Vegetative propagation, growth and metabolite response of Myrsine africana to photoselective shade netting

Abstract

Natural products derived from indigenous plant species continue to gain interest in the medicinal and cosmetics industries. This places pressure on plant populations and ecosystems. Therefore, this necessitates domestication and clonal propagation to regenerate superior varieties with high potency, quality and uniformity for commercialisation purposes. Myrsinaceae species of South Africa were reviewed, including the distribution, biochemistry, bioactivity, propagation and cultivation of each. Myrsine africana L. was found to possess commercially important secondary metabolites and bioactivity for applications in the cosmeceutical industry. However, challenges for commercialisation of M. africana include relatively slow growth and vegetative propagation, in addition to low rooting percentage and slightly low bioactivity of raw extracts for economically feasible cosmetic production. Light properties have been exploited in industry to enhance plant growth and development, in addition to secondary metabolite production. Photoselective shade netting is a technique used to manipulate light spectral composition and intensity on a large scale. Furthermore, endogenous carbohydrates and phenolic compounds are sensitive to light manipulation and have been shown to influence adventitious root (AR) development in some species. However, the influence of light quality on AR development in relation to carbohydrates and phenolic compounds is unclear. Therefore, this study involved the manipulation of the stock plant environment through photoselective shade netting to investigate the effects of light intensity and spectral composition on primary and secondary metabolites of M. africana. Furthermore, the subsequent influence on vegetative growth, rooting competence and duration, and bioactivity was investigated. This was accomplished by cultivating M. africana shrubs under green, black, white and blue shade net of 50% density, red and black shade net of 80% density and under full sun exposure as the control. Shoots were harvested in Spring and Autumn to investigate adventitious root development and quantify non-structural carbohydrates, total phenolic content and anti-wrinkle activity. Plant height and canopy size were measured physically and using the Canopeo mobile application, respectively, carbohydrate proportions were measured using appropriate assay kits, total phenolic content was quantified through the Folin-Ciocalteu method, and bioactivity was analysed through the elastase inhibition assay. Data was transformed appropriately and analysed for significance using GraphPad Prism 4 statistical analysis software. xiv The results revealed the influence of respective light spectral treatments on environmental conditions and parameters measured on M. africana. Vegetative growth was enhanced significantly by cultivation under green, black and red shade net and inhibited under blue shade net. This may be a result of the manipulation of starch content by the shade net. Total phenolic content was significantly higher under blue and white shade net treatments, in comparison to the control, and higher in Spring than in Autumn. In contrast, elastase inhibition was higher in Autumn than in Spring and was significantly higher under green and red shade net in comparison to the control, with IC50 values of 18.59 μg/mL and 19.28 μg/mL, respectively, in comparison to the control 37.93 ± 3.61 μg/mL. Furthermore, donor plant manipulation through photoselective shade netting did not influence adventitious root development significantly, although positive trends were observed under red shade net, which may motivate further investigation. Therefore, it is recommended that M. africana should be cultivated under green photoselective shade net and harvested in Autumn for cosmeceutical production.