Influence of nitrogen fertilizer micro-dosing on phenolic content, antioxidant, and anticholinesterase properties of aqueous extracts of three tropical leafy vegetables

Abstract

This work investigated the effect of fertilizer micro‐dosing on in vitro antioxidant and anti‐acetylcholinesterase (AChE) properties of aqueous extracts of leaves of three vegetables (Solanum macrocarpon L., Amaranthus viridis L., and Telfairia occidentalis f. Hooke). Urea was combined (0, 20, 40, and 60 kg/ha) with cow manure (5 t/ha) or without cow manure (80 kg/ha) to grow the leafy vegetables. Significantly higher (p < .05) polyphenol extract yield was obtained with 60 kg N/ha when compared with other fertilizer doses. Total polyphenol (510.70–521.50 mg gallic acid equivalent/g) and total flavonoid (609.51–742.50 µg rutin equivalent/g) contents were reduced as fertilizer dose increased. The 2,2‐diphenyl‐1 picrylhydrazyl radical scavenging and iron reducing properties were enhanced by organic manure while metal ion chelation and anti‐AChE activity was highest at the 60 kg N/ha. We conclude that the combined use of organic manure with urea fertilizer led to enhanced antioxidant and anti‐AChE activities of the leaf polyphenolic extracts. PRACTICAL APPLICATIONS: The search for natural scavengers of free radicals as food preservatives and nutraceutical agents has intensified in the past decade because of increasing negative reactions to synthetic compounds by consumers. Vegetable leaf polyphenolic compounds used in this work showed free radical scavenging and other antioxidant properties that are comparable to those of butylated hydroxytoluene, a synthetic antioxidant agent. The polyphenol extracts also showed acetylcholinesterase (AChE)‐inhibitory property that is similar to that of galanthamine, a drug used for the treatment of Alzheimer's disease. Since oxidative stress is also associated with the development of chronic diseases, the vegetable leaf extracts are potential agents that can be used both as effective food preservatives and bioactive agents against neurodegenerative diseases.