Cutting and peeling of onions brings tears to the eyes because of the presence of

Cutting or peeling an onion causes tears because it ruptures the plant's cells, releasing enzymes and sulfur-containing compounds [t1][t2]. Onions absorb sulfur from the soil to form amino acid sulfoxides, specifically S-1-propenyl-L-cysteine sulfoxide [t4][t7]. When the cell is damaged, the enzyme alliinase reacts with these sulfoxides to produce sulfenic acids [t3][t6]. A second enzyme, lachrymatory-factor synthase, then converts these into a volatile gas called syn-propanethial-S-oxide [t1][t9]. This sulfur-based compound is a lachrymator; when it reaches the eyes, it reacts with the moisture on the cornea to form a mild sulfuric acid [t2]. This irritation triggers the lachrymal glands to produce tears as a protective mechanism to dilute the acid [t1][t6]. Therefore, the presence of sulfur in the cell is the fundamental reason for this reaction.

Sources

1. [1] https://www.chm.bris.ac.uk/motm/pso/psov.htm
2. [2] https://www.loc.gov/everyday-mysteries/food-and-nutrition/item/why-does-chopping-an-onion-make-you-cry/