## 2 Phytotoxic Effects of Volatile Terpenes from the Lamiaceae Family  
Terpenes are a widespread group of specialized metabolites, with hydrocarbon structures consisting of five carbon units--isoprene units (CH2=C(CH3)CH=CH2)--that can be converted into cyclic forms [42]. Two key C\({}_{5}\) isopenroid precursors, isopentenyl diphosphate (IPP) and its double bond isomer dimethylallyl diphosphate (DMAPP), play a central role in terpene biosynthesis [43; 44]. In plants, these precursors are synthesized via two different metabolic pathways: the cytosolic mevalonate (MVA) pathway, which originated from acetyl-CoA, and the plastidial 2-C-methyl-D-erythnit 4-phosphate pathway (MEP), which is derived from pyruvate and glyceraldehyde 3-phosphate [45; 46]. Through a series of enzyme-directed reactions involving rearrangements, elongations, and cyclizations, IPP and DMAPP are converted into different classes of terpenes [47; 48; 49]. Terpenoids are chemically modified terpenes that often contain oxygenated groups such as hydroxyl (-OH), carboxyl (-COOH), ketone (-C=O), or aldehyde (-CHO) groups. These modifications provide them with greater chemical diversity and various bioactive properties [50]. According to their isoprene units, terpenes are divided into hemiterpenes (one unit, C\({}_{5}\)), monoterpenes (two units, C\({}_{10}\)), sesquiterpenenes (three units, C\({}_{15}\)), diterpenes (four units, C\({}_{20}\)), triterpenes (six units, C\({}_{30}\)), and tetraterpenes (eight units, C\({}_{40}\)) [51]. Geranyl pyrophosphate synthase (GPPS) and farnesyl pyrophosphate synthase (FPPS) catalyze the production of geranyl pyrophosphate (GPP), a precursor for C\({}_{10}\) monoterpenes, and farnesyl pyrophosphate (FPP), the precursor for C\({}_{15}\) sesquiterpenenes. Similarly, geranyl-geranyl pyrophosphate synthase (GGPPS) and farnesyl geranyl pyrophosphate synthase (GGPP) facilitate the formation of geranyllgeranyl pyrophosphate (GGPP), a precursor for C\({}_{20}\) diterpenes, and farnesyl geranyl pyrophosphate (FGPP), which leads to C\({}_{25}\) sesquiterpenes. These intermediates (GPP, FPP, GGPP, and FGPP) serve as substrates for the terpene synthase enzyme family, which catalyzes a series of cyclization and rearrangement reac  
Figure 1: Cultivated and weed species used to assess the phytotoxic effects of neeptalactone or phloretin. tions that diversify the structural and functional properties of terpenoids and contribute to the wide range of terpenoid classes in nature [1; 52; 53]. Monoterpenes and sesquiterpenes, along with their oxygenated derivatives, are VOCs that significantly contribute to the distinctive aroma and olfactory profile of plants. Species within the Lamiaceae family are especially abundant in these compounds, which are integral to the characteristic fragrances exhibited by plants in this family. The Lamiaceae family comprises more than 250 genera and about 7000 species [54], which are widely used in the pharmaceutic, cosmetic, and food industries due to their antioxidative, antibacterial, antifungal, antiviral, cardioprotective, anti-inflammatory, antidiabetic, and anticancer properties [55; 56; 57; 58; 59; 60; 61]. In addition to numerous pharmacological and biological activities, species from the Lamiaceae family also exhibit strong phytotoxic effects against various target plants, which are attributed to the monoterpenes and their derivatives contained in the EOs (Figure 2) [62; 63; 64; 65]. The monoterpenes have been described as the main constituents of the EOs of eight Mediterranean species from the Lamiaceae family (_Hysospus officinalis_ L., _Lavandula angustifolia_ Mill., _Majorana hortensis_ Moench., _Melissa officinalis_ L., _Orimum basilicum_ L., _Origanum vulgare_ L., _Salvia officinalis_ L., and _Thymus vulgaris_ L.). The EOs of _M. officinalis_ and _T. vulgaris_ showed a strong phytotoxic effect on the seed germination and growth of the radicules of _Raphanus sativus_ L., _Lactuca sativa_ L., and _Lepidium sativum_ L. via different mechanisms [66].