## 3 Discussion  
The chemical profile of four EOs from Pinaceae--A. alba, P. abies, P. cembra and P. mugo--tested for phytotoxic interactions with weeds was investigated by SPME-GC/MS. From a qualitative point of view, the EOs vapor phase was less rich in components than the respective liquid phase while, quantitatively, the percentage values of the more volatile compounds, i.e., the lower boiling temperature, prevailed. Nevertheless, in all samples, both phases were characterized by the prevalence of monoterpene compounds in accordance with the composition data of Pinaceae EOs [10; 11; 12; 13; 14; 15]. A wide variety of activities, including allelopathic activity, are attributed to monoterpenes [16]. In particular, they have a marked synergistic phytotoxic action when used in combination, supporting a greater effect of crude EOs than single components [17; 18; 19], some of which, however, have shown a remarkable ability to suppress weed germination and growth. For example, [PERSON] et al. [20] demonstrated that most of the 30 investigated monoterpenes possessed significant inhibitory activity with species-specific effects and that the oxygenated ones were more active than the hydrocarbon ones. Among the most effective compounds and with comparable phytotoxicity were limonene, myxcene, \(\alpha\)-pinene and \(\beta\)-pinene, the four main constituents of the EOs investigated herein. The weed-suppressing potential of limonene was later also assessed by [PERSON] and co-authors [21], who discovered that even at low concentrations, it was able to inhibit germination and seedling growth in terms of height, dry weight and root elongation of _Amaranthus viridis_ L. [PERSON] et al. [22] studied the impact of limonene on _Chloella vulgaris_ cell growth reporting that it could play allelopathic roles in cyanobacterial VOCs by reducing the photosynthetic capabilities of algae. Moreover, [PERSON] and collaborators [23] identified limonene as the second most abundant compound in the EO of _Dracoccephalus kotschyi_ [PERSON], capable of reducing seed germination and seedling growth of two important weeds such as _Amaranthus retroflexus_ L. and _Chenopodium album_ L. The phytotoxic activity of limonene and \(\beta\)-myrcene was further investigated, against both monocotyledonous and dicotyledonous weeds. The obtained data confirmed their potential as allelochemicals [17]. [PERSON] et al. [18] found \(\beta\)-myrcene as the most toxic constituent of _Artemisia scoparia_ [PERSON]. & [PERSON]. leaf EO, followed by \(p\)-cymene and limonene, suggesting that further exploration is warranted in terms of its phytotoxicity against weeds. On the other hand, [PERSON] [24] showed that the germination of _Echinochla crus-galli_ L. was not significantly affected by exposure to limonene and \(\beta\)-myrcene, which, however, consistently and proportionally inhibited its root development to the used concentrations. \(\beta\)-Myrcene was also identified as one of the major volatile constituents of _Artemisia frigida_ [PERSON]. released from fresh leaves crushed and tested for their allelopathic activity with positive feedback [25]. Recently, [PERSON] et al. [26] reported a strong phytotoxic activity of \(\alpha\)-pinene against _Elymus nutans_ Griseb., an important forage and ecological restoration plant species. This monoterpene was able to severely stress the target seedlings at very low concentrations. Different effects of pineine isomers against _Zea mays_ L. were also documented by [PERSON] et al. [27], observing a more incisive action of \(\beta\)-pinene compared to \(\alpha\)-pinene. The same information was previously recorded by [PERSON] et al. [28]. Theyalso verified that these monoterepenes may cause inhibition of cell proliferation in the root apical meristem of the test plants. Finally, high allebotathic activity was recorded for the EOs of _Prangos pabularia_ Lindl. leaves, whose main compound was _a_-pinene [29]. As a whole, the results of this work confirmed the efficacy of Pinaceae EOs as well as their chemical content.