5.1.6. Phytohormones and plant volatiles

Plant hormones (also known as phytohormones) are chemicals that regulate plant growth, development and functioning. When stress is perceived, plants emanate volatile hormones such as ethylene, methyl jasmonate and methyl salicylate. This release can function as a stress signal between plant organs and between plants (Caparrotta et al. 2018). Enhanced stress hormones synthesis leads to numerous gene expression changes, and ultimately, to altered plant growth (Nemhauser et al. 2006; Vandenbussche et al. 2012). Ethylene and methyl jasmonate negatively affect plant growth; hence, they need to be avoided in biomass production systems. Especially because ethylene can induce the production of more ethylene. Aside from ethylene, plant produce many volatiles. Their function in the life of the plant is not always clear. As reviewed by Dudareva et al (2004), plant volatiles are known to serve as stress signal, have anti-microbial or anti-herbivore activities, and attract and guide pollinators to their flowers. Proper ventilation and the use of scrubbers can reduce the amount of plant volatiles in a protected plant cultivation system. In order to do research on plant volatiles in closed cultivation systems the inlet and outlet of the air circulation system should be measured. For proper measurement the air inlet should be hydrocarbon free, preferentially lower than 0.01 ppb, but certainly sub-ppb level.