From orchard to table, citrus quality hinges on the balance between sugar production in leaves and storage in fruit—a dynamic known as the source–sink relationship. Potassium, an essential plant nutrient, influences this balance by regulating photosynthesis, carbohydrate transport, and sugar metabolism. While annual crops have revealed much about potassium’s role, the intricate transport systems in perennial fruit trees remain less understood. In citrus, especially, the mechanisms connecting potassium application to sugar delivery are poorly defined. Due to these gaps, a detailed investigation is needed to uncover how potassium shapes sugar production, movement, and storage, paving the way for strategies to consistently improve citrus sweetness and market competitiveness.
Wuhan, China — September 9, 2024 — Citrus growers may have a new ally in the quest for sweeter harvests. In research published (DOI: 10.1093/hr/uhae240) in Horticulture Research, scientists from Huazhong Agricultural University reveal that potassium fertilization fine-tunes the citrus plant’s internal logistics, moving more sugars from leaf to fruit. Through a combination of field trials, pot experiments, and molecular analyses, the team discovered that potassium strengthens both the “source” (leaves) and the “sink” (fruit), with the symplastic transport pathway playing a starring role. Their findings offer a science-backed roadmap to boosting fruit flavor and yield.
In trials with five potassium levels, the researchers identified 1.5% fruit potassium concentration as the sweet spot for maximum soluble sugar content, with higher levels reducing sweetness. Potassium application increased fruit weight, yield, and concentrations of sucrose, fructose, and glucose at multiple developmental stages. In leaves, potassium boosted photosynthesis and sucrose synthesis by enhancing the activities of sucrose phosphate synthase (SPS) and sucrose synthase (SS-S), while reducing acid invertase activity to limit sugar breakdown. In fruit, it upregulated sugar-metabolizing genes (CsSPS1, CsSPS4, CsSUS4) and transporters (CsSWEET15, CsTMT2), improving sugar unloading and storage.
A ^13C-isotope labeling experiment showed greater ^13C-sucrose and ^13C-glucose in fruit under potassium treatment, confirming increased carbon flow from leaves. Microscopy revealed denser plasmodesmata and stronger fluorescent signals in treated plants, indicating enhanced symplastic sugar transport. Potassium also expanded the cross-sectional area of leaf vascular bundles, further improving transport capacity. Together, these results show that potassium orchestrates a more powerful and efficient sugar delivery network, driving sweeter, higher-quality oranges.
“Potassium is not just plant nutrition—it’s an active manager of the citrus supply chain,” said Professor Songwei Wu, senior author of the study. “By strengthening the sugar-producing power of leaves and the storage capacity of fruit, potassium application reshapes how carbon moves through the plant. Our work pinpoints the precise conditions—especially the 1.5% fruit potassium threshold—that growers can target to achieve the best flavor and yield. This is about using science to fine-tune orchard management for both quality and profitability.”
For citrus growers, the study delivers clear, actionable guidance: aim for around 1.5% potassium in fruit pulp and time fertilization to coincide with the color-turning stage, when sugar transport peaks. By aligning nutrient supply with the fruit’s developmental needs, growers can maximize sweetness without compromising other quality traits. Beyond citrus, the findings spotlight a general principle—managing potassium to enhance source–sink efficiency—that could benefit other perennial fruit crops. The approach supports higher market value, stronger consumer appeal, and more sustainable orchard practices, turning a common fertilizer into a precision tool for flavor engineering.
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References
DOI
10.1093/hr/uhae240
Original Source URL
https://doi.org/10.1093/hr/uhae240
Funding information
This research was supported by the Fundamental Research Funds for the Central Universities (2662022ZHQD002), the National Natural Science Foundation of China (No. 32001986), the National Key Research and Development Program of China (2019YFD1000103), and the Modern Citrus Industry Technology System of China (CARS-26).
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.