Traits of seasonal dimorphism associated with adaptation to cold stress in Origanum dictamnus L. (Lamiaceae)

Abstract

Phryganic plants are herbs or subshrubs with erect shoots that grow at low altitudes under hot and arid environmental conditions. Many phryganic plants are aromatic/medicinal plants and display the phenomenon of seasonal dimorphism (Thymus sibthorpii, Satureja thymbra, Cistus incanus, Phlomis fruticosa, Teucrium polium, etc.). Phryganic plants of the Mediterranean region are reported so far to be affected by summer drought stress. However, in the present study, the phryganic plant Origanum dictamnus appears to be affected by winter cold stress rather than by summer drought stress in the specific area. This triggered the investigation of the functional and structural traits associated with adaptation of O. dictamnus to cold stress. Winter plants of O. dictamnus in order to stand up to the cold, shed all of their large leaves and keep only the small apical leaves, which are covered with a thick insulating layer of dendroid non-glandular hairs. Epidermal and mesophyll cells of apical leaves contain antioxidant vacuolar phenolics protecting from oxidative stress due to low temperatures. Protection from oxidative stress is also provided by the antioxidant essential oils secreted by the numerous glandular hairs. Concentrations of stress indicators like proline and soluble sugars, are always higher in winter leaves compared to summer leaves. Principal components of the O. dictamnus essential oil are p-Cymene, Carvacrol, γ-Terpinene, and Borneol. The major constituent of the winter oil is p-Cymene (59.2%) and that of the summer oil Carvacrol (42.4%). The presented results provide a structural and functional elucidation for the largely deviating appearance of winter and summer plants of O. dictamnus, and favour the interpretation of adaptation of the plant to cold stress.