Pungency in onion is due to

Pungency in onion is due to Allyl propyl disulphide.
Pungency is largely due to the level of sulphur - based non-protein amino acids they contain. Pungency is basically due to the presence of a volatile oil - allyl-propyl-disulphide. It is formed by an enzymatic reaction when tissues are broken.
Although measurement of these elements can be difficult, it is well known that good nutrition is essential to managing the taste and pungency of onion bulbs.