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.
A research team from Huazhong Agricultural University in China has unveiled how potassium fertilization enhances sugar accumulation in Newhall navel oranges by stimulating internal sugar transport.
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 and sucrose synthase. In addition, potassium elevated the expression of key sugar transport genes such as CsSUT1 and CsSWEET15.
The 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.”
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.
See a Horticulture Research article about the study here.
Source: Horticulture Research