Phytochemical composition, characterization and termiticidal activity of Acacia tortilis heartwood extracts

Abstract

Termites are commonly known as “silent destroyers” as they have the ability to attack wood undetected. Globally, the economic loss incurred from termites’ destruction in the agricultural sector is approximately US $ 50 billion annually. In Kenya, termites cause 11-20 % crop yield losses and synthetic chemicals used in control of termites’ infestation are non-eco-friendly contributing to lots of environmental problems. This study therefore aimed at investigating phytochemical composition and anti-termiticidal properties of Acacia tortilis heartwood extracts. The specific objectives were to (1) determine the phytochemical composition of A. tortilis heartwood extracts, (2) characterize A. tortilis heartwood extractives using spectroscopic methods and (3) evaluate termiticidal activity of A. tortilis heartwood extracts. The ground heartwood samples were extracted using hexane, ethyl acetate (EtOAc), acetone and ethanol solvents by the Soxhlet method. Phytochemical screening of these extracts was done using standard methods. Characterizations of the extractives were done using Ultraviolet-Visible (UV-Vis), Gas chromatography Mass spectroscopy (GC-MS), Liquid Chromatography-Photodiode-array-Mass spectroscopy (LC-PDA-MS) and Fourier transform Infrared (FTIR). To determine termiticidal activity, wood blocks of A. tortilis and Pinus sylvestris measuring 10 cm × 1.5 cm × 0.5 cm (longitudinal × radial × tangential) were prepared. Extracted, unextracted wood blocks of A. tortilis (negative control) and Pinus sylvestris (positive control) was used to provide evidence if the experimental setup was working. Wood blocks were randomly exposed to Macrotermes natalensis for 5 months at Cheptebo in Kerio valley, Kenya. Phytochemical screening showed presence of phenols, tannins, flavonoids, cardiac glycosides, terpenoids and alkaloids. UV-Vis spectra showed presence of chromophores due to peaks at 338, 338, 339 and 341 nm. Total phenolic content (TPC) was of the order 10.7 ± 0.10 < 157.6 ± 0.06 < 169.3 ± 0.15 < 350.1 ± 0.10 mg GAE/g for hexane, EtOAc, acetonic and ethanolic extracts. Total flavonoid content (TFC) was of same order of solvent polarity as mentioned in TPC; 0.7 ± 0.06 < 14.7 ± 0.15 < 19.2 ± 0.15 < 21.1 ± 0.17 mg QE/g respectively. GC-MS with NIST library revealed presences of catechin, epicatechin and pinitol in acetonic, EtOAc and ethanolic extracts. Pinitol was the prevalent compound exhibited in ethanolic (84%), acetonic (59%) and EtOAc (28%). Fatty acids and terpenoids were also identified in EtOAc and hexane extract. LC-PDA-MS, further confirmed the presence of catechin with charge/mass ratio (m/z) of 289 with maximum wavelength of 278 nm. Afzelechin was observed with m/z of 273. Two compounds with m/z of 817 and 801 were identified as hydrolysable tannins. FTIR analyses showed peaks between 3254 - 3272 cm -1 (OH stretch), 1701 cm -1 (C=O stretch) and 1604, 1513 and 1445 cm -1 due to vibration of C=C of aromatic skeleton. The weight loss of unextracted A. tortilis heartwood blocks was 0.4 % and P. sylvestris (100%) after 5 months of exposure. Weight losses of extracted A. tortilis blocks were dependent on the extractive; hexane (1.9%), EtOAc (3.7%), acetone (3.9 %) and ethanol (4.8 %). In conclusion, A. tortilis contains useful secondary metabolites and exhibited anti- termiticidal activity which the study attributes to the identified compounds. The study recommends that isolation of identified compounds from A. tortilis and testing the termiticidal activity of isolated compounds to be done.