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 three 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 (from the main stem). Bark contains 15-35 kDa vegetative storage proteins (VSPs), which are highly abundant and accumulate seasonally, while mature leaves contain 45-55 kDa 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, bark total N decreased by 1.30 times from winter to early spring, while leaf N decreased only 1.01 times. Under high N supply, bark arginine decreased significantly by 2.66 times in early spring, whereas mature leaf arginine remained unchanged. Under low N supply, mature leaf proline decreased by 17.52 times in late spring, while proline in bark decreased by 5.50 times. Thus, under high N, bark is the primary early spring arginine source, 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.