Histo-phytochemical evaluation and characterisation of the foliar structures of Tagetes minuta L. (Asteraceae).

Abstract

Plants have been used as ethnomedicine for millennia. In recent years, there has been an upward surge of interest in the use of plants as medicine due to the interest in drugs with fewer side effects as well as the fight against antibiotic resistance. This study is based on Tagetes minuta, an aromatic essential herb that is cultivated for its high percentage essential oils which have been used in the treatment of various ailments. In addition, T. minuta contains a myriad of secondary metabolites that serve in numerous industrial and clinical applications. The aim of this study was to characterise the foliar structures responsible for the production, storage, and exudation of these useful compounds, as well as to examine the chemical constituents of the crude organic solvents derived from the leaves of T. minuta. The potential for green synthesis of silver nanoparticles from the crude methanolic extract and its potential as an antibacterial was also determined. Stereomicroscopy and scanning electron microscopy revealed the presence of uniseriate non-glandular trichomes on the foliar surfaces, as well as large pellucid secretory cavities. Histochemical analyses on the non-glandular trichomes showed that they are capable of storing various bioactive compounds, which is a novel discovery for this species. The development of the subdermal secretory cavities show that the cells undergo autolysis in order to form a schizolysigenous cavity in mature leaves, which was revealed using light microscopy. The ultrastructure of the secretory epithelium within the secretory cavity was analysed using transmission electron microscopy, which displayed the changes of the plastids to contain lipid molecules as well as an increase in vesicles indicating the presence of essential oils. Phytochemical analysis on the crude organic solvents derived from the leaves of T. minuta revealed the presence of alkaloids, sterols, saponins, terpenoids, phenols, and lipids. Gas-chromatography mass-spectrometry was carried out to reveal that the constituents with the highest percentage were 9-octadecen-1-ol (4.51 %), β-sitosterol (6.07 %), olean-12-en-3-one (7.47 %), and 3-methyl-1-butanol (14.77 %), all of which cause bacterial growth inhibition, as well as showing acaricidal activity, and anticancer properties in studies focussed on clinical applications. Silver nanoparticles were successfully synthesised from the methanolic leaf extract, which was confirmed using UV-visible spectroscopy and energy dispersive x-ray analysis. UV–visible spectrum of synthesised silver nanoparticles showed maximum peak at 442 nm, and transmission electron microscopy revealed the silver nanoparticles to be spherical in shape, ranging from 10 to 50 nm in diameter. Preliminary antimicrobial activity was determined using the agar well diffusion method, which showed growth inhibition against E. coli, S. aureus, methicillin-resistant S. aureus, B. subtilis and P. aeruginosa. This study has shown that T. minuta contains numerous bioactive compounds that have pharmacological and medicinal uses, as well as characterising the non-glandular trichomes present on the adaxial and abaxial leaves for the first time. The synthesis of silver nanoparticles from the methanolic extract of T. minuta in this study is novel, and shows promise for cheaper, more effective, and less risky nanotechnological applications.