Distribution and Importance of Sempervivum
The genus Sempervivum (Crassulaceae) contains approximately 80 species and several subspecies. The name is indicative of their evergreen, sempervirent nature (semper = always, vivum = living). Houseleeks (Sempervivum spp.) grow mainly on sunny, barren hillsides, mostly at 1000-2000 m. They are favourite plants in rock-gardens, because they grow on walls and roofing tiles.
One of the most important species, Sempervivum tectorum L. is native to the Alps, but it can be found sporadically as far as the Pyrenees and the northern regions of the Balkan Peninsula, in central Anatolia. It grows well under extreme conditions, usually in calcareous soil. It is a cosmopolitan species living in dry circumstances (Hegnauer 1964). It can be used on extreme sites (notably in urban environments), if its basic ecological and growth requirements are respected.
One of the most important ecophysiological features of Sempervivum – in which it is similar to other members of Crassulaceae family – is nocturnal C02 fixation; this physiological adaptation to a dry environment enables tolerance of water deficiency.
This metabolism, known as CAM (crassulacean acid metabolism), is an alternative pathway for C4-type CO2 fixation. This route results in the diurnal disconnection of the Calvin cycle and CO2 fixation. In CAM plants phosphoenolpyruvate (PEP) carboxylation occurs nocturnally – oxaloacetic acid is reduced to malate by malate dehydrogenase, and the malate is accumulated in the vacuole. In this phase the stomata are open, thus CO2 flows into the cytosol. Without ATP and NADPH2 the direct fixation of CO2 is impossible in the Calvin cycle, because in the dark the decarboxylation of malate stops rapidly. Under illumination, malate is returned to the cytosol and decarboxylated, the CO2 being donated into the Calvin cycle, so that light-dependent CO2 fixation can occur under closed conditions. Desiccation of these plants is, consequently, prevented when exposure to daylight causes warming and dryness.
Thomas and Andre examined the effect of water stress on hourly and daily patterns of Sempervivum growth. Water stress reduced gross photosynthesis and modified its division between the reduction of CO2 and O2. Schuber and Kluge examined CO2 exchange and measured the diurnal rhythms of organic acids in situ in some species of Sempervivum. According to measurements by Pilou et al., PEP carboxylase (CAM key enzyme) activity fluctuated seasonally, with maxima during the summer which were 8- to 30-fold higher than winter values. Daytime temperature seemed to be the most influential factor; light intensity had a small but significant effect. Increasing the temperature increased efflux from the tissue in a nonlinear manner. In tissue which had accumulated malate (acidified state), a temperature increase caused reduction of the rate of dark fixation, whereas the rate of CO2 fixation in the light remained largely unaffected. This result indicates that in the acidified tissue increasing the temperature increases the efflux of malate from the vacuole.
In folk medicine Sempervivum is called ‘ear-herb’, because the juice squeezed from the plant can effectively cure diseases of the ear. It is believed to be effective for treatment of inflammation and skin burns. According to Nicolae et al., the gel used by them for treatment of burns contains 3-5% methyl cellulose, 5-8% pectin, and 20-25% propolis extract, and the balance is Sempervivum tectorum juice. The gel can also be used for treatment of ear inflammations. The leaf infusion is suggested for treatment of bronchitis and mouth abrasions.
Leaves (diameter 6-10 cm) from fully developed plants are used to prepare the drug ‘Sempervivi tectori herba’. Because it is not an official pharmacopoeial drug, it has no standard. In 1974 it was proposed that Sempervivum tectorum be included in the list of protected plants (); all Sempervivum spp. are protected plants in Hungary, two cantons of Switzerland (Nidwalden, Obwalden), Austria, and some provinces of Germany.
Conventional Practices for Its Propagation, and for the Production of Medicinal Components
The leaves of this perennial plant are flat and succulent, and form rosettes. Houseleek develops young sprout rosettes on short or long runners (stolons); these form roots and in 3-4 years evolve into flowering plants. The inflorescence forms from the middle of the rosette, and is of ramifying structure. After flowering and seed ripening the flowering rosettes perish.
It is recommended that the plant be propagated by means of leaf cuttings or runners, in the conventional manner, because propagation by sowing seeds results in slow development.
Propagation is usually performed with rooted, young daughter rosettes, which develop on the runners, with a runner 2-3 cm in length.
Leaves from 2-year-old plants are cut, squeezed, and the freshly squeezed juice is used. The succulent leaves can be dried by lyophilization, a method which is always successful. A kilogram of the drug can usually be extracted from 7-8 kg fresh leaves; these are of light green color and the original, star-shaped form of the leaf is preserved.
Sempervivum has rarely been investigated phytochemically. Carbohydrates, organic acids, alkaloid, and flavonoids have been isolated from the whole plant. Of the polysaccharides, accumulation of sedoheptulose is characteristic; sucrose and fructose also occur in the leaves of the plants. Significant acid metabolites are isocitric (5-9%), citric, and malonic acids.
A method for isolation of isocitric acid has been described by Vickery and Wilson. Fumaric acid has also been detected in Sempervivum species.
