3.1.1  Constitutive release of infochemicals as information sources
Plants continuously release a profile of volatile organic compounds (VOCs) and non-VOCs to the root and atmosphere. These chemical profiles, concentrations, and release time courses function as signals of plant presence and health status. Constitutive release of specific chemical signatures can be exploited by neighbor organisms to determine the presence and identity of neighboring plants. Receiver plants recognize the degree of phylogenetical relationship of neighbor plants to discriminate kin and non-kin plants (Biedrzycki, Jilany, Dudley & Bais, 2010, Yang, Li, Xu & Kong, 2018). In most cases, signals from healthy plants can affect neighboring plants by modifying biomass allocation, lateral root formation, and chlorophyll and phenolic compound biosynthesis, or directly inhibiting seed germination (Delory, Delaplace, Fauconnier & du Jardin, 2016, Takigahira & Yamawo, 2019) . The sagebrush volatile compounds methyl jasmonate, camphor, 1,8-cineol, α-thujone, and nerol inhibit the germination of Nicotiana attenuate seeds (Jassbi, Zamanizadehnajari & Baldwin, 2010). Exposing potato to healthy onion changes the profile of potato volatiles is repellent to an aphid pest and attractive to the aphid predator ladybird beetle (Ninkovic, Dahlin, Vucetic, Petrovic-Obradovic, Glinwood & Webster, 2013, Vucetic, Dahlin, Petrovic-Obradovic, Glinwood, Webster & Ninkovic, 2014). (E )-nerolidol and (3E ,7E )-4,8,12-trimethyl-1,3,7,11-tridecatetraene were the primary effective volatiles released by exposed potato plants (Vuceticet al. , 2014).
Some plant infochemicals are released primarily by healthy plants and can be considered as a marker of plant health status. Isoprene emission levels are linked to plant health and photosynthetic efficiency. The reduction of isoprene emission after pest and disease attacks is considered as a marker for plant stress (Copolovici, Kannaste, Pazouki & Niinemets, 2012, Jiang, Ye, Veromann & Niinemets, 2016, Toome, Randjärv, Copolovici, Niinemets, Heinsoo & Luik, 2010). Isoprene alleviates plant oxidative stress (van Doorn, Merl-Pham, Ghirardo, Fink, Polle, Schnitzler & Rosenkranz, 2020).