## 1 Introduction
Nevertheless, for the pharmacological activity of _Sideritis_ species, terpenes and flavonoids seem to be the constituents that are most important. Other chemical components which have been identified within the genus are iridoids, coumarins, lignans, phenylpropanol glycosides and sterols [15]. Only three taxa of this genus are reported to be native to Croatia: _Sideritis romana_ L., _S. romana_ L. subsp. _purpurea_ (Talbot ex Benth.) [PERSON] and _S. montana_ L. [17]. While several reports on EOs of _S. romana_ L. [18; 19], _S. romana_ subsp. _purpurea_[20; 21] and _S. montana_ L. [18; 22; 23; 24; 25; 26] isolated by hydrodistillation have already been published, as well as that of _S. romana_ subsp. _purpurea_ EO constituents obtained by steam distillation [27], the composition of the headspace volatile components extracted using HS-SPME has never been evaluated for these species. Furthermore, to the best of our knowledge, the only report on using the HS-SPME technique for the detection of volatile components within the genus _Sideritis_ has been published for _S. ozturkii_ [PERSON] and [PERSON] [28]. Additionally, the volatile HS profiles of _S. scardica_ [PERSON] and _S. raseeri_ Boiss. and Heldr. were analyzed using GC-FID/MS [29], while EO composition of _S. albiflora_ Hub.-Mor. was analyzed by both HS GC-MS and thermal desorption GC-MS [30]. The aim of the present study was to compare the VOCs composition of the aerial parts of _S. romana_ L. and _S. montana_ L. populations from different parts of Croatia and to determine characteristic volatile compounds for each species. Having in mind that the composition of volatile compounds may vary depending on environmental conditions [31; 32], in comparison to similar studies conducted on hydrodistilled EOs of these species, which were based on the analysis of single populations [18; 19; 22; 23; 24; 25; 26], the advantage of our study is a greater number of investigated _S. romana_ (four) and _S. montana_ (two) populations. Considering that, on one hand, _Sideritis_ species are generally poor in essential oil, and the limited availability of plant material in natural habitats from which the samples had been collected, and, on the other hand, the multiple advantages of HS-SPME, which have been mentioned above, instead of the conventionally used hydrodistillation, the latter technique was chosen for the isolation of volatile components, which were further analyzed by using GC-MS, with the aim to obtain more reliable results. Previous studies have shown that hydrodistillation and HS-SPME can detect similar major volatile constituents of individual plant species, although their relative percentages may vary depending on the technique used [33]. Having in mind that HS-SPME may, among other things, depend on the type of fiber used for extraction [34], the volatile components were extracted by HS-SPME using two different fibers suitable for untargeted analysis [35], divinylbenzene/carboxylate/polydimethylsiloxane (DVB/CAR/PDMS) and polydimethylsiloxane/divinylbenzene (PDMS/DVB) fiber. To account for possible genetically and/or ecologically dependent variabilities in VOCs composition, several populations of _S. romana_ (four populations) and _S. montana_ (two populations) have been included in the analysis and principal component analysis (PCA) was conducted on the major volatile constituents (average relative percentage \(\geq\) 1.0% in at least one of the samples) obtained by each fiber to examine the interrelationships among the investigated populations. Furthermore, the obtained results have been compared to previous findings for the same two species that were mostly based on hydrodistillation. Finally, the VOCs profile was compared to related species and discussed considering the reported biological activities of major constituents. To our knowledge, this study is the first report on the VOCs profile of natural populations of _S. romana_ and _S. montana_ from Croatia.