Horticultural and phytomedicinal investigation of dragon fruit (Hylocereus spp.) a crop new to South Africa

Abstract

Dragon fruit is a crop produced from a number of species of vining, subtropical members of the exotic genus of cactus Hylocereus (A. Berger) Britton & Rose hailing from South and Central America where they have been considered a source of food and medicine for generations. Interest in the commercial farming of dragon fruit has been building in the South African farming community since the early 2010’s due to ease of cultivation and efficient use of water for a fruit crop. During this project literature research and engagement with farmers from associated industries assisted towards documenting the crop, its history, the potential in South Africa, sources of improved varieties and current developments in the local farming community, including local perspectives of research and usage of this specific emerging crop. Basic seed lot evaluation and germination experiments under different photoperiods were undertaken on selected commercial selections of Hylocereus, focusing on analysis of selected germination performance parameters towards optimisation of seedling production, vital in plant breeding efforts which assist with improved cultivars. It was found that photoperiod length affects both final weight of seedlings and Germination Index in all accessions studied. Analysis of seed weight and final seedling weight led to the suspicion that the selection ‘Voodoo Child’ is, in fact a polyploid. The ploidy level of ‘Voodoo Child’ was determined using DNA flow cytometry (FCM) and DAPI (diamidino-2-phenylindole) staining using young aerial root material and bracketing with known diploid, Hylocereus costaricensis ‘Ruby Red’ and Selenicereus megalanthus, a closely related species of tetraploid vine cactus also grown for its fruit. Using a reference standard of known absolute genome size, Lycopersicum esculentum cv. ‘Stupické polní tyčkové rané’, calculations were undertaken to estimate absolute genome size of all specimens tested. The genome size of the diploid accession was determined to be 3,41 pg 2C-DNA and the cultivar tested showed a 2C-DNA content of 6,91 proving that it is, indeed a tetraploid plant, which has implications to its current use as a commonly planted pollinator and in future cultivar breeding efforts using this plant. Dragon fruit is considered a “superfood”, with high concentrations of anti-oxidants, and it has been claimed to have claimed anti-diabetic, anti-aging activity and a range of other health benefits. The phyto-medicinal portion of this study aimed to explore and compare the medicinal and nutritional properties of selected commercially important Hylocereus varieties, of identified functional classes; white fleshed fruit, red fleshed fruit and purple fleshed fruit not only of the fruit but also by-products of the crop, namely the cladophylls and fruit peels. The study investigated the phytochemical content and the biological activity of the ethanolic extracts prepared from the different plant parts. Antiproliferative activity on human epidermoid carcinoma cells (A431), anti-oxidant activity against DPPH and nitric-oxide, enzyme inhibitory assays on elastase and tyrosinase, antimicrobial activity on Candida albicans and Cutibacterium acnes, potential cytotoxic effects on normal human keratinocyte cells (HaCat) and hepato-protective activity using human liver carcinoma cells (HepG2) were evaluated. The nutritional content of selected ethanolic extracts and cladophyll powdered material was also analysed. The ethanolic extract of Hylocereus fruit, fruit peels and cladophylls displayed moderate antioxidant potential with the highest ranges being the cladophyll extracts, with IC50 ranges below 500 µg/mL for the NO anti-oxidant assay, (the best being ‘Zamorano’ with 159,73 µg/mL ) and below 100 µg/mL for the DPPH assay (the best being ‘Ruby Red’ with 66,31 µg/mL) and selected fruit extracts (the white fleshed ‘Lemonade’ and red fleshed ‘Jackye-Lee’ and ‘Trisha’) showed antimicrobial activity against C. albicans at 7.5 mg/mL. The in vitro activity of ethanolic extracts of Hylocereus stem, peel and cladophyll extracts could be due to the presence of phenols and betacyanin pigments. This study has highlighted new opportunities for the development of novel products from an emerging fruit crop, and has aided in identifying promising new research directions as well as developed one new, simpler methodology towards ploidy assessment in the group.