Diana Howard says, "Many botanical extracts have been studied for their ability to inhibit tyrosinase. These include kiwi, mulberry, bearberry, and, more recently, giant fennel. While the precise mechanism of how they work is not clearly understood, studies have been conducted looking at the flavonoid components of these plants. Many of these botanical extracts may work on two fronts; not only do they inhibit the tyrosinase enzyme, but they also compete with the enzyme's substrate."

Reference
Organic Facts. (n.d.). Health Benefits of Fennel.