Development of conservation methods for Gunnera perpensa L.: an overexploited medicinal plant in the Eastern Cape, South Africa.

Abstract

In South Africa, many plants which are used in traditional medicines are collected from wild populations. The high demand for trade and use of these medicinal plants place an enormous pressure on their natural populations, especially because they are indiscriminately harvested. The most affected of these plant species are those harvested from their underground parts, among which is Gunnera perpensa L. Gunnera perpensa is of considerable ethnobotanical interest in traditional medicine because of its wide usage. The rhizomes are widely used and indiscriminately collected in large quantities from the wild to meet the ever increasing demand in traditional medicine markets. As a result, this valuable medicinal plant species is being endangered. According to the Red List of South African Plants, the conservation status of G. perpensa has been listed as ‘declining’. The ethnobotanical survey conducted as part of this research confirms the plant species as threatened. It is, therefore, important to develop propagation and conservation strategies for this medicinal plant. Clonal propagation of G. perpensa was conducted using varying lengths of the rhizome (1, 2, 3, 4 and 5 cm) segments as propagules. While regeneration was possible with all the rhizome lengths, most of the growth parameters were significantly higher in the 5 cm rhizomes than the other rhizome segments. The appropriate planting depth for the rhizomes was also determined and 4 or 5 cm planting depths were found appropriate. No significant difference was observed in the growth parameters amongst the planting depths; nevertheless, 4 cm depth gave higher growth and yield. The results of this study show that regenerating G. perpensa through rhizome segments is an efficient method for obtaining plant material for medicinal purposes. Seed germination is an important determinant in the distribution and survival of a plant species. It therefore becomes essential to study the germination requirements of G. perpensa seeds. The effects of light, temperature conditions, leaching, scarification, pre-chilling and chemical substances such as gibberellic acid and KNO3 were investigated in the germination of the seeds. The optimum temperature for germination in this study was 25oC under light conditions. Overall, germination of G. perpensa seeds was poor and irregular, but mechanical scarification significantly improved the percentage germination from 4% to 32% compared to untreated seeds (control), with a mean germination time (MGT) of 56 days. This was followed by 7 day leaching with 26% germination and MGT of 44 days. Antimicrobial and antioxidant activities were evaluated between two different plant parts of G. perpensa, for their potential in plant part substitution. The plant parts (roots and leaves) were extracted in differing polarity solvents. These were screened for antibacterial activity against ten bacterial species including Bacillus subtilis and Staphylococcus aureus. The plant extracts presented some activity against the bacterial strains with zones of inhibition varying from 8 to 25 mm and MIC values ranging from 2.5 to 10 mg/ml. The highest activity against the tested bacteria was obtained with the methanol leaf extract with inhibition zone of 25 mm against Serratia mercescens. Gunnera perpensa extracts were also screened for antifungal activity against four fungal species. All the plant extracts exhibited high antifungal activity against Candida krusei and Absidia corymbifera with MIC values ranging from 0.15 to 0.62 mg/ml. The ability of the extracts of G. perpensa to inhibit the growth of several bacteria and fungi is an indication of the broad-spectrum antimicrobial potential of this plant that further validates its use for the treatment of various ailments. The phytochemical evaluation of the studied plant species indicated that the observed activities might largely be due to their high flavonoid and phenol contents, with a contributing effect from their alkaloids and saponins. The highest flavonoid content (434.09 mg Quercetin/g) was recorded in acetone rhizome extract, followed by methanol leaf extract (432.22 mg Quercetin/g). In the antibacterial and some of the antioxidant assays, the leaves of this plant demonstrated higher activity than the rhizomes, while in the antifungal assay the two plant parts exhibited similar activities suggesting their potential in plant part substitution. The harvesting of leaves as a conservation strategy is certainly more sustainable than the destructive use of the roots of this threatened plant species. Micropropagation is a useful technique for rapid clonal multiplication of plant material which could alleviate the pressure on the wild plant populations as well as potentially provide plant materials for use. This study did develop a successful decontamination method and the efficient means for eliminating detrimental browning of G. perpensa explants and media. The result demonstrated that ascorbic acid or activated charcoal was required as media supplements to reduce the browning effect. This information provides an important starting point for the development of a successful micropropagation protocol for the conservation of G. perpensa. This research further highlights the need to conserve our indigenous plant resources before they become extinct, since some of them could be pharmacologically active and perhaps contain novel compounds that are biologically active against some diseases.