You measure the sugar content of a plant with a brix meter......very simple to do. You take a small leaf sample. So the sugar are in the fan leaves and "sugar" leaves as we called them ( old school I guess)...lol!
So the sugar is getting into the leaves "somehow"..... and photosynthesis makes sense for the mechanism.
Advanced Nutrients is one of the only companies I know that did actual studies on it, and developed the product
Bud Candy which does have sugars (for the microbes as food) and then the magic bullet......magnesium.
Sulfur is the other additive used.....as mentioned.
If they did uptake sugars for a benefit, I'm sure they would have put it in there formulas.