Variations in growth, yield and metabolites of African ginger (Siphonochilus aethiopicus) in responses to irrigation regimes and nitrogen levels.

Abstract

Medicinal plants are valuable natural resources used as traditional medicine and have economic significance. African ginger (S. aethiopicus) (Schweinf.) B.L. Burtt is one of the most important rhizomatous plants, highly-valued for its medicinal properties and wide distribution in many regions of southern Africa. The plant is currently listed on the Red List of South African endangered species due to overharvesting. The increased demand for plant material has led to extinction in other areas of South Africa. The loss of wild populations harvested will destroy the natural habitats and genetic diversity in the long term. The demand for S. aethiopicus plant parts, particularly the rhizome is associated with the medicinal remedies possessed by the plant. The rhizomes have been traditionally used for the treatment of coughs, colds, asthma, headaches, pain, inflammation and malaria. Currently, there is limited scientific evidence on the cultivation and response of secondary metabolites of S. aethiopicus to agronomic practices. Cultivation of medicinal plants is a good approach to conserve species biodiversity and meet current demands for plant based products. This study investigated the variations in growth, yield and metabolites of S. aethiopicus in response to cultivation practices for commercial production and further development of medicinal products. In this study, total phenolic content, flavonoid content and antioxidant activity of S. aethiopicus leaf, rhizome and root from varying areas (Mpumalanga, KwaZulu-Natal, Limpopo and North West) were evaluated. Total phenolic and flavonoid contents were investigated by Folin-Ciocalteu and aluminium chloride (AlCl3) colorimetric methods, respectively. Antioxidant activity in different parts of S. aethiopicus was evaluated by 2, 2-diphenyl-1-picrylhydrazyl (DPPH) free radical scavenging activity and ferric reducing power (FRAP). Furthermore, the study determined the variations in soluble sugars in the leaf, rhizome and root as influenced by varying growing areas. The results showed high concentration of sucrose, glucose and fructose in the leaf and root as influenced by different growing areas. A higher content of both total phenolics and flavonoids were found in the root from Mpumalanga (54.5±2.0 mg GAE/g and 14.83±0.06 μg QE/g, respectively) compared to the leaf and rhizome from other growing areas. KwaZulu-Natal also exhibited high flavonoids in the leaf (12.72±1.18 μg QE/g), rhizome (14.21±1.98 μg QE/g) and root (12.88±0.57 μg QE/g) compared to other growing areas. In both methods, the leaf exhibited higher antioxidant activity than the root and rhizome. The high antioxidant activities exhibited in the leaf from Mpumalanga suggest its adaptive capabilities to different environments. S. aethiopicus parts could be used as a potential source for antioxidant properties and encourage cultivation under different growing areas to conserve its biodiversity and increase species populations. The effect of nitrogen levels and irrigation regimes on biomass yield, stomatal conductance, chlorophyll content and leaf area index was investigated under the rainshelter for two growing seasons. The results of this study conclusively reveal that the plant height and number of leaves per plant were significantly higher towards maturity. Plants grown with 50 and 100 kg N/ha had greater plant height, number of leaves per plant, LAI, SPAD values and biomass yield that eventually resulted in higher dry matter production. Stomatal conductance was higher throughout the growing period and decline in response water stressed treatment. The high amount of water utilized from well watered treatment (30% ADL) compared to moderate (50% ADL) and severe (70% ADL) treatments could be attributed to improved water availability and superior plant canopies. Further experiments should be conducted to evaluate different combinations of agronomic practices to fully exploit the growth of S. aethiopicus under different conditions. The high amount of water utilized from the well watered treatment (30% ADL) compared to moderate (50% ADL) and severe (70% ADL) treatments could be attributed to improved water availability and superior plant canopies. The well watered treatment (30% ADL) had a significantly higher total biomass, fresh and dry rhizome yield compared to other water stressed treatments. The response of water stress and nitrogen levels showed significant accumulation of plant flavonoids and phenolics in leaf, rhizome and root. In plant carbohydrates, root had high sucrose content with the application of low N under severely stressed (70% ADL) treatment. The investigation of volatile components of leaf, rhizome and root in response to irrigation regimes and nitrogen levels were analysed by GC-MS. The results showed that the highest volatile components in the root and rhizome were terpenes, as compared to the increased components of aliphatic acids, benzenoids and aliphatic aldehydes in the leaf. In all treatments and parts, the odorant sesquiterpene (1E)-5-Methyl-1-(2, 6, 6-trimethyl-2, 4-cyclohexadien-1-yl)-1, 4-hexadien-3-one was the most abundant volatile compound. The 4-Hydroxy-4-methyl-2-pentanone was detected under severely stressed (70% ADL) treatment with the application of 100 kg N/ha. Severely stressed (70% ADL) treatment with minimal application of N induced the terpenes components in all plant parts. The study showed that volatile components of S. aethiopicus vary with plant sources, water stress and mineral nutrient deficiency. Knowledge on the impact of S. aethiopicus parts will provide a useful guide for selection towards identifying profiles of volatile compounds and explore the additional bioactive compounds for therapeutic use. Taken together, this study represents the importance of cultivation methods as an alternative approach to wild harvesting, conserving S. aethiopicus for commercial production and exposure to water stress conditions for high secondary metabolites.