The effect of salt stress on the nutraceutical, physiological and phytochemical properties of Trachyandra ciliata (L.f.) Kunth: an edible halophyte from the Western Cape, South Africa

Abstract

Water scarcity and high salinity in agricultural lands pose serious threats to food security amid exponential population growth. This necessitates the cultivation of salt and drought tolerant plant species as alternatives to mainstream vegetable crops. Trachyandra ciliata (L.f.) Kunth is a wild underutilized halophyte that is endemic to the Western Cape coastal dunes in South Africa. Trachyandra genus belongs to Asphodelaceae (Aloe) family, which is widely known for its pharmaceutical importance. The literature on this plant is limited to non-existent, and there are currently no Trachyandra species in commercial cultivation. However, it is known that this plant is edible and that Khoe-San people who lived around the South African Cape coast utilized it as food, although its edibility and nutritional composition remain undocumented. The only study on this plant focused on the vegetative growth of T. ciliata in response to different growth media and salinity to develop its growth protocol. However, its precise salt tolerance mechanisms are unexplored. Furthermore, the presence of some antioxidants in the plant especially when subjected to salt stress has been reported. This prompts studies on the pharmacological potential of this plant amid global quest to discover more plant-based pharmaceuticals for the treatment of chronic diseases. Thus, this study was carried out to evaluate the nutraceutical, phytochemical, and physiological properties of T. ciliata under varying degrees of salinity to promote its consumption, therapeutic use, and commercialization. The propagation experiment was carried out at the greenhouse nursery of the Cape Peninsula University of Technology, Bellville campus, Cape Town, South Africa. Plants were subjected to five salinity (NaCl) concentrations (0, 50, 100, 150, and 200 mM). After 15 weeks of salinity treatments, plants were harvested, dried, and pulverized for extraction and further analyses. Chapter One introduces the general overview of the research, which includes significance of the research, its aims, and the overall list of objectives, which guided the study. On the other hand, Chapter Two explored nutraceutical, agricultural, and economic potential of Trachyandra ciliata through extensive review of literature. This chapter unravels the overall potential of T. ciliata as a food source and a therapeutic agent for the treatment of chronic diseases. It further discusses the potential salt tolerance mechanisms and the potential of T. ciliata in desalination and phytoremediation. Chapter three explored the effect of salinity stress on growth parameters, leaf hydration, photosynthetic pigmentation, cation content, oxidative stress markers and concentration of antioxidant enzymes to understand precise mechanisms involved in its salt tolerance. Results revealed that salinity positively influenced growth parameters in all plant parts at low concentration (50 mM NaCl) compared to the control. Moreover, the total chlorophyll content negatively correlated with increasing salinity. T. ciliata maintained equivalent Relative Water Content (RWC) from control to 100 mM treatment, which then efficiently decreased with increasing salinity, while the plant maintained an unchanged leaf succulence among all treatments. On the other hand, high salinity stimulated oxidative stress as indicated by high MDA, cell death, and superoxide radicals, which were more expressive under 200 mM treatment. To counter the catastrophic effects of excessive ROS, T. ciliata activated antioxidative defence mechanisms as indicated by high SOD and CAT antioxidative enzymes which were more prevalent at high salinity (200 mM). Furthermore, the high proline content under higher salinity treatments ensured further scavenging of ROS. Chapter four examined the leaf surface and cross-sectional properties, elemental composition and anatomical responses in T. ciliata using Scanning Electron microscopy (SEM) and Energy Dispersive X-ray spectroscopy (EDX) to clarify salt tolerance mechanisms in T. ciliata. From the SEM micrographs, salt glands were observed protruding from the epidermis along the vascular system under low salinity and salt crystals appeared under higher concentrations, which makes this plant maintain cellular homeostasis under high salinity, and the plant can be classified as a recretohalophyte. In addition, stomatal distribution, stomatal density and the number of open stomata decreased with increasing salinity. EDX revealed the presence of some important elements such as Potassium, Magnesium, Phosphorus, Calcium and more in the leaves. The results showed that increased salinity led to a decrease in the percentage composition of P, K and Ca2+, while Mg2+ was high under control and low salinity (50 mM), decreased under 100 mM and increased again with increasing salinity. On the contrary, increasing salinity caused an increase in Na+ and Cl- in a stable manner. Chapter five investigated the edibility of T. ciliata by exploring its proximate, antinutrient, mineral, and phytochemical composition when cultivated under different levels of salinity. Salinity significantly influenced the mineral, proximate, antinutrient, and phytochemical composition of T. ciliata. Control and 50 mM treatments recorded significantly higher macro and micronutrient content in the flower buds and leaves, except for heavy metals such as Zn and Cu, which increased with increasing salinity and significantly higher in the roots. Leaves under low salinity treatments recorded higher moisture and protein content, while leaves also recorded higher ash content under high salinity. On the other hand, flower buds under low salinity recorded significantly high fat and NDF composition. Phytochemicals and antinutrients increased with increasing salinity concentrations. The low antinutrient content and high nutritional, mineral and phenolic contents validate the edibility and suitability of T. ciliata for human consumption. Chapter 6 quantified and characterised phytochemicals present in leaves, roots, and flower buds of T. ciliata through UHPLC-MS to discover novel bioactive compounds with potential therapeutic applications. The UHPLC-MS identified 71 compounds, which were grouped into flavonoids, anthocyanins, alkaloids, nucleobase, nucleosides/tide, saccharides, fatty acids, amino acids, and coumarins. The diverse identified compounds indicate that the extracts of T. ciliata may have biological activities against chronic diseases, including diabetes, oxidative stress, neurodegenerative disorders, cancer, cardiovascular diseases, and gastrointestinal disorders. Results from this study further show the potential of this plant in the treatment of other ailments such as skin problems, viral diseases, inflammation, oxidative stress, as well as plant plant-based food additives and preservatives. Chapter 7 evaluated crude extracts of T. ciliata as a natural therapeutic agent for the amelioration of cancer, acetylcholinesterase inhibitory activity, and ROS scavenging activity in the liver for the first time. The yellow dye 3-(4,5-dimethyltiazol-2yl)-2,5-diphenyl tetrazolium bromide (MTT), Ellman's colorimetric method, and the 2',7'-dichlorodihydrofluorescein diacetate assay (H2DCF-DA) were respectively employed to evaluate cytotoxicity, acetylcholinesterase, and ROS scavenging activity of the plant extracts. Results revealed that flower bud extracts prepared from 0 mM and 100 mM salinity treatments at 1mg/mL concentration showed strong cytotoxicity to cancer cells, while they had moderate and weak cytotoxicity to non-cancer cells respectively. All extracts showed high acetylcholinesterase inhibitory activity, except for root and flower bud extracts from 100 mM salinity treatment. Moreover, ROS scavenging activity was mainly observed in the leaf extracts from all treatments, and in the root extract from 0 mM salinity treatment. These findings suggest that T. ciliata could be a therapeutic agent for the treatment of cancer, Alzheimer's disease, and liver disorders amidst global quest to develop more plant-based pharmaceuticals for the treatment of chronic diseases. From the results gathered in this study, T. ciliata is recommended for human consumption, domestication and commercial cultivation to mitigate the looming food shortage and hidden hunger due to increasing population and water scarcity. Findings from this study serve as points of reference to commercial farmers, pharmaceutical industries, medical practitioners, communities, scholars, policy makers, and aspiring researchers whose interests are on the potential of easily accessible underutilized wild edible crops to develop new strategies to address global issues concerning human health and wellness.