarily store N in bark; evergreen trees utilize both mature leaves and bark. Citrus is an evergreen species; leaf N storage and remobilization are well studied, but inner bark remains poorly understood. This study used pot experiments with N supply rates (low, moderate and high) to examine seasonal (winter, early, and late spring) N storage and remobilization between mature leaves (developed in autumn) and bark (main stem). Bark contains 15-35 kDa of vegetative storage proteins (VSPs), which are highly abundant and accumulate seasonally, while mature leaves contain 45-55 kDa of VSPs. Proteomic analysis revealed the oxygen-evolving enhancer protein as a key bark VSP, with Rubisco and others predominant in leaves. Under high N supply, the reduction ratio of total N content in bark from winter to early spring was higher than that in mature leaves. Under high N supply, bark arginine decreased significantly in early spring, whereas mature leaf arginine remained unchanged. Under low N supply, the decrease in proline content from winter to late spring was significantly greater in mature leaves than in bark. Thus, under high N, bark supply more arginine in early spring, whereas under low N, leaves supply more proline later. Bioinformatics indicate that ribosomal proteins may be involved in N remobilization in bark under high N and in both bark and leaves under low N. These results demonstrate that bark and mature leaves exhibit different seasonal N remobilization patterns.