Effects of chilling stress on leaf morphology, anatomy, ultrastructure, gas exchange, and essential oils in the seasonally dimorphic plant Teucrium polium (Lamiaceae)

The Mediterranean region (and globally also other regions) is characterized by the presence of phryganic plants, i.e. subshrubs that grow under hot and arid environmental conditions. These plants are reported to be affected by summer drought stress. However, in the present study the phryganic plant Teucrium polium (mountain germander) appears to be affected by winter chilling stress rather than by summer drought stress in a specific area. Winter leaves of the plant are smaller and thicker compared to summer leaves, have more stomata and glandular hairs, and their chloroplasts are larger, more numerous, with voluminous starch grains. Moreover, epidermal and mesophyll cells of winter leaves contain in their vacuoles dark phenolics and calcium oxalate crystals. Summer leaves are devoid of vacuolar phenolics and their chloroplasts possess many large plastoglobuli. Leaf gas exchange parameters (photosynthesis, transpiration, stomatal conductance) are significantly higher in winter leaves. Concentrations of osmoprotectors (stress indicators) like proline and soluble sugars are similarly higher in winter leaves. Essential oil assessments showed a significantly higher oil yield of winter leaves compared to summer leaves. Percentages of the major oil components (linalool, terpinen-4-ol, germacrene D, and spathulenol) are remarkably higher in winter oils than in summer oils. In conclusion, low environmental temperatures (1-10 A degrees C) appear to decisively influence the structure and function of winter leaves compared to summer leaves. Winter plants undergo chilling stress to which they respond by developing various mechanical and chemical defensive strategies.