Rheum australe (Himalayan Rhubarb): Identification, Anthraquinones and Uses

Rheum australe, Himalayan rhubarb, is a large perennial herb of the high Himalaya and a classic example of a plant whose chemistry does two opposite things depending on dose. Understanding that dual action is the key to almost every exam question about it.

Scientific classification

Kingdom	Plantae
Clade	Angiosperms, Eudicots
Order	Caryophyllales
Family	Polygonaceae
Genus	Rheum
Species	Rheum australe D. Don
Common names	Himalayan rhubarb

Do not confuse it with garden rhubarb (Rheum rhabarbarum or R. rhaponticum), which is cultivated for its edible leaf stalks. Same genus, different species, different use.

How to identify it

• Habit. A robust perennial herb, 1–2 m tall in flower, grown from a thick woody rootstock.
• Root. A large, thick, branched rhizome and root — the part harvested. Cut surfaces are typically yellow to orange.
• Leaves. Very large, borne on long thick stalks from the base. Blades are broadly ovate to heart-shaped with wavy or lobed margins and conspicuous veins. The leaf stalks are often reddish.
• Inflorescence. A tall, much-branched panicle of numerous small flowers, appearing after the leaves are well developed.
• Flowers. Small, greenish-white to dark purple, with six tepals.
• Fruit. A three-winged achene — the winged fruit is characteristic of the genus.

Habitat and distribution

The species grows on open, rocky, grassy slopes and among boulders at high altitude, generally between 3,000 and 4,500 metres. It favours moist, well-drained, humus-rich soil and tolerates the short, cold growing season of the subalpine and alpine zone.

Chemical constituents

Two groups dominate, and knowing both is what makes a complete answer:

• Anthraquinones — the principal group. The five usually named are emodin, chrysophanol, physcion, aloe-emodin and rhein. They occur free and as glycosides.
• Stilbenoids — including piceatannol and resveratrol, plus the glucoside rhaponticin.
• Also present: tannins, flavonoids and calcium oxalate.

The dual action — this is the important part

The anthraquinone glycosides pass through the stomach unchanged, are hydrolysed by gut bacteria in the colon, and the liberated anthraquinones stimulate peristalsis and reduce water reabsorption. That is a purgative or laxative effect at higher doses.

The tannins do the opposite — they are astringent and constipating. At low doses the tannin effect can predominate. So the same root can act as either a mild astringent or a purgative depending on dose and preparation. If you are asked why dose matters for this plant, this is the answer.

Reported and traditional uses

The root is used in traditional systems as a purgative, astringent and stomachic, and for a wide range of digestive complaints. Reported pharmacological activities include laxative, antioxidant, anti-inflammatory, antimicrobial, antifungal, hepatoprotective and nephroprotective effects in laboratory studies. Young leaf stalks are eaten locally, including as a preserved product.

This is study material, not health advice. Anthraquinone laxatives are not harmless — prolonged use can cause electrolyte disturbance and dependency. Never consume or use any plant on the basis of what you read here. See the Medical Disclaimer page.

Conservation

The species is regarded as endangered across much of its range. As with other rhizome-bearing Himalayan medicinals, the cause is straightforward:

• Harvesting the root kills the plant.
• Plants are often collected before seed set.
• Growth is slow at high altitude.
• Commercial demand is sustained.

Cultivation, in-vitro propagation and managed harvesting are the main responses being developed.

Cultivation and harvesting

Cultivation is now the main conservation tool for this species, and it works well because the plant is robust once established.

• Propagation. Either from seed, sown in nursery beds and transplanted as seedlings, or by dividing the rootstock, each piece carrying a bud. Vegetative division is faster and preserves the characteristics of the parent.
• Site. Open, sunny, well-drained slope with deep humus-rich soil at high altitude. The plant needs a cold dormant period and does not thrive at low elevation.
• Harvest. Roots are lifted in autumn, once the aerial parts have died back and the plant has moved resources below ground — this is when anthraquinone content is highest. They are cleaned, sliced and dried.

Roots are generally taken only after several years of growth, and harvesting after seed set is strongly preferred so that wild-stand regeneration is not blocked. In cultivation, leaving some plants unharvested to flower and set seed maintains the planting stock.

In-vitro propagation protocols have also been published, aimed at producing uniform planting material and preserving germplasm without further wild collection.

Key points for the exam

• Family Polygonaceae; order Caryophyllales.
• Very large basal leaves on thick stalks; tall branched panicle; three-winged fruit.
• Part used: root and rhizome.
• Main compounds: anthraquinones (emodin, chrysophanol, physcion, aloe-emodin, rhein) and stilbenoids (piceatannol, resveratrol).
• Dual action: anthraquinones are purgative, tannins are astringent and constipating.
• Altitude roughly 3,000–4,500 m. Status: endangered.

Practice questions

1. Name five anthraquinones from this species. Emodin, chrysophanol, physcion, aloe-emodin and rhein.
2. Why can the same root act as both laxative and astringent? Anthraquinone glycosides are hydrolysed by gut bacteria into stimulant laxatives, while the tannins are astringent and constipating. Which effect predominates depends on dose.
3. How do anthraquinone laxatives work? The glycosides pass to the colon, where bacteria hydrolyse them; the free anthraquinones stimulate peristalsis and reduce water reabsorption.
4. What type of fruit does the genus produce? A three-winged achene.
5. How is it distinguished from garden rhubarb? They are different species: R. australe is the wild high-altitude medicinal species harvested for its root, while garden rhubarb is cultivated for its edible leaf stalks.

Frequently asked questions

Can you eat the leaf stalks?
Young stalks are eaten locally, but do not forage on the basis of what you read here. Identify with certainty first.

Why is the cut root yellow?
The anthraquinone pigments.

How does it differ chemically from other Himalayan medicinals?
Its chemistry is dominated by anthraquinones and stilbenoids, whereas species like Valeriana and Nardostachys are dominated by terpenoids.

What altitude should I write?
Roughly 3,000–4,500 metres.