## 1 Introduction
Thus, the VOCs released by plants have certain key ecological functions. They participate in plant-to-plant communication [13] and have an impact on the growth, disease resistance, dormancy, enzyme activity, photosynthesis, respiration, and the content of reactive oxygen species [12; 14; 15]. They can also be used to enhance weed biological control [16]. The largest group of such VOCs are terpen, comprising isoprene, monoterpenes, and sesquiterpen. Isoprene is associated with combatting abiotic stress,while the mono- and sesqui-terpenes are primarily responsible for chemical plant-to-plant communication [13]. Plants react by sensing environmental stimuli. One of the most abundant groups of such chemical stimuli are plant-associated bacteria, which are part of the plant microbiome, comprising all microorganisms. Plants can acquire early warning information through their microbiome and plant-plant signals such as volatiles. The perception of VOCs generates intra- and inter-cellular signals that induce changes in a plant's physiology and metabolism [17]. In this way, the plant is connected to its plant-associated microbial community (microbiome), which has been shown to play an important role in plant-to-plant communication. However, a different set of VOCs is secreted by the plant during infection by pathogenic bacteria or fungi, and those volatiles play an important role in a plant's defense system [18]. Sensing the environmental stimuli is also a way for a plant to react to herbivores, with and without physical contact. Plants affected by a herbivore infestation produce VOCs, which induce microbiome adaptation in receiver plants, eliciting the defense response and improving the plant's fitness against stresses [17]. A plant's response to a pest attack is a subject of considerable interest in the agricultural industry [12; 14; 17; 19]. Herbivore attack induces plants to produce specific herbivore-associated plant volatiles, that not only prime a plant's defense mechanism, but also influence the behavior of a plant's herbivores and natural enemies, by making them more susceptible or more resistant to parasitoids and pathogens [19]. The mutual interaction between plants and herbivores is a complex process involving constant modifications in the chemical responses between insects and plants. Insects utilize plants for oviposition and as larval food sources in order to complete their life cycle. In response, plants utilize various mixtures of VOCs to improve their defense system and to help overcome the damages caused by the insects [20]. Plants use VOCs as a primary mechanism to repel rather than resist herbivore attacks [21]. In the event of an herbivore attack, plants defend themselves by using a set of VOCs specific for the particular herbivore. It has been shown, for example, that for the _Solanum tuberosum_ (potato), the chewing Colorado potato beetle (_Leptinotarsa decemlineata_) and the piercing-sucking green peach aphid (_Myzus persicate_) pests induce clearly different volatile signatures [22]. Such studies prove that the VOC emissions influence plant resistance in the field and modulate the herbivore fitness, which can be used for pest control and ecology-based approaches in sustainable agriculture [19]. Within the VOCs there is a group of chemical compounds called the green leaf volatiles (GLVs) that are produced constantly at low levels by almost all unstressed plants [23], and immediately in high levels during mechanical damage and under biotic and abiotic stresses [14]. They also have direct toxic effects on bacteria and fungi, induce defense mechanisms of plants in the vicinity, and can be used to attract or repel herbivores and their natural enemies, which has been explored in detail by [PERSON] et al. [14]. An example of such studies is the use of the Y-tube olfactometer for testing the behavior of herbivores to various sets of VOCs released by plants because of herbivore attack, which are called herbivore-induced plant volatiles (HIPVs).