Potassium plays a vital role in plant health, affecting photosynthesis, sugar movement, and crop quality. In citrus, sugars such as sucrose, fructose, and glucose not only determine taste but also influence market success. While it's known that potassium boosts sugar levels in fruit, the underlying mechanisms—especially in perennial trees like citrus—are not fully understood. The balance between too much or too little potassium can either elevate or degrade flavor. Meanwhile, the route sugars take from leaves to fruit—whether through cell-to-cell pathways or extracellular channels—remains a key question. Due to these challenges, deeper exploration into how potassium modulates sugar transport and accumulation is essential for advancing both science and farming practices.
A research team from Huazhong Agricultural University has unveiled how potassium fertilization enhances sugar accumulation in Newhall navel oranges by stimulating internal sugar transport. Published (DOI: 10.1093/hr/uhae240) on September 9, 2024, in Horticulture Research, the study combined field and greenhouse experiments with advanced imaging and isotope tracing techniques. The findings reveal that potassium strengthens both the sugar source (leaves) and the sugar sink (fruits), primarily by activating the symplastic phloem loading pathway—a highly efficient sugar delivery route. This breakthrough offers a molecular explanation for potassium's impact on fruit sweetness and provides a practical roadmap for growers seeking quality-driven fertilization strategies.
To uncover potassium's role in citrus sweetness, researchers conducted a series of controlled experiments using varying levels of potassium in 10-year-old Newhall navel orange trees. They observed that optimal potassium application significantly increased fruit weight, yield, and total sugar content—particularly sucrose. The sweet spot, they found, was a 1.5% potassium concentration in fruit pulp; beyond this level, sugar accumulation declined.
Further investigations revealed that potassium boosted the activity of sugar-metabolizing enzymes like sucrose phosphate synthase (SPS) and sucrose synthase (SS), and elevated the expression of key sugar transport genes such as CsSUT1 and CsSWEET15. Using 13C-labeled sucrose, the team tracked enhanced carbon flow from leaves to fruits in potassium-treated trees. Under the microscope, they saw intensified carboxyfluorescein signals and increased plasmodesmata density in leaf veins—evidence that potassium facilitated symplastic sugar transport.
Importantly, this symplastic route allows sucrose to move directly from cell to cell, bypassing energy-intensive steps. The most active period for this transport was during the fruit's color-turning stage, where potassium's effect on vascular structure and transport proteins was most pronounced. Together, these findings demonstrate a coordinated potassium-driven enhancement of sugar flow, from leaf to fruit, culminating in sweeter, higher-quality oranges.
“Sweetness isn't just a matter of genetics or sunshine—it's also about how well a tree moves its sugars,” said Professor Songwei Wu, corresponding author of the study. “Our research shows that potassium acts as a conductor in the orchestra of sugar transport, optimizing both the energy flow and the physical pathways that deliver sugar to fruit. By revealing how potassium enhances symplastic loading in citrus, we provide a more nuanced understanding that growers can turn into action for better-tasting, market-ready fruit.”
This research offers both scientific insight and practical guidance for improving citrus fruit quality. By identifying 1.5% potassium content in fruit as the ideal threshold, the study helps farmers fine-tune fertilization strategies to maximize sweetness without over-applying nutrients. The discovery that potassium promotes symplastic phloem loading could also influence breeding and management approaches in other perennial fruit crops. Looking forward, the authors suggest investigating how environmental conditions and other nutrients interact with potassium to further optimize source–sink dynamics. With global demand for high-flavor fruit rising, this potassium-powered pathway may help citrus producers deliver not just yield, but quality.
###
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.