Glandular and non-glandular hairs in the seasonally dimorphic Origanum dictamnus L. (Lamiaceae) as a means of adaptation to cold stress

Abstract

<em>Origanum dictamnus</em> is a seasonally dimorphic plant having different appearance in winter and summer. Shoots of winter plants are leaf-naked except for their apical region which bears a cluster of small leaves covered with a thick indumentum of non-glandular hairs. This indumentum highly contributes to the avoidance of cold penetration into the leaf mesophyll, allowing thus plants to endure winter low temperatures. Shoots of summer plants are vigorous with large green leaves. Non-glandular hairs are dendroid with a 5-celled vertical stub and several lateral branches. Glandular hairs are of two types, large peltate hairs and small capitate hairs. Peltate hairs are numerous and consist of a 12-celled head, a unicellular stalk, and a basal epidermal cell.. They constitute the sites of essential oil secretion. Capitate hairs occur in a small number and are composed of a unicellular head, a unicellular stalk and a basal epidermal cell. They are not secreting essential oil, but a hydrophilic material. The oil secreted by the peltate hairs has antioxidant properties, opposes the oxidative stress resulted from low temperatures, and contributes to the adaptation of the plant to winter cold stress.