Evaluating the effect of 40% and 0% shading levels on the secondary metabolites, antifungal and anti-insect activities of extracts of Allium porrum cultivated hydroponically under greenhouse conditions

Abstract

Background: Allium species are generally reputed for their anti-pest properties; however, few studies have focused on optimising the quality and yield of anti-pest bioactive materials from these medicinal plants. Setting: Laboratory and greenhouse experiments were carried out on the Bellville campus of the Cape Peninsula University of Technology, Cape Town. Aims: This research study aimed to evaluate the effect of light intensity on the volatile constituents, antifungal and anti-insect activities of extracts obtained from Allium porrum L. cultivated hydroponically under greenhouse conditions. Methods: Seedlings of A. porrum were hydroponically grown under 40% shading and 0% shading conditions for 12 weeks. The phytochemical constituents of the aerial parts (leaf and bulb) of A. porrum were analysed. The antifungal activity against Fusarium oxysporum and the anti-insect activity on the grapevine mealybug (Planococcus ficus) was evaluated by microdilution and repellency bioassays, respectively. Results: Remarkably, the total polyphenol content was statistically higher (DF = 1, 6; F = 9.17; p < 0.05) in plants exposed to 40% shade treatment. The gas chromatography-mass spectrometry (GC-MS) analysis revealed that the volatile compounds varied significantly (DF = 1; χ2 = 3.435; p > 0.05) between the two treatments. However, although a higher number of compounds (73) occurred in plants exposed to 40% shade than in those exposed to 0% shade (58), the shading effect on the number of compounds was not significant (DF = 1; χ2 = 69.551; p > 0.05). The acetone extracts of A. porrum that were cultivated under lower light irradiance showed a higher fungistatic activity against F. oxysporum in the antifungal bioassay. Conclusion: Broadly, this study revealed that lowering light intensity from 313 µmol m-2 s-1 to 153 µmol m-2 s-1 favoured a higher phenolic content, volatile constituents and higher anti-F. oxysporum activities in leeks.