Further Observations on the Occurrence of Protocatechuic Acid in Pigmented Onion Scales and Its Relation to Disease Resistance in the Onion.*

In a paper recently published in this Journal (1) we reported the isolation of the phenolic acid commonly known as protocatechuic acid (3,4-dihydroxybenzoic acid) from the outer bulb scales of pigmented onions. The occurrence of this acid in the outer scales has a significant relationship to disease resistance in the onion. Protocatechuic acid is apparently one of the chemical entities that enables pigmented onions to resist the invasion of Colletotrichum circinans (Berk.) Vogl., the fungus organism responsible for the disease commonly known as smudge. In view of the fact that the white varieties of onion are susceptible to the disease and as far as our chemical investigations have been able to show do not contain protocatechuic acid in the outer bulb scales, it appears that we have established for the first time a specific chemical difference between a resistant host (the pigmented onion) and a non-resistant host (the white onion). We emphasized that the toxicity experiments conducted along with the chemical investigations indicated that not all of the inherent