Toxic, Radical Scavenging, and Antifungal Activity of Rhododendron tomentosum H. Essential Oils

Abstract

The chemical composition of eight (seven shoot and one inflorescence) essential oils (EOs) of Rh. tomentosum H. plants growing in Eastern Lithuania is reported. The plant material was collected during different phases of vegetation (from April to October). The oils were obtained by hydrodistillation from air-dried aerial parts (leaves and inflorescences). In total, up to 70 compounds were identified by GC-MS and GC (flame-ionization detector, FID); they comprised 91.0 ± 4.7%-96.2 ± 3.1% of the oil content. Sesquiterpene hydrocarbons (54.1 ± 1.5%-76.1 ± 4.5%) were found to be the main fraction. The major compounds were palustrol (24.6 ± 2.6%-33.5 ± 4.4%) and ledol (18.0 ± 2.9%-29.0 ± 5.0%). Ascaridol isomers (7.0 ± 2.4%-14.0 ± 2.4% in three oils), myrcene (7.2 ± 0.3% and 10.1 ± 1.3%), lepalol (3.3 ± 0.3% and 7.9 ± 3.0%), and cyclocolorenone isomers (4.1 ± 2.5%) were determined as the third main constituents. The toxic activity of marsh rosemary inflorescence and shoot oils samples was evaluated using a brine shrimp (Artemia sp.) bioassay. LC₅₀ average values (11.23-20.50 µg/mL) obtained after 24 h of exposure revealed that the oils were notably toxic. The oil obtained from shoots gathered in September during the seed-ripening stage and containing appreciable amounts of palustrol (26.0 ± 2.5%), ledol (21.5 ± 4.0%), and ascaridol (7.0 ± 2.4%) showed the highest toxic activity. Radical scavenging activity of Rh. tomentosum EOs depended on the plant vegetation stage. The highest activities were obtained for EOs isolated from young shoots collected in June (48.19 ± 0.1 and 19.89 ± 0.3 mmol/L TROLOX (6-hydroxy-2,5,7,8-tetra-methylchromane-2-carboxylic acid) equivalent obtained by, respectively, ABTS^•+ (2,2'-amino-bis(ethylbenzothiazoline-6-sulfonic acid) diammonium salt) and DPPH^•(2,2-diphenyl-1-picrylhydrazyl) assays). Agar disc diffusion assay against pathogenic yeast Candida parapsilosis revealed the potential antifungal activity of EOs. An alternative investigation of antifungal activity employed mediated amperometry at yeast Saccharomyces cerevisiae-modified electrodes. The subjection of yeast cells to vapors of EO resulted in a three to four-fold increase of electrode responses due to the disruption of yeast cell membranes.

Keywords: Ericaceae; Rhododendron tomentosum H.; amperometry; antifungal activity; antioxidant tests; essential oil composition; toxicity in vivo.

Figure 1

Inhibition of C. parapsilosis growth by Rh. tomentosum essential oil (EO) (8 Sh) (a) and nystatin (b).

Figure 2

Scheme of amperometric measurements.

Figure 3

Amperometric responses of yeast-modified electrodes to 0.2 mmol/L L-lactic acid: not affected by vapors (short-dashed line), after 20 min (long-dashed line) and after 40 min (solid line) of subjection to vapors of Rh. tomentosum EO (8 Sh); phosphate buffer pH 7.3 contained 0.5 mmol/L K₃[Fe(CN)₆]; operating potential 0.3 V.

Figure 4

Dependence of yeast-modified electrode responses to 0.2 mmol/L L-lactic acid on the time of electrode subjection to vapors of Rh. tomentosum EO (8 Sh), phosphate buffer pH 7.3 contained 0.5 mmol/L K₃[Fe(CN)₆]; operating potential 0.3 V.