General Morphology and Distribution
Glutinous rehmannia (Rehmannia glutinosa Libosch., Scrophulariaceae), with the Chinese name Dihuang, is one of the most common and important Chinese medicinal herbs. It is a perennial herbaceous plant, 10-37 cm in height, covered with long, soft, gray-white, glandular hairs over the whole plant. The plant grows as a rosette before flowering, with leaves 3-10 cm in length and 1.5-4 cm in width. The inflorescence is a raceme, over 40 cm long, flowering in April-May, setting capsular fruits with 300-400 seeds and maturing in May-early June. The plant part for medicinal use is the root tuber (Rhizoma Rehmanniae).
Wild Rehmannia plants are distributed on hillside, field ridge and roadside. Cultivated varieties or strains are mostly selected from R. glutinosa Libosch. f. hueichingensis (Chao et Schih) Hsiao. The Rehmannia plants for medical use are mainly cultivated and produced in most areas of China, especially in the provinces of Henan and Shandong. Both fresh or dried rhizome (Rhizoma Rehmanniae) and prepared rhizoma of Rehmannia (Rhizoma Rehmanniae Praeparatae) have been used as traditional Chinese medicine. Wild Rehmannia mostly growing in the provinces of Liaoning, Hebei, Shandong and Zhejiang, is also harvested, but only fresh root tuber is used.
In Shen Nong’s Canon of Materia Medica, one of the most ancient books of Chinese traditional medicine (before the Eastern Han Dynasty, about 100-180 A.D.), Rehmannia was already recorded. In the worldwide well-known Compendium of Materia Medica (Li Shi-zhen 1578 A.D.), there is a more detailed description of this plant and its medicinal uses.
Fresh or dried roots of Rehmannia (Rhizoma Rehmanniae) are “cold” (All the terms in brackets are special terms of Chinese medicine. Details can be seen in A Concise Dictionary of Chinese Medicine [ed. by Editing Committee of Chinese Medicine Dictionary]) in nature, with sweet-bitter flavor. It is mainly used to “clear up the heat”, “cool the blood”, stop bleeding, promote the production of body fluid and nourish “Yin”. Prepared rhizome of Rehmannia (Rhizoma Rehmanniae Praeparatae) is made after a process of steaming, slicing, and sunlight drying of Rehmannia root tubers. It is slightly “cold” in nature, with sweet flavor, and is an important Chinese medicine to enrich the blood. Its principle functions include to “promote the essence of life” and benefit the kidneys, and it is used for curing the diseases as follows: dizziness due to blood deficiency, palpitation and insomnia, night sweat and seminal emission, diabetes, etc. When Rehmannia roots are used as medicine, they are made as a medicinal broth after decocting, honey-bolus, or tonic wine usually together with other medicinal herbs. The Rehmannia root tuber is also used as medicinal herb in Japan, Korea and some other Far-Eastern countries. In Japan, Rehmannia root tubers are mainly imported from China and Korea. Annual demand in China, including home and export need, is about 1.5×104 t.
Conventional Practices for Propagation and Cultivation, and Problems
According to the record, Rehmannia cultivation practice gradually changed from sexual propagation by seeds to vegetative propagation by root tubers during the Ming-Qing Dynasties (1368-1911). In the present practice for Rehmannia cultivation, root tubers normally sprout in the spring, with optimal temperature 18-25 °C. New-formed roots start to enlarge after 35-45 days of sprouting from the soil. Root tubers grow rapidly during late July – middle October, and are harvested in the late autumn or the early winter. From sprouting to harvesting tubers needs over 140 days.
The herb has suffered serious deterioration, possibly as a result of virus infection during successive vegetative propagations. The tuber yield is reduced, also due to farmers usually picking the large tubers for sale and having thin, small ones as seeds. Self-incompatibility during inbreeding is another difficulty in Rehmannia breeding.
Review of Tissue Culture Studies on Rehmannia glutinosa
Since Jiang and Mao (1979) first reported that callus tissues and plant regeneration were induced from root, bud, stem, and leaf explants of seedlings, or directly induced from cultured seeds, some attempts have been made to apply tissue culture techniques to Rehmannia (Table In vitro studies on Rehmannia glutinosa), including callus formation, plant regeneration and clonal propagation, meristem culture, protoplast culture, anther culture, and in vitro fertilization.
Table In vitro studies on Rehmannia glutinosa
|1. Sterile seedlings||Callus, shoots and plants from root, bud, stem and leaf|
|2. Leaf||Callus, shoots and plants from leaf explants|
|3. Leaf||Callus, shoots and plants|
|4. Meristem||Virus-tested plants and clonal propagation|
|5. Shoot tips||Multishoots and plants|
|6. Root and young stem||Callus, shoots and plants|
|7. Anther||Pollen plants|
|8. Unpollinated ovules with placenta||Plants from test-tube fertilization for overcoming self-incompatibility|
|9. Mesophyll protoplasts||Callus, shoots and plants|
Rehmannia glutinosa: Conclusions and Prospects
Rehmannia glutinosa is one of the few medicinal herbs that could be manipulated by the various plant tissue culture techniques. The successes obtained with this herb so far, including meristem culture, protoplast culture, anther culture, in vitro fertilization, callus induction, and subsequent plant regeneration from the different parts of the plant have already produced some encouraging results for Rehmannia breeders and farmers.
A new fine and virus-tested strain from meristem culture, Mericlone No. 16, has been released and is now being planted in some main production areas of Rehmannia root tubers, which saves its parent variety, Golden No. 1 Scholar (a fine cultivar in China, but much attacked by virus diseases). Although the optimum conditions for Rehmannia anther culture still need to be defined, some selected homozygous lines obtained from anther culture and the inbred plants from in vitro fertilization are being used to make new hybrids. The somaclonal variations appearing in the plants regenerated from the culture of leaf explants is also of importance. Actually, some new strains have been selected from these regenerated plants. The success of protoplast culture of this herb will help the Rehmannia breeders to find more useful somaclones with different variation. The cell suspension culture has not been established, and mutagenesis not tested in Rehmannia tissue culture so far. When all those in vitro techniques mentioned above are integrated into the Rehmannia improvement program, it will certainly change the breeding and production of this traditional Chinese herb.
Selections from the book: “Medicinal and Aromatic Plants I”, 1988.