2.7.1. Chemical Communications

Plants, similar to animals, do not exist in isolation. Plant individuals and populations maintain population relationships and resist external stress through chemical communication. VOCs volatilized from some plants are involved in plant–plant communication. Through more than 20 years of research, it has been found that when plants were stressed by insect feeding, microbial infection, and mechanical damage, they could use volatile organic substances to carry out inter-chemical and intra-chemical chemical communication [125,126]. Plants sent VOCs signals under attack or stress, then neighboring plants received these VOCs signals directly or indirectly to turn on the chemical defense mechanisms and produce phenolic alkaloids chemical defense compounds including directly terpenoids and other defense substances. Indirect chemical defense the use of VOCs to attract the natural enemies of pests, through the methods of predation and parasitism to eliminate pests. For example, the VOCs signals released by injured *A. tidentata* could induce direct chemical defense by protease inhibitors in *Nicotiana attenuate*, and the VOC signals released by maize leaves when insects fed could induce plants to rapidly release monoterpenoids that attracted insect predators for indirect defense [127,128]. Wild lima bean quickly synthesized and secreted excess nectar to attract natural enemies after receiving VOC signals from plants that have been harmed by foraging [129]. In recent years, more and more research has gone deep into the mechanism of action of these chemical communication signaling molecules. Baldwin et al. [125] found when in response to the attack, infested leaves released (E)-β-ocimene (typical signaling chemicals of volatile terpenoids) to increase the resistance of un-infested leaves and induce the expression of defense-related genes in neighboring un-infected leaves [130]. In future research, based on the clarification of the biochemical and metabolic processes of the phytochemical signal substances that have been discovered, the response mechanism of the recipient plants to these signal substances should be further explored. Therefore, although the research of phytochemical communication faces huge challenges, the clarification of phytochemical identification and communication relationships will broaden the horizons of the interspecific and intraspecific relationships of plants.