The total alkaloid content of the leaves of Sempervivum tectorum has been found to be 0.01-0.03%. Nicotine was identified among the alkaloids by Frigot.
Among the free amino acids asparagine is dominant; caffeic acid and chlorogenic acid are the dominant phenolic acids.
The houseleek also contains approximately 0.7% flavonoids (quercetin, kaempferol, and flavone glycosides), 3-4% tannins (including procyanidines), and significant ascorbic acid and mucilage. Gumenyuk et al. detected complex phenolic compounds in the flowers of S. ruthenium and also isolated and identified five flavonoid glycosides. Flavonoid variation in Eurasian Sempervivum was minimal (kaempferol was the principal flavonol); this reflects the morphological uniformity of the genus and the similar ecological preference of the species.
Stevens et al. compared alkaloid and tannin levels in 36 species of Crassulaceae including three Sempervivum spp. Proanthocyanidins (condensed tannins) and galloyl esters were found in all the genera, but Sempervivum spp. were free from piperidine alkaloids.
Lotti et al. detected fatty acid compounds in Sempervivum chrysanthemum.
Gumenyuk and Borisenko established that Russian houseleek contained a complex mixture of phenolic substances including quercetin, kaempferol, isorhamnetin, and scutelarein derivatives. The complex had an inflammatory activity.
According to Kery et al., phytochemical screening of the plant extract proved the presence of flavonoids (0.7% w/w), polyphenolic compounds (4.2% w/w) and polysaccharides (11.2% w/w). Water-soluble polysaccharides of plant origin are of interest as potential biostimulators with immunomodulating and antitumor activity. The biologically active polysaccharides of the plants have antitumor activity. Plant extracts containing low molecular mass and antioxidative compounds have been successfully used in phytotherapy since ancient times, because reactive oxygen-containing species are involved in several diseases.
It has been demonstrated that many naturally occurring antioxidants have activity as radical scavengers and lipid peroxidation inhibitors. Stevens et al. detected no triterpenes in waxes of Sempervivum.
Satory et al. have reported experiments on lyophilized extracts of Sempervivum tectorum L. in which they discovered marked dose-dependent topical antiinflammatory activity which inhibited the edematous response of rabbit ears to Croton oil (28-64%) within 6-24 h.
The pressed juice of S. tectorum is a well-known antiphlogistic medicine.
Sempervivum spp. (Houseleek): Conclusion and Prospect
As previously described, ‘Sempervivi tectori herba’ contains isocitric and citric acids, alkaloids, flavonoids, and polysaccharides. The drug is used in folk medicine for curing skin burns, inflammation of the throat, and ear diseases.
Because the plant is protected in several European countries, collecting houseleek growing in the wild is prohibited, so production of Sempervivum tectorum L. plants is of great importance.
The disadvantage of generative proliferation is that plants grown from seeds develop very slowly; the disadvantage of vegetative multiplication is that a limited number of young rosettes can be obtained from the runners.
Soft brownish-yellow and hard bright green calluses have been developed in callus cultures of Sempervivum tectorum L. of leaf and shoot origin, respectively
During the redifferentiation process only the hard green type of callus had morphogenetic activity. Shoot organogenesis was induced by KN only sporadically, whereas shoot development could be observed in 62-78% of callus cultures when BAP was combined with IAA.
Although root organogenesis could not be detected in callus cultures, the isolated shoots could be rooted on plant-growth-regulator-free medium.
Callus cultures cannot, however, be used for 100% successful production of large quantities of propagation material.
There is still much uncertainty in the successful induction of morphogenesis, because the causes of regeneration and the molecular changes involved are not completely understood. We do not know which genes regulate the induction of morphogenesis, nor the endogenous conditions which must be maintained in the cells if this gene is to be activated.
The engineering of plant regeneration will be planned and reproduced easily only when conditions necessary for realization of cell totipotency are known on a molecular level.
In contrast to plant regeneration from calluses, the technology of in vitro micropropagation of Sempervivum tectorum L. can be applied even under farming conditions. In the propagation phase, six to eight new shoots can be isolated within 6-7 weeks under the action of BAP and KN in combination with IAA.
The efficiency of propagation can be improved by 50-70% by adding the reduced form of nitrogen, adenine sulphate, to the medium.
New shoots can be rooted on media containing IBA, but without the use of growth regulator; after adaptation plants can be placed out of doors.
This method of micropropagation is suitable for producing homogeneous propagation material from Sempervivum tectorum L. Shoot cultures maintain their capacity to regenerate during a long period of culture, in contrast to calluses.
The possibility of genetic changes during the propagation of axil-buds and axil-shoots is minimal, so this method is a highly suitable means of preserving genotypes and maintaining an in vitro gene-bank. This is very important in countries where Sempervivum tectorum L. is a protected plant.
Besides producing homogeneous propagation material, micropropagation is suitable for the production of standard pharmaceutical products of uniformly high quality for use in phytotherapy.
Selections from the book: “Medicinal and Aromatic Plants XII” (2002).