Distribution and Importance
Polygonum hydropiper L. (family Polygonoceae) is a member of a genus of some 175 species. It is a semi-erect (25-75 cm) annual herb with a branched stem and lance-shaped leaves, carrying its greenish-pink flowers in slender racemes (). The species is widespread in most parts of Europe, temperate Asia, and North America, and it also occurs at scattered sites in North Africa. Across its main range it is abundant in the verges of ponds and ditches and on waterlogged grasslands and water meadows. Polygonum hydropiper is not grown commercially but has found an exceptionally impressive range of uses in folk medicine and also as a culinary herb, and this has led to the adoption of a rich variety of apt local names, e.g. fireweed, arsemart and smartweed are examples of some 20 English regional names in addition to the accepted vernacular name of “water pepper“. The flower heads have little odour but all the aerial parts have a bitter acrid taste and contain vesicant compounds that blister the skin upon repeated handling ().
Medicinal use of Polygonum hydropiper goes back to Dioscorides (ca. 60 a.d.) and tinctures of foliage are used as diuretics, diaphoretics, and to arrest gynecological bleeding (). Water effusions are astringent and anti-inflammatory and have been claimed efficacious for colds, coughs, dysentery, gout, sores and ulcers, toothache, dropsy, menstrual pains, and hemorrhoids. Foliage can be applied to arrest bleeding or be secreted in beds to kill fleas, cf. German synonyms Flohkraut and Flohpfeffer (). Linneaus recorded that the herb yielded a yellow dye with an alum mordant. The peppery taste of the foliage has led to widespread culinary use in the Far East, in particular as a garnish for fish dishes.
These varied biological effects, if not anecdotal, must be a consequence of the secondary metabolism of the herb. The essential oil from foliage (ca. 0.43% dry wt.) contains monoterpenoids with a variety of skeletons, e.g. 1,4-cineole, car-3-ene, a and β-pinenes, fenchone, borneol, camphor, linaloyl acetate etc., but is otherwise unexceptional (). More noteworthy is the sesquiter-penoid component of the flower heads. The main compound (ca. 0.05% wet wt.) is (—)-polygodial () accompanied by its hydroxy-derivative ( — )-warburganal in small amounts. The former (= 9β-drim-7-en-11, 12-dial) has also been isolated from liverworts and named tadeonal (). Both polygodial and warburganal are responsible for the hot taste of the plant (). Polygodial has been assigned widespread physiological activities. It is a fungicide, and as it acts by damaging cell membranes, it also facilitates the entry of antibiotics and thus is synergistic for them (). It also acts as a mutagen (); an allergen (); a plant growth inhibitor, preventing germination of rice at 100 ppm (); is moluscidal, piscicidal and phytotoxic (); and is a viral-genome inactivator (). Best studied is its antifeedant activity towards aphids () and army worms (). Thus, leaves of Zea mays dipped into 0.1 ppm solutions of the dialdehyde are uneaten and the insects perish of starvation. Warburganal is also antifungal () and is an antifeedant for army worms () and aphids (). Other drimane derivatives isolated from foliage (usually in small amounts) are () isopolygodial (); polygonic acid; which shows weak anti-coagulant activity; 11-ethoxycinnanolide; polygodial acetal; valdiviolide (); fuegin (); cinnanolide (a fungicide); and drimenol (); in addition to the nor-sesquiterpenoids (C14-compounds) isopolygonal; polygonal (); and polygonone. Seeds contain polygodial, together with several of the above and concertifolin, a plant growth regulator, and up to 4% linolenic acid ().
Foliage also contains anthocyanidins and anthocyanins (), flavonoids including guercetin and kaempferol, and glycosides such as isorhamnetin and rhamnazin (); caffeic, gallic, sinapic and many other aromatic acids (); and other aromatics (); p-sitosterol and a variety of high molecular-mass linear hydrocarbons and waxes (); and tannins (). Of especial interest are isocoumarins that possess anti-inflamatory activity () and an unidentified fraction of extract from foliage that is a potent anti-fertility agent for rats ().
Polygodial would seem ripe for exploitation in order to protect agricultural and horticultural crops, not only in view of its antifeedant properties which could prevent aphid infestation, but the consequent prevention of the distribution of plant viruses by the insects. The known different responses of aphidian and lepidopteran larvae to the compound would also auger well. However, despite extensive studies on the dialdehyde and analogues () few concrete control proposals have been reported (Res. Corporation Patent 1981). The compound is volatile and unstable and its small but significant phytotoxicity and fish toxicity may well restrict its use in pest control. It seems likely that future control systems may rely on several compounds, in low concentrations, acting synergistically; polygodial may well become a component of such an integrated and environmentally acceptable approach (). Pharmaceuticals containing warburganal for use in combating fungal infections have been patented (). Use of Polygonum hydropiper as a cocrop to provide protection for aphid-susceptible crops such as beans and potatoes has not been reported: possibly the damp conditions suitable for successful cultivation of the former cannot easily be achieved in cultivated environments.
Clearly, tissue culture studies are unlikely to be competitive for the commercial production of polygodial and its relatives. In addition to Polygonum hydropiper, this compound occurs in several other natural sources that could be exploited (); and several total and partial chemical syntheses are available (). However, gene transfer whereby commercially important plants are endowed with the ability to synthesize and secrete the anti-feedant compound seems an encouraging approach and the first steps have been laid in the above described establishment of suitable biomass and the partial purification of the appropriate synthetase.
A great deal of work is also necessary to understand the sources of the other biological activities of extracts of Polygonum hydropiper. An important line of work could follow the claimed use of polygodial as a virus genome inactivator, as this could lead to the compounds, use as a viricide and perhaps as an antitumour compound.
Selections from the book: “Medicinal and Aromatic Plants IV”, 1993.