**5. Prospective**

VOCs may have a wide range of potential allelopathic effects in agriculture. VOCs can not only manage weeds and pests as natural substance, but also regulate plant growth, competition, dormancy, respiration and photosynthesis, ROS content, enzyme activity, and diseases resistance. The most important thing is that they come from plants and meet our requirements for developing green agriculture. It has more economic value and is a more environmentally friendly and effective measure to use the allelopathic effects of plant natural products to develop agricultural industry. At the same time, there are some limitations in using allelopathy of VOCs of plants; for example, in field trials, VOCs are not easy to control. The volatilization of VOCs of plants is specific to species, cultivars, genotypes, and organs, as well as environments. In fact, the release of VOCs by plants are not single VOCs, but a complex mixture [150]. In agricultural production, the release of plant VOCs is affected by environmental and meteorological conditions, such as wind speed and direction, humidity and rain, and temperature, among others. These factors make the release of VOCs difficult to control. Moreover, the concentration of VOCs in open field experiments is often lower than in laboratory experiments. Therefore, the allelopathy of plant VOCs in agricultural production is more suitable for an easily controlled greenhouse.

With the development of allelochemicals identification technology and the participation of more and more chemists, the identification of allelochemicals is no longer a difficult problem in the study of allelopathy. However, the molecular mechanism of VOCs has not been studied, and it is not yet clear how VOCs are perceived by plants, and little is known about the dynamics of the active release pathway of VOCs of plants. Therefore, the focus of future research is to explore the nature and regularity of plant VOCs allelopathy and dynamic process of VOCs release, mainly to decipher the perception mechanism of VOCs within plant tissues. Over the past ten years, a large number of studies have proved that VOCs are involved in signal transduction among plants, and many allelochemicals involved in chemical communication have been identified. Now the problem we are facing is how the mechanism of chemical recognition and communication and the mechanism of allelopathy interacts. The general and specific chemical identification and the research on the identification and transfer mode of the communicating substance are urgent issues to be solved. Because the VOCs of plants are not easy to control, identifying the natural concentration of VOCs is also a major problem we face with.

At present, with increasing demands for environmental protection and sustainable development, VOCs have a dominant position in agricultural development and will become more competitive in the future, especially in the greenhouse. The theoretical research and practical application of allelopathy of plants VOCs have profound significance for sustainable development of agricultural production, for maintenance and improvement of natural resources, and for the rational arrangement of rotation and intercropping, the construction of efficient planting systems, and the improvement of natural resources' utilization efficiency, for the construction of efficient planting systems and rational arrangement of rotation and intercropping or controlling pests and weeds. Not only that, the use of allelopathy of plant volatiles can also affect plant life activity by regulating plant growth, dormancy, respiration and photosynthesis, ROS content, and enzyme activity, or by the chemical communication between plants. In summary, the allelopathy of VOCs of plants has inestimable potential in agricultural development. Based on this, it is of great significance to develop and practice the application potential of allelopathy of plant VOCs.

**Author Contributions:** Writing—review and editing, Y.X., X.H. and L.T.; funding acquisition, X.H. and Y.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was funded by the Youth Program of National Natural Science Foundation of China (No. 31601747), High-Level Talents Project of Hainan Natural Science Foundation (No. 320RC723), Innovation Team Project of Chinese Academy of Tropical Agricultural Sciences (No. 1630032017029), Departmental Budget Project of Ministry of Agriculture and Rural Affairs

of the People's Republic of China, and Major Science and Technology Project in Hainan Province (No. ZDKJ2017001).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Acknowledgments:** We acknowledge Ziji Liu for comments on a previous version of the manuscript.

**Conflicts of Interest:** The authors declare that they have no conflict of interest.
