*2.2. VOCs and Weed Control*

Allelopathy gain extensive attention in biological weed control [3–6,71]. Boydston et al. [3] found that mustard seed meal has the potential for weed control in organic production systems. The release of volatile organic compounds from the leaves can cause allelopathic effects and damage the growth of other competitive plants [72]. The allelopathy of the volatile allelochemicals may perform a vital part in inhibiting the competitive ability of weed species, be one of the alternatives to control weed infestation, and it has excited the greatest interest [67,73–75]. Therefore, the best way to control weeds is to use the crops' own competition and allelopathy. Wei et al. reported that the volatile oil released by *Atriplex cana* Ledeb. significantly inhibited the growth of seedlings of four weed species, including *Amaranthus retroflexus* L. and *Poa annua* L., and it had a high value for further use as a biological herbicide [65].

Brassicas produce the allelochemicals glucosinolates throughout their plant parts and released them into the environment by volatilization [76]. In the natural environment, glucosinolates are broken down into several compounds, and the most important compound of them is isothiocyanate [77], which can inhibit the growth and development of plant or weed [78]. *Digitaria sanguinalis* is a common non-irrigated weed that severely affects crop yields. Pardo-Muras et al. showed that the oxygenated monoterpenes in VOCs produced and released by both *Ulex europaeus* and *Cytisus scoparius* inhibited the germination and early growth of two weeds, *A. retroflexus* and *D. sanguinalis* [79]. Many studies have reported that in addition to independent effects of VOCs, VOCs also have

synergistic or antagonistic effects. These synergistic and antagonistic effects lead to the final allelopathy [80,81].

Besides, VOCs from invasive species can also reduce interspecific performance and improve the performance of invasive species [82]. VOCs might also be perceived by neighboring plants to adjust their defensive phenotype according to the present risk of attack. Allelochemicals as natural herbicide have an attractive prospect, and some plants may be expected to develop into a new generation of herbicides or fungicides.
