The pungency in radish is due to

The pungent flavor in Radish is due to the presence of Isothiocyanates.
Isothiocyanates are the chemical group formed by substituting the oxygen in the isocyanate group with a Sulphur.
It is the chemical group formed by substituting the oxygen in the isocyanate group with a Sulphur. Naturally, it is produced by enzymatic action and conversion of metabolites called glucosinolates. It is produced in cruciferous vegetables, like bok choy, broccoli, cabbage, cauliflower, radish, kale, etc.