## 2 Phytotoxic Effects of Volatile Terpenes from the Lamiaceae Family
### Phytotoxic Effects of Monoterpenes from the Genus Nepeta
Earlier studies on _Nepeta_ species have revealed organ-specific expression patterns of genes involved in iridoid biosynthesis [86; 91; 92]. Closed buds and open flowers of _N. cataria_ and _N. mussinii_ exhibited higher transcript levels of _NEPS_, _GSH_, and _ISY_ genes compared to mature leaves [86]. Among the _NEPS_ genes, _NEPS1_ showed the highest expression in the open flowers and closed buds of _N. cataria_, while _NEPS5_ transcripts weremost abundant in the closed buds of _N. mussini_[86]. In _N. nuda_, genes responsible for the steps directly preceding the formation of nepetalactol and NL (_NtnNEPS1_, _NtnNEPS2_, _NtnNEPS3_, and _NtnMLPL_) showed significantly higher expression levels in infloreserences than in leaves [84]. Additionally, a major latex protein-like (MLPL) enzyme catalyzes the formation of the _cis,trans_- isomer [86]. Enzymes such as NEPS, NEPS5, and NEPS1 subsequently oxidize these nepetalactols into NL [86; 89]. In species that do not have detectable amounts of iridoids in their leaves, such as _N. nervosa_, there is likely a silent biosynthetic platform for iridoids [92]. It is assumed that some components are missing in the iridoid biosynthetic pathway of _N. nervosa_, which impairs the biosynthesis of iridoids. This includes the absence of GES-like activity due to gene silencing, leading to the absence of both iridoid aglycones and iridoid glycosides, as well as the essential role of other NEPS and MLPL enzymes, in addition to NnNEPS1 and NnMLPL, in the biosynthesis of iridoid aglycones [92]. It has been well-documented that EOs of _Nepeta_ species have shown strong inhibitory effects on seed germination and seedling growth in different target species. Table 1 provides a detailed summary of the phytotoxic activity of the EOs of different _Nepeta_ species, highlighting the dominant compounds detected in the EOs, the plant species tested, the experimental design with the doses of the EOs or dominant compounds used, and the phytotoxic activities observed. [PERSON] et al. [40] showed that treatment with _N. tranjensis_ EO affected various physiological and metabolic processes in _A. thaliana_. [PERSON] et al. [93] reported that _N. meyeri_ EO completely suppressed the seed germination of four weed species: _Amaranthus retroflexus_ L., _Chenopodium album_ L., _Crisium arvense_ (L.) Scop., and _Sinapis arvensis_ L. The most sensitive weed to _N. mayeri_ EO spraying was _A. retroflexus_. [PERSON] et al. [94] demonstrated that the weed _Taraxacum officinal_ was the most affected by _N. catararia_ EO, while the crop species _Lepidium sativum_ was the most tolerant. In addition, [PERSON] et al. [95] found that _N. meyeri_ EO inhibited seed germination and altered the antioxidative activity of seven weed species, with _Bromus danthoniae_ [PERSON], _Bromus intermedius_ [PERSON], and _Lactuca serriola_ L. being the most perceptive. Furthermore, [PERSON] et al. [96] examined the phytotoxic properties of EOs from _N. gluccephalata_ [PERSON] and _N. ispahanica_ [PERSON]. on four weed species, finding that _A. retroflexus_ and _C. album_ were the most affected. In addition, [PERSON] et al. [97] investigated the bioherbicidal activity of EOs from _Nepeta menthoides_ [PERSON], _N. mahanensis_ Jamzad & [PERSON], _N. eymaitia_ [PERSON], and _N. binaludensis_ [PERSON] on two weed species: wild mustard (_Sinapis arvensis_) and winter wild oat (_Avena ludoviciana_ [PERSON]). Although wild mustard was more sensitive to the _Nepeta_ EOs than winter wild oat, the study concluded that _Nepeta_ EOs have phytotoxic effects and could be used as a bioherbicide to control both weeds tested. The diverse nature of monoterpenoids, coupled with their wide range of biological activities and proven phytotoxic effects, highlights their significant potential for further investigation and the development of monoterpenoid-based formulations as effective bioherbicides in sustainable agriculture.