The effect of light intensity on the growth and anti-pest activities of leek (Allium porrum) against the grapevine mealybug (Planococcus ficus) and a phytopathogenic fungus (Fusarium oxysporum)

Abstract

Pest infestation is a disturbing factor in the agricultural industry. Grapevine mealybug (Planococcus ficus) and fusarium wilt, (Fusarium oxysporum) are the major pests of economic importance. Control of these pests mainly relies on chemical-based pesticides. However, synthetic pesticides pose severe threats to human health and the environment. Hence, the search for safer alternative pest control measures is intensifying. Allium porrum (Alliaceae) is a medicinal plant of the Allium genus extensively recognized to contain many important bioactive agents with remarkable biological activities. Research focusing on optimizing yield and modification of bioactive constituents is intensifying. This study was designed to determine the effects of varying light intensities on plant growth, bio-screen insecticidal, and antifungal activity of A. porrum and to establish the optimum light intensity level for enhancing yield and medicinal properties. The study was divided into two parts to address these objectives. The first part focused on the physiology of of the plant and is presented in chapter two. The second part, covered in chapters 3 and 4, focused on the evaluation of the effect of light intensities on the volatile constituents, antifungal and anti-insect activities of A. porrum extract cultivated under greenhouse conditions. The results of the study demonstrated varied effects of light intensities on plant growth parameters. Seasonal changes significantly influenced plants subjected to low light and high light intensities. Aerial-part height was significantly (P < 0.05) increased with low light intensity, whereas number of leaves, fresh and dry weights decreased under low light intensity. The association between light intensity and tissue nutrients were examined closely; shading elicited a significant (DF = 6; P < 0.05) positive response in N, P, and Ca accumulation in the plant tissue. This finding suggests that decreased light intensity favoured the growth of A. porrum in height by enhancing macronutrient uptake. In the second part of the study, presented in chapter three, the seedlings of A. porrum were hydroponically grown under low light (40% shade) and high light intensity (0% shade) in 3 different seasons, each running for 12 weeks. The phytochemical analyses of constituents in dry aerial parts of A. porrum showed that total polyphenol content was statistically higher (DF = 1, 6; F = 9.17; P < 0.05) in plants exposed to reduced light intensity than in those subjected to high light intensity at 12 weeks post-treatment. Following the gas chromatography mass spectrometry (GC-MS) analysis, the number of known antifungal and anti-insect volatile compounds plant constituents did not vary significantly, however, higher number of compounds occurred in plants subjected to low light intensity (DF=1; ꭓ2 =0.44; P > 0.05). The acetone extracts A. porrum subjected to lower irradiance exhibited fungistatic effects against F. oxysporum. In the grapevine mealybug repellency bioassay, there was no significant difference in different solvent extracts of A. porrum subjected to low light and high light intensity at diffent concentrations. The insect repellency tended to be higher at higher concentrations irrespective of the solvent. This study's key findings are: low light intensity has positive effects on volatile constituents, total polyphenol content, fungistatic activities of extracts of A. porrum. Broadly, the results obtained in this study will contribute to the literature of growth, physiological responses, and nutrient content of A. porrum subjected to varying light intensities. This study also contributes to the present knowledge on the effect of light irradiance on plant growth and physiology and the biosynthesis of bioactive compounds. This study establishes new protocol for optimised cultivation of leek plants with increased medicinal values. Furthrmore, it opens up new market opportunities to improve profits for farmers.