Massey (1925) suggested that the toxic component of walnuts might be juglone. This idea was further supported by Davis (1928). Today this concept is widely held. Chemically this substance is known as 5, hydroxy-1, 4, naphtho-quinone and belongs to a group of strong oxidizing agents with commercial uses, including tanning agents, medicinals, poisons, etc.
A knowledge of the physiology of juglone in the walnut is essential to an understanding of the divergent results obtained by various experimenters. Juglone, as such, occurs probably only in minute quantities in the inner root bark, and in the green husks of the nuts. These regions are, however, rich in a substance known as hydrojuglone. This compound, the colorless, non-toxic, reduced form of juglone is immediately oxidized to its toxic form upon exposure to the air or some oxidizing substance from the roots of other plants. Upon standing in the air juglone again disappears, being either changed back to hydrojuglone or broken down into other non-toxic substances.
This sequence of events may be noted in a fresh green husk of a black walnut. When the fresh husk is cut, the interior is white but immediately turns yellow as the colorless hydrojuglone is transformed into the yellow juglone. Upon standing or drying the husk becomes black as further chemical changes occur. It is impossible to extract juglone from these dried husks without first reoxidizing them.
It now becomes possible for us to understand some of the discrepancies in the studies on walnut toxicity. If walnut bark or other plant parts are allowed to become desiccated, no toxicity may be found. If the roots of plants do not contact plant parts containing juglone or hydrojuglone, their oxidizing ability can not produce the toxin. Further the relative amounts of juglone in various species of Juglans has not been completely investigated. It does occur definitely in J. nigra and J. cinerea and has been reported as being in J. regia. Other species need investigation before being included as sources of juglone.
It is known that many plants are not adversely affected when grown under or near walnut trees. Some of these have root systems too shallow to contact the roots of the walnuts, especially in plowed ground. Some plants may send out sufficient surface roots to keep the plant alive in spite of injury to the deeper roots. The possibility that the roots of some plants are capable of withstanding the oxidizing power of the juglone is currently under study.
In early American folklore, the inner bark and the husks of the nuts were used as a source of a yellow dye for cloth. This yellow dye is juglone. The ancients also used this method of dying both cloth and hair.
Another property of juglone is its toxicity to fish. A few years ago it was a common practice in the South to cut the husks from young nuts and throw them immediately into a still pond of water. The fish, stunned by the juglone, would rise to the surface and were collected and eaten. No one seemed to worry about the effects of such poisoned food on the consumers.
Juglone is toxic to fungi and bacteria. Of all the medicinal powers attributed to walnuts by the Greeks and Romans, its use in curing certain skin diseases including ringworm has held up through the ages until many today can recall the use of the green husks for control of ringworms. Brissemoret and Michaud (1917) reported the use of juglone in clinical cases for the cure of eczema, psoriasis, impetigo and other skin diseases and concluded that juglone deserves extensive use in dermatology. To our knowledge the medical profession has not followed up the possibilities which this substance offers. The author is familiar with one case in which pure juglone was applied to a persistent ringworm infection. The infection disappeared within a month after treatment was begun. Though conclusions can not be drawn on a single case, certainly this observation lends credence to the medicinal lore of the ancients and the American pioneers.
During the fall and winter of 1942-43, investigations on juglone were started at the Connecticut Agricultural Experiment Station in conjunction with studies of the effect of other plant toxins on the roots of higher plants. When the toxicity of this oxidizing compound was established, it was produced in some quantity both by extraction from walnuts after the method of Combes (1907) and by synthesis after the method of Bernthsen and Semper (1887). Working on the assumption that the killing of germinating fungus spores and root hairs are similar phenomena, juglone was subjected to standardized laboratory tests for fungicidal value. In a series of experiments, this compound proved to be equally toxic with the copper in Bordeaux mixture. Such a high degree of toxicity was deemed worth further investigation, so juglone was tested as a seed protectant and as a spray in field trials for the control of black spot of roses.
As a seed protectant, juglone failed miserably. It's toxicity to the noncutinized surfaces of root tissues was so great that germination was abnormal and greatly impaired. The injury noted here was apparently the same as that discussed by Brown (1943) and that which occurs normally in the field.