Variation of volatiles in Tunisian populations of Thymbra capitata (L.) Cav. (Lamiaceae)

Volatiles from 14 wild Tunisian populations of Thymbra capitata (=Thymus capitatus Hoffmanns. et Link=Coridothymus capitatus Rchb.f.), sampled in five ecological areas (sub-humid, upper semi-arid, mean semi-arid, lower semi-arid, and upper arid areas) were analyzed by gas chromatography (GC) and gas chromatography/mass spectrometry (GC/MS) techniques. Thirty-nine constituents representing 94.2 to 99.5% of the total oil were identified. Carvacrol (38; 51.1-75.9%), p-cymene (13; 3.7-15%), γ-terpinene (12; 1.4-11.9%), and trans-β-caryophyllene (22; 2.9-4.6%) are the major compounds. A significant variation among populations and population's bioclimatic stage for the majority of compounds was shown. The chemical population structure, estimated using a principal-component analysis (PCA) and an unweighted pair group method with arithmetic mean (UPGMA) cluster analysis performed on all populations and compounds and based on Euclidean distances among populations, was high. Both methods allowed separation of the populations into distinct groups defined rather by minor than by major compounds. The spatial compound distribution is linked to ecological factors, indicating that local selective environmental factors influence chemical-composition diversity. Conservation strategies should involve all populations because of their low size and their high level of destruction. Populations exhibiting particular compounds other than the major ones should first be protected. In situ conservation of populations should be accomplished appropriately according to bioclimate.