As one of the “Big 3”—the name for the three main macronutrients needed by plants—it has long been known that potassium is vital to plant health. We know that it affects photosynthesis, crop quality, and sugar movement; that potassium affects both the sweetness and the taste profile of citrus. However, up until recently, it wasn’t understood how potassium affects sugar in citrus, like oranges. Thankfully, recent research from Huazhong Agricultural University in China has filled in some of the gaps in our knowledge.
The Mechanism Between Potassium and Sugar in Citrus
The research team at Huazhong utilized field and greenhouse experiments with laboratory testing like advanced imaging and techniques for isotope tracing to find out how potassium helps sugars to move through a citrus tree. They found that potassium activates the symplastic phloem loading pathway; this is a sugar-delivery route in plants whereby sugar that is made in the leaves pass through tiny tunnels, called plasmodesmata, and into the phloem of the plant. This pathway is powered by diffusion—from a high concentration in the leaves to a lower concentration in the phloem—so it is highly efficient as it requires no additional energy from the plant.
This finding adds another piece to the puzzle on how potassium affects citrus sweetness at the molecular level. The research team also found that potassium increased the activity of enzymes that metabolize sugar, such as sucrose phosphate synthase and sucrose synthase. Furthermore, the researchers also found that potassium also increased the expression of vital sugar transport genes, like CsSUT1 and CsSWEET15.
Essentially, potassium helps a citrus tree to improve how well it moves sugar from where it’s created in its leaves to where it’s needed in the fruit. The corresponding author of the study, Professor Songwei Wu, eloquently likened potassium to a conductor of an orchestra that is playing the music of sugar transport.
The Potassium Sweet Spot
The Huazhong research team also conducted controlled experiments utilizing 10-year-old Newhall navel orange trees to find the perfect amount of potassium for the oranges. In addition to seeing the application of potassium increase fruit weight, yield, and total sugar content—especially sucrose—they also found that a 1.5% of potassium concentration in the fruit pulp was ideal; any amount of potassium added beyond 1.5% in the fruit pulp led to a decline in sugar accumulation.
Uncovering the mechanisms that lead to potassium improving the sugar content and taste of oranges will, with further research, turn into actionable steps for citrus growers to create sweeter, more desirable fruit.