Radish flavor

The “spicy” flavor radishes are known for comes predominantly from their skin. This is due to defense chemicals in the skin, called Allyl isothiocyanates, for warding off herbivores. The same compounds are what make mustard and horseradish spicy.

Source: https://www.themeateater.com/wild-and-whole/gardening/why-your-radishes-are-spicy-and-how-to-cook-with-them