Onions produce sulfur compounds as part of their natural defense strategy against pests and threats. These compounds, which include thiopropanolsulfonic acids, are stored within the onion cells and are generally harmless when the onion is intact. However, when the onion is cut, these compounds are released and undergo a chemical transformation.

When the onion is attacked by herbivores, such as insects or animals, the cells are ruptured, triggering the release of the enzyme alliinase. This enzyme catalyzes the breakdown of sulfur compounds into sulfenic acids, which are unstable and quickly convert into syn-Propanethial-S-oxide-a volatile compound that evaporates into the air.

Syn-Propanethial-S-oxide is not only irritating to human eyes but also has a strong odor. This sharp scent can be a deterrent to herbivores, signaling that the onion may not be safe to eat. Additionally, the irritant can harm the eyes of small pests, possibly driving them away from the plant.

In a broader ecological context, the release of these sulfur compounds can act as a warning signal to nearby plants. The presence of sulfur compounds in the air could attract other insects or animals that are beneficial to the onion plant by preying on the herbivores, thus offering a form of indirect protection.

Onions, therefore, use sulfur compounds as a chemical defense to discourage herbivores and protect themselves from being eaten. These compounds make the onion less palatable and harder to digest for Sulfur compounds in onions are a defense mechanism against herbivores and pests. By producing irritants like syn-Propanethial-S-oxide, onions discourage animals from eating them and increase their chances of survival.