*2.1. VOCs and Plants Growth*

Numerous results showed that VOCs have an effect on plant cell growth and differentiation [50–53], such as diallyl disulfide (DADS) of garlic VOCs, which can affect mitotic activity and cell length of tomato roots by impacting on cell division, endogenous plant hormone levels, expansin gene expression, and sulfate assimilation and glutathione (GSH) metabolism [53,54].

Similarly, many studies confirm that plant volatiles can inhibit seed germination and the growth of root and seedling [50,55–60]. β-terpineol, linalool, eugenol, and tetradecanoic acid are the VOCs released from tomato (*Solanum lycopersicon* Mill.) foliage, and they could inhibit seed germination of the tropical plant *Amaranthus mangostanus* L. [55]. The brassica species exude allelochemicals, which are glucosinolates [61] that could break down into several biological action compounds, such as isothiocyanates, which have biologically active and inhibit germination and growth of exposed plant species [57,62]. VOCs released from pine needles and the roots of *Pinus halepensis* L. mainly inhibited the seed germination and root growth of two herbaceous target species *Lactuca sativa* L. and *Linum strictum* L. [63]. VOCs affect plant growth, mainly to change plant morphology and reduce plant biomass. Therefore, seed germination, seedling root length, and seedling height are often used as intuitive indicators to evaluate allelopathy. Low concentrations of DADS promoted the growth of cucumber roots and induce elongation of the main roots by up-regulating the expression of CsCDKA and CsCDKB genes and regulating the hormone balance of the roots [64]. The VOCs released by *Atriplex cana* Ledeb. (Amaranthaceae) significantly inhibited seedling growth of *Amaranthus retroflexus* L. and *Poa annua* L., and 5 μg/mL essential oil completely inhibited the seed germination of *A. retroflexus*, *Medicago sativa* L., *P. annua*, and *Echinochloa crusgalli* L. [65]. Besides, Effah et al. found that plant VOCs mediate multiple ecological networks, and they may mediate the allelopathic effects of the germination or growth of competitors seeds [66]. Monoterpenoids are considered as effective inhibitors of seed germination and seedling growth [67,68].

VOCs affect plant growth, mainly to inhibit plant growth, but some studies have found that VOCs have dual effects on germination and plant growth, both promoting and inhibiting; for example, Arroyo et al. found that volatile chemicals from *Artemisia herba-alba* Asso. inhibited the germination of *Pinus halepensis* Mill. seeds, promoted the growth of *P. halepensis* seedlings, and reduced the root biomass of *Salsola vermiculata* L. seedlings [69]. VOCs also have an effect on plants growth direction. Runyon et al. found that *Cuscuta pentagona* seedlings were favored by not only the growth of host tomato plants, but also the direction of tomato VOCs [70]. The extracted VOCs of tomato and wheat were placed on both sides of *C. pentagona* seedlings, and the *C. pentagona* seedlings continued to grow in the direction of tomato VOCs. It was also found that β-phellandrene and β-myrcene of tomato VOCs could significantly attract the growth of *C. pentagona* seedlings.
