Crop yields, secondary metabolites and anti-fungal activities of extracts of Helichrysum cymosum and Helichrysum odoratissimum cultivated in aquaponics

Abstract

The growing demand for herbal remedies has resulted in the significant over-harvesting of wild plants, exacerbating the problem of species extinction. This study evaluates two cultivation approaches that could be explored to cultivate medicinal plants. Production crops in aquaponics, which is a system that combines the growing of fish (aquaculture) and soilless cultivation of plants (hydroponics) under controlled conditions, is an alternative, innovative approach. The plants recycle the fish wastes from the fish tank by using up the nutrients in the wastewater for growth, and the clean water is then recirculated in a closed system. Hydroponics is another cultivation method widely used in the precision cultivation of vegetables. It minimizes water wastage and optimises the fertilization of plants. Thus far little evidence exists of the use of these technologies for cultivation of medicinal plants. Helichrysum odorattisimum and Helichrysum cymosum, which belong to Asteraceae, are among the most sought-after medicinal plant species in South Africa. Herbal remedies are widely used and are becoming more popular. The harvesting of medicinal plant materials from wild populations is destroying their genetic diversity and their habitats. Helichrysum spp. are popular for their therapeutic values in traditional medicine. Extracts from numerous species of the same genus have been used to treat skin diseases, respiratory disorders, and circumcision procedures. This study was conducted to evaluate the antifungal activities, secondary metabolite content, and crop yields of Helichrysum cymosum and Helichrysum odoratissimum extracts cultivated in aquaponic and hydroponic systems to assess the prospects of using both systems in the sustainable cultivation of medicinal plants. To evaluate the effectiveness of the aquaponic and hydroponic systems, plants were grown using three cultivation methods: aquaponic, hydroponic and field systems, and the data on antifungal and antioxidant activities, secondary metabolite content, and crop yield were compared. Six-week-old H. odorattisimum and H. cymosum seedlings were cultivated in an aquaponic (T1) and hydroponic systems (T2). Field-growing plants (T3), which were planted three months earlier were also screened for antifungal and antioxidant activities and secondary metabolite content. The bioactive chemicals and antifungal activities of the plants cultivated in these two systems were then compared to plants grown in the field (T3). In the aquaponic system, plants were cultivated on silica sand in a deep culture system and Carassius auratus (goldfish) produced the fish wastes. In the hydroponics, plants were cultivated in a substrate blend of Coco coir, vermiculite, and perlite as substrates at a ratio of 2:1:1, respectively and supplied with a hydroponic fertilizer (Nutrifeed®). Fifteen replicates of each species (H. odoratissimum and H. cymosum) seedlings were used in this study for both treatments. The results for H. odoratissimum yield (plant height, fresh and dry weights) and the tissue nutrient contents did not change substantially (p > 0.05) between aquaponic and hydroponic treatments. Gas chromatography–mass spectrometry (GC–MS) analysis showed that monoterpenes and sesquiterpenes were the most abundant compounds in H. odoratissimum; however, no statistical difference was observed among the field, hydroponic, and aquaponic plants (DF = 2; χ2 = 2.67; p > 0.05). While there was no significant difference in polyphenol contents among the three treatments, remarkably, the flavonol contents in the leaves varied significantly (DF = 2; χ2 = 6.23; p < 0.05) among the three treatments. A higher flavonol content occurred in leaves from the hydroponic system than in leaves from the aquaponic (p < 0.05) and field (p > 0.05) systems. The MIC results showed that the ethanolic extract of H. odoratissimum was fungistatic against F. oxysporum; however, this effect was more prominent in the ethanol extracts of plants grown in the aquaponic system, with a mean MIC value of 0.37 ± 0.00 mg/mL. Helichrysum cymosum plants did not vary significantly (DF=1,28; χ2 =1.63; p = 0.21) in plant heights which ranged from (22.83 ± 1.59 cm – 26.46 ± 0.94 cm) and dry weight among the treatments; however, T2 produced slightly better results. The fresh weight of H. cymosum differed significantly (p < 0.05) between the treatments. The results showed that there was no significant difference between aquaponics and hydroponics treatments on crop yields (plant height, fresh and dry weight). Interestingly, there were significant differences (DF = 2; χ2 = 19.76; p = 0.00) in total polyphenol contents between the three cultivation techniques; however, the field-collected plants yielded higher polyphenol contents (452.10 ± 53.37 mg GAE/g) than hydroponics (433.49 ± 11.95 mg GAE/g) followed by aquaponics (136.46 ± 42.09 mg GAE/g). The flavonol contents differed significantly between the three cultivation techniques (DF = 2; χ2 = 6.31; p = 0.03), with higher flavonol contents in the field-collected plants (250.62 ± 58.12 mg QE/g), followed by hydroponics (164.05 ± 14.89 mg QE/g) and aquaponics (71.60 ± 14.45 mg QE/g). The volatile chemical contents did not differ significantly according to the gas chromatography-mass spectrometry (GC-MS) analysis (DF = 2; χ2 = 3.53; p = 0.17; p > 0.05) among the three treatments. However, plants grown in aquaponics had a higher number of compounds (106) than those grown in hydroponics (104) or field plants (103). The antifungal bioassay showed that the ethanol extracts of H. cymosum harvested from the field also had a higher fungistatic activity against F. oxysporum. The highest antioxidant capacity was obtained in plants cultivated in hydroponics followed by field-collected plants although there was no statistical difference among the treatments. In conclusion, aquaponics and hydroponics performed better or similar to field cultivation and are viable alternative methods for cultivating H. odoratissimum plants. The results of the present study suggest that commercial cultivation of H. odoratissimum and H. cymosum using hydroponic and aquaponic systems may be feasible.