Nutraceutical, phytochemical, intercropping and morpho-physiological response of Tetragonia decumbens Mill. to salt-stress: a promising wild leafy vegetable for bio-saline agriculture in South Africa

Abstract

Tetragonia decumbens Mill., a neglected South African edible halophyte has shown promising production returns for bio-saline agriculture. However, in-depth studies on the micro-morphological, physiological, biochemical and antioxidative mechanisms of this species to salinity remains underexplored for precise bio-saline agriculture. Furthermore, the lack of scientific knowledge on its nutritional profile, pharmacological activity, and its potential use as a phytoremediator in salt affected soils has contributed to its underutilisation. Therefore, this study examined the nutraceutical, phytochemical, intercropping and morpho-physiological response of T. decumbens to salt-stress to further support its domestication, consumption, and cultivation among South African households and in regions affected by salinity. To elucidate the salt-tolerance mechanisms in T. decumbens, the effect of salinity on micro morphological, physiological, biochemical and antioxidative mechanism was evaluated. Plants were subjected to varying salinity doses (0, 50, 100, 150, 200 and 250 mM NaCl). Results revealed a substantial enhancement in plant growth, relative water content, as well as total fresh weight in plants irrigated with 50 and 100 mM NaCl in comparison to the control, while higher saline concentrations (150-250 mM NaCl) reduced plant growth, chlorophyll content and stomatal density. Similarly, these high salt concentrations induced more severe oxidative stress indicated by high amounts of superoxide, cell death viability and malondialdehyde. Nevertheless, T. decumbens modulated various defence mechanisms with increasing salinity stress; these include the upregulation of superoxide dismutase, catalase, polyphenols, flavonoids, proanthocyanidins and the build-up of sodium ions in the leaves. Moreover, micromorphological examination of the adaxial layer of the epidermis with scanning electron microscope revealed distinctive up-rooted glandular peltate trichomes with increasing salinity, thus suggesting this species may have utilized these specialised structures to store or dilute excessive sodium ions. These results showed that T. decumbens can withstand salinity by modifying its anatomical features, morpho physiological traits, antioxidant defence systems, and managing ion toxicity and oxidative stress efficiently. To justify its nutritional potential for consumption, cultivated and wild collected samples were analysed for nutritional composition, secondary metabolites, and anti-microbial potential. Wild samples were collected from three coastal areas (Strand beach, Muizenberg beach and Blouberg beach) during the dry (summer) and wet (winter) seasons in Cape Town, and the greenhouse cultivated which were subjected to varying salinity doses (0, 50, 100, 150, 200 and 250 mM). Results revealed a considerable increase in minerals (N, P and Mg) and proximate composition (Ash, moisture, and carbohydrates) in greenhouse cultivated plants subjected to 50 mM of salinity, while the highest crude fat and neutral detergent fibre were recorded in wild samples. Moreover, heavy metal uptake, phytochemicals, anti-nutrients and anti-microbial activities were more pronounced in wild plants than in cultivated samples. These findings validated for the first time, the relevance of nutritional quality of T. decumbens in assessing its suitability as a source of nutrients. Moreover, the efficacy of antimicrobial metabolites of the crude extract of wild samples T. decumbens, suggest that this halophyte might be suitable for both anti-bacterial and anti-fungal applications. Therefore, for optimal yield of minerals and proximate constituents, it is recommended to cultivate this species under 50 mM of salinity while for antimicrobial purposes, wild samples were recommended. To validate its phytoremediation potential, an intercropping system under saline condition was conducted. Spinach (Spinacia oleracea) seedlings were grown alone and in consociation with this halophyte under various salt stresses (50, 100, 150 and 200 mM NaCl). Results showed that increasing salinity reduced crop growth, relative water content, chlorophyll, and nutritional quality of spinach in monocultured system. Interestingly, intercropped spinach irrigated with 50 and 100 mM of NaCl revealed a substantial enhancement in crop performance, reduction in oxidative stress and had improved nutritional quality depicted by high amounts of minerals, proximate constituents, and vitamins. These results validate its phytoremediation potential of saline soils, support the introduction of T. decumbens in vegetable farming systems and highlighted its positive impact on improving the overall crop performance of salt sensitive vegetables under saline condition. The pharmacological potential including cytotoxicity, acetylcholinesterase inhibitory activity, anti-cancer and anti-inflammatory activity of diverse chemicals aggregated in the crude extracts of T. decumbens was evaluated. For the first time, the ultra-performance liquid chromatography-mass spectrometry (UPLC-MS) identified 98 compounds in saline cultivated samples (0, 50, 100, 150, 200 and 250 mM NaCl) most of which were stilbenoids (E-Piceatannol), hydroxybenzoic acid derivatives (6-Gingerol and 6-Gingerol-o- pentose), tannin (3,3'-di-O-methylellagic acid), coumarins (Bergapten and Xanthotoxin), Phenylsulfate (catechol sulphate), Sesquiterpene (Zederone), and alkaloid (Trigonelline) among others. Potent anti-inflammatory activity in lipopolysaccharide (LPS)-induced cells was noted in the dose of 100 μg/mL in crude extract of 100 mM NaCl treatment, and this was significantly higher than cells treated with Aminoguanidine (positive control). There was also a significant inhibitory activity of acetylcholinesterase in Human SH-SY5Y neuroblastoma cells treated with crude extracts of 0 to 100 mM NaCl, when compared with untreated cells. These findings suggest that the identified compounds in T. decumbens could have potential anti-inflammatory and acetylcholinesterase inhibitory activities. Overall, this study has shown the potential value of T. decumbens as a nutritious vegetable with numerous pharmacological values for bio-saline agriculture. Moreover, its phytoremediation potential of saline soils supports its introduction in vegetable farming systems in saline affected areas.