Review: Marsh rosemary (Rhododendron tomentosum Harmaja (ex Ledum palustre Linn) growing in Lithuania) essential oils and their properties

The paper reviews already (in the past fifteen years) published data from research articles on essential oils (EOs) and their biological properties (such as antioxidant, antifungal, anti-inflammatory and toxic activity) of marsh rosemary (Rhododendron tomentosum (Stokes) Harmaja (ex Ledum palustre Linnaeus, Ericaceae Juss.) growing wild in Lithuania. Rh. tomentosum is a perennial woody shrub (up to 1 m in height) with evergreen leaves and small white or white-pink sticky flowers grouped in racemes. The plant emits a strong specific smell that affects the central nervous system, and may cause nausea, headache or aggressive behaviour to some people. Only one species of the plant (ex L. palustre var. palustre) grows in Lithuania, mainly forming colonies in limited areas over all the territory. Marsh rosemary are widely used in folk medicine and homeopathy for treatment of various sickness, externally and internally as well. Most applications of Rh. tomentosum and pharmacological properties have been validated by scientific researches. In Lithuania, the plant is used for healing rheumatism, different pains, insect bites, eczema and other skin problems, infections, bronchitis, asthma, cold, tuberculosis, to block bleeding, etc. Rhododendron tomentosum H. (ex Ledum palustre L.) plants, the essential oils (EOs) of which are already investigated, were collected mostly in the Eastern part of the country (Rokiškis, Utena, Vilnius and Šalčininkai districts). Monoterpene hydrocarbons, p-cymene, myrcene and limonene, bicyclic monoterpenoid ascaridole and oxygenated sesquiterpenes, ledol, palustrol and cyclocolorenone isomers, were found to be principal compounds in the investigated EOs. Some oils contained appreciable quantities of heterocyclic compound lepalol. Most of the oils could be attributed to the ledol+palustrol or ledol+palustrol+ascaridole chemotype. Antifungal activity of Rh. tomentosum EOs was evaluated by several different techniques: against Penicillium cyclopium Westling, Trichoderma harzianum Rifai and Candida parapsilosis using an agar diffusion method; and amperometricaly, using Saccharomyces cerevisiae yeast-modified electrodes. Anti-inflammatory activity of Lithuanian marsh rosemary EOs has been revealed by subcutaneous carrageenan injection-induced hind paw oedema tests. However, antioxidant activity (tests using ABTS˙+, DPPH˙ and TROLOX) and toxic properties (against brine shrimp (Artemia sera) larvae) of Rh. tomentosum EOs were summarized.