The University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) recently published “Brix Basics: Where Plant Sugars Come From and Why They Matter.” Edwin Gutierrez Rodriguez, UF/IFAS citrus Extension agent for DeSoto, Hardee and Manatee counties, authored the article. Edited excerpts follow:
Degrees Brix is the measurement of the total dissolved solids in a liquid, based on how much the liquid bends light. One degree Brix equals about one gram of sucrose dissolved in 100 grams of solution. The number is measured with a refractometer.
Brix is not just a sugar unit. It is best understood as a quick, practical stand-in for sweetness and ripeness.
A plant builds its own sugar. Sugar is manufactured in the leaves through photosynthesis, using sunlight, water and carbon dioxide.
Once sugar is made in a leaf, it is delivered via the phloem to the fruit or root where it will be stored. Sugar is actively loaded into the phloem in the leaf, transported throughout the plant, and then unloaded into “sink” tissue, where it accumulates and eventually appears as a Brix reading. Fruit, roots, seeds and other growing tissues that receive sugar are called “sinks” because they are primarily where sugars are deposited.
RISE REASONS
A rising Brix number can mean two very different things:
- More sugar is arriving. Strong sunlight, warm weather, healthy leaves and a well-fed root system all increase the amount of sugar a plant produces and successfully delivers to fruit.
- Less water dilutes the same amount of sugar. When a plant is mildly water-stressed, fruit tissue loses water faster than it loses sugar, so the same amount of sugar becomes more concentrated. This is sometimes called the concentration effect, and it is the mechanism behind deficit-irrigation strategies that growers use to raise Brix before harvest.
NUTRIENTS AND BRIX
Potassium, commonly associated with Brix, may support the carbon “loading” into fruits. Nitrogen and phosphorus deficiencies reduce photosynthesis, causing sugar to back up in the leaves rather than move outward. They also shift the plant’s growth priority toward the roots. Magnesium deficiency can leave a leaf sitting on sugar it is physically unable to export.
HLB EFFECTS
A struggling root system (as caused by HLB) limits a plant’s ceiling for sweetness long before any fruit symptoms appear.
In Florida’s heavily HLB-infected citrus industry, as the HLB infection progresses, the phloem accumulates callose (a highly dense polysaccharide) and other compounds triggered by the bacteria-host interaction. That clogs the phloem, physically blocking the sugar pathway from leaves to fruit and roots.
HLB, combined with hurricane damage, has driven a decline in Florida’s average fruit Brix. Due to the Brix decline, the Florida Citrus Commission in October 2025 reduced the minimum level required for oranges and grapefruit juice. Likewise, this July, the U.S. Food and Drug Administration changed the federal standard for pasteurized orange juice, lowering the minimum Brix requirement from 10.5 to 10%.
Source: UF/IFAS
Share this Post