Juglone

Juglone is a naphthoquinone with the molecular formula C10H6O3 and a molar mass of 174.15 g/mol. It is a yellow-orange crystalline solid that sublimes readily and is sparingly soluble in water but soluble in organic solvents. The compound is synthesized in walnut trees via the shikimate and acetate pathways, with the precursor 1,4-naphthoquinone being hydroxylated at the 5-position. Juglone is stored in the tree's roots, bark, leaves, and nut hulls as a non-toxic glycoside called hydrojuglone, which oxidizes to the active quinone when exposed to air and soil microbes.¹

Juglone inhibits the growth of many herbaceous and woody plants by interfering with mitochondrial electron transport, specifically by accepting electrons from complex I and III, thereby disrupting ATP production. This leads to reduced root respiration and nutrient uptake, causing wilting, yellowing, and eventual death of susceptible species. The compound is most concentrated in the root zone, where it can persist for years after a walnut tree is removed. Tomatoes, potatoes, peppers, and many ornamentals are highly sensitive, while grasses and some legumes show tolerance.²

Beyond its allelopathic role, juglone exhibits antimicrobial, antifungal, and insecticidal activities. It has been studied for its potential in controlling plant pathogens and pests, and for its cytotoxic effects on cancer cells in vitro. Juglone also inhibits the enzyme Pin1, a peptidyl-prolyl isomerase implicated in several cancers, making it a lead compound for drug development. In traditional medicine, walnut hull extracts containing juglone have been used as a topical treatment for skin conditions, though its toxicity limits systemic use.

Juglone is also found in other members of the Juglandaceae family, including hickory (Carya) and pecan, though in lower concentrations. The compound is used as a natural dye, producing a rich brown color on wool and silk. In forensic science, juglone has been identified as a marker for walnut wood in archaeological samples. Interestingly, juglone is phototoxic: it generates reactive oxygen species when exposed to UV light, which may enhance its allelopathic and antimicrobial effects. Some insects, such as the walnut caterpillar, have evolved detoxification mechanisms to feed on juglone-rich foliage.

Allelopathy

The release of chemicals by one plant that inhibits the growth of neighboring plants.

Naphthoquinone

A class of organic compounds derived from naphthalene, characterized by a quinone structure.

Hydrojuglone

The reduced, non-toxic form of juglone found in walnut tissues.

Pin1

A peptidyl-prolyl isomerase enzyme that regulates protein conformation and is implicated in cancer.