The effect of different potassium concentrations and electrical conductivity on hydroponic- grown Trachyandra divaricata, as a green vegetable

Abstract

The shortages of freshwater supply, inadequate groundwater, and high salinization levels in soil have become major challenges worldwide. Salinization, which refers to the presence of salts in the soil or water, is a common challenge in half of cultivated soil in South Africa and that makes it difficult for conventional crops to absorb nutrients and water. However, the salinity phenomenon has hampered the quality of arable soil and promoted the use of lowquality water (brackish or saline water) due to the lack of fresh water. Saline water has proved to harm the quality of crops and soil as it encourages soil erosion and causes a decline in soil nutrients for plants. This problem has facilitated the consideration in cultivation of coastal plants as potential alternatives as they are naturally evolved salt-tolerant species that can survive in a saline environment. Trachyandra divaricata is an indigenous coastal plant native to the southern African region and distributed in this hot and dry Mediterranean type climate and in high saline conditions along the coast. Trachyandra divaricata flower buds were historically harvested in coastal regions by indigenous people and eaten raw or boiled as a nutritious inflorescent vegetable. However, there has been insufficient scientific documentation supporting biochemical properties and nutritional content of this plant besides the knowledge collected from indigenous people. Cultivating this plant in hydroponics could be efficient as it will utilise minimal resources and nutrients but cutting plants back to encourage new growth. This aim of this study was to investigate the nutrient profile, antioxidants potential, and flower bud development of the hydroponically grown T. divaricata in reaction to different potassium dosages and levels of cutting heights to discover the appropriate protocol of the plant in a hydroponic system. Four identically constructed Nutrient Film Technique (NFT) systems arranged in a complete block design were used, with each system on a separate metal mesh steel table (900 mm x 1 250 x 25000 m) having 10 replicates of each treatment. Nutrifeed™ was used as a basic nutrient in all systems, however, different potassium concentrations (0.0216, 0.0144 and 0.0072 M of K2SO4) were used as treatments added in into each sump, while the system with Nutrifeed only considered as a control. The different treatments comprised of plants with three different cutting height levels and non- cut plants considered as control, 5 cm (minimal), 10 cm (moderate) and 15 cm (maximum cutting). The plant height was measured using a standard ruler and the number of flower buds counted manually during and after experiment. A laboratory scale was used to measure fresh and dry weight of the plant. SPAD-502 Konic-Minolta meter was used to measure the chlorophyll content of the plants. The phytochemicals and antioxidants were analysed using dried samples of flower buds by means of assays for total flavonols, total polyphenols, 2,2'-azinobis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), ferric reducing antioxidant power (FRAP), and 2,2-diphenyl-1-picrylhydrazyl (DPPH). Similarly to nutrient contents and proximate analysis, the dried flower buds were assessed by means of assays for carbohydrates, crude proteins, crude fat content, Ash content, moisture content, Non fiber carbohydrates (NFC), Neutral detergent fibre (NDF), energy content, macro-elements (( sodium (Na), phosphorus (P), magnesium (Mn), potassium (K and nitrogen (N)) and micro nutrients (Magnesium (Mn), Zinc (Zn), Cupper (Cu) and Iron (Fe)). The plants irrigated with 0.0072 M of K2SO4 without pruning showed a substantial improvement in floral bud, Neutral detergent fiber, fresh and dry weight of the shoots and roots, ash, as well as height. Contrarily, calcium and chlorophyll content were equivalent among treatments, but the treatment 0.0114 M of K2SO4, and 10 cm of pruning produced the maximum production of nitrogen, sodium, potassium and Zinc. The plants that were pruned by 10 cm and irrigated with 0.0072 M of K2SO4 had the maximum antioxidant value (Flavonol, DPPH and Polyphenols). The results suggest that T. divaricata is a viable leafy vegetable since a minimal concentration of K2SO4 (0.0072 M) and moderate cutting back the plants optimized its productivity in relation to the biomass characteristics, growth, antioxidant potential and nutritional content.