Bark, not leaves, fuels early spring growth in evergreen citrus trees
image: Experimental design of nitrogen (N) fertilizer application of potted citrus seedlings. From July 2020 to March 2021, potted seedlings were acclimated to moderate N, followed by the application of low, moderate, and high N treatments. The experiment spanned three periods: winter, early spring, and late spring. After collecting bark, mature leaves, senescing leaves, branches, and roots across seasons, gel electrophoresis was performed. Based on the results, bark and mature leaves were selected for total N, soluble protein N, and FAA N analysis. Proteome profiling was conducted on bark and mature leaves from winter and early spring under low and high N treatments, followed by integrated analysis of proteome and N partitioning data.
Credit: Horticulture Research
Efficient nitrogen reuse is essential for sustaining growth in evergreen fruit trees, yet how different tissues contribute to seasonal nitrogen supply remains unclear. This study reveals that evergreen citrus trees rely on distinct nitrogen remobilization strategies between bark and mature leaves during spring growth. By tracking nitrogen pools, storage proteins, and free amino acids across winter, early spring, and late spring under varying nitrogen supply, the research shows that bark mobilizes nitrogen earlier and more intensively than leaves. The findings uncover tissue-specific storage proteins and amino acid preferences that shift with nitrogen availability, highlighting bark as a previously underappreciated nitrogen reservoir that plays a dominant role in supporting early spring shoot development.
In perennial trees, nitrogen stored during dormant periods is critical for supporting new growth before root uptake resumes. While deciduous trees mainly store nitrogen in bark, evergreen species are thought to rely primarily on mature leaves. In citrus trees, leaf nitrogen storage has been extensively studied, but the role of inner bark remains poorly understood. This knowledge gap is particularly important because citrus orchards often receive excessive nitrogen fertilizer, leading to low nitrogen-use efficiency and environmental losses. Understanding how different tissues store and remobilize nitrogen under varying nutrient conditions is essential for improving fertilization strategies and sustainability. Based on these challenges, it is necessary to conduct in-depth research on tissue-specific nitrogen remobilization mechanisms in evergreen citrus trees.
Researchers from Southwest University (China), in collaboration with the University of Freiburg (Germany), reported these findings (DOI: 10.1093/hr/uhaf103) in Horticulture Research,, published on 21 April 2025. Using controlled pot experiments with low, moderate, and high nitrogen supply, the team examined seasonal nitrogen dynamics in mature leaves and bark of evergreen citrus trees from winter through spring. By integrating physiological measurements with proteomic and amino acid analyses, the study provides a comprehensive view of how different tissues contribute to nitrogen recycling during critical growth transitions.
The study demonstrates that bark and mature leaves exhibit fundamentally different nitrogen remobilization patterns. During early spring, bark showed a larger reduction in total nitrogen and soluble protein nitrogen than leaves, particularly under high nitrogen supply, indicating that bark serves as the primary nitrogen source at this stage. In contrast, mature leaves retained much of their nitrogen until later in the season.
Protein profiling revealed clear tissue specificity. Bark accumulated storage proteins mainly in the 15–35 kDa range, with the oxygen-evolving enhancer protein emerging as a dominant bark storage protein that increased in winter and declined rapidly in spring. Mature leaves, however, stored nitrogen predominantly in 45–55 kDa proteins, including Rubisco and other metabolically active proteins.
Amino acid analysis further highlighted contrasting strategies. Under high nitrogen supply, bark preferentially mobilized arginine during early spring, providing a nitrogen-rich and efficient source for rapid growth. Under low nitrogen supply, mature leaves delayed nitrogen export and later supplied large amounts of proline in late spring, likely balancing stress protection and nutrient recycling. Proteomic network analyses identified ribosomal proteins as key regulators of nitrogen remobilization, with their involvement shifting depending on nitrogen availability and tissue type.
“This work challenges the traditional view that mature leaves dominate nitrogen recycling in evergreen trees,” said one of the study’s corresponding authors. “Our results show that bark is not just a passive structure but an active and efficient nitrogen reservoir, especially during early spring when root uptake is limited. The tissue-specific use of storage proteins and amino acids reflects a finely tuned strategy that allows citrus trees to balance growth, metabolism, and environmental stress under different nitrogen conditions.”
These findings have direct implications for orchard management and sustainable fertilization. By recognizing bark as a major nitrogen source in early spring, growers may reduce early-season nitrogen inputs without compromising growth, especially when low temperatures restrict root uptake. Later in the season, targeted nitrogen supplementation could better align with leaf-driven nitrogen supply. Beyond citrus production, the study provides a conceptual framework for understanding nitrogen recycling in other evergreen woody crops. Improving nitrogen-use efficiency through tissue-informed management could lower fertilizer use, reduce environmental pollution, and support more sustainable perennial agriculture systems.
###
References
DOI
Original Source URL
https://doi.org/10.1093/hr/uhaf103
Funding information
This work was supported by the National Natural Science Foundation of China (32172676) and special fund for youth team of the Southwest University(SWU-XJPY202308).
About Horticulture Research
Horticulture Research is an open access journal of Nanjing Agricultural University and ranked number one in the Horticulture category of the Journal Citation Reports ™ from Clarivate, 2023. The journal is committed to publishing original research articles, reviews, perspectives, comments, correspondence articles and letters to the editor related to all major horticultural plants and disciplines, including biotechnology, breeding, cellular and molecular biology, evolution, genetics, inter-species interactions, physiology, and the origination and domestication of crops.