The effect of different growth media and salinity on the vegetative growth of Trachyandra ciliata (Wild cabbage) in hydroponics

Abstract

Water scarcity and increasing salinization of agricultural lands will pose a huge threat to food production in the future. Increasing agricultural production, therefore, becomes a challenge to keep up with the increasing population and sustain nutrition and food security. The Western Cape Province of South Africa has been the most affected by water scarcity and it has been predicted that in a few years, this province will be unable to supply water for its agricultural needs. This phenomenon necessitates the development of more innovative techniques and the cultivation of halophytes to enhance sustainable crop production. Trachyandra ciliata is a halophytic plant with limited literature and it belongs to the Asphodelaceae family that is popular for the medicinal properties of its members. The existing literature suggests that it is edible but there is no scientific evidence of its biochemical properties and nutrient content. Hydroponically grown plants use 10 times less water than conventionally grown plants because in soil-grown plants water quickly leaches to the soil, while water is collected and circulated again in hydroponic cultivation. The aim of this study was to determine the influence of salinity and soilless media on the vegetative growth, phytochemical and antioxidant capacity of hydroponically grown T. ciliata under a greenhouse environment, in order to determine the growth protocol for this plant. Four identically constructed Nutrient Film Technique (NFT) systems were used, with each system on a separate metal mesh steel table (2.5 m long). NutrifeedTM served as a general hydroponics feed while different salt concentrations of Sodium chloride (NaCl) (100mM, 200mM, 400mM of NaCl) were used as treatments added into each sump, while nutrient solution without addition of NaCl was considered as control. Twenty treatments were evaluated with 10 replicates per treatment. All nutrient solutions containing NaCl were replaced and the system flushed every week to avoid the buildup of salts in the medium, pots, gutters, and reservoirs. The treatments were comprised of 3 different soilless media; Leca clay (LC), silica sand (SS), and a combination of peat: perlite: vermiculite (PPV) at a 1:1:1 ratio. A standard laboratory scale was used to measure the weight and the shoot length was manually using a standard measuring tape. The chlorophyll content of the plants was determined using a SPAD-502 Konica-Minolta meter. Antioxidant content and capacity of metabolites within the mixture of dried and ground roots and shoots were assessed 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). Salinity significantly (P≤0.05) affected shoots wet weight, root wet weight, shoot dry weight, and both total wet and dry weight. The highest mean values for shoots wet weight, root wet weight, and total wet weight were observed at 100 mM salinity level, while high salinity levels recorded the lowest mean values. Growth media had no significant effect on biomass accumulation. Both salinity and soilless media showed a significant effect on shoot length and number of leaves with a combination of 100mM+PPV recording the highest mean values. Both salinity and soilless media significantly affected the inflorescence weight and the number of flowers with 100mM+PPV interaction recording the highest significant mean values. The chlorophyll content was significantly affected by salinity while soilless media did not affect chlorophyll accumulation. 0mM and 100mM recorded the highest SPAD values although there was no significant effect between the treatments. Salinity-induced oxidative stress and soilless media are significantly (P≤ 0.05) effective for the accumulation of polyphenols. The interaction of moderate (20mM) and PPV recorded the highest significant mean values for polyphenols, FRAP, ABTS and DPPH. The combination of 100mM+PPV recorded the highest significant mean value for the accumulation of flavonols. It was concluded that low salt levels (100mM) promote biomass accumulation while high salt levels (400mM) are toxic and decrease growth significantly. Soilless media has no significant effect on plant biomass except for the inflorescence, in which the combination of 100mM+PPV is recommended. Moderate salt levels (200mM) in conjunction with PPV are recommended for the accumulation of polyphenolic compounds(antioxidants) except for flavonols, where 100mM is recommended. It was concluded that moderate salt levels may not promote biomass accumulation but effective for antioxidant content.