Plant VOC emissions: making use of the unavoidable.

All plants emit substantial amounts of phytogenic volatile organic compounds (PVOCs), which include alkanes, alkenes, alcohols, aldehydes, eters, esters and carboxylic acids. Defence, communication and/or protection against extreme conditions have been proposed as reasons for these emissions. However, Rosenstiel and colleagues have recently proposed that emission of PVOCs represents a metabolic 'safety valve' by preventing the unnecessary sequestration of phosphates. Additionally, Niinemets and colleagues suggest that the emission rates of some PVOCs are determined by the principal physicochemical characteristics of the emitted compounds, such as their solubility, volatility and diffusivity, rather than by physiological mechanisms, such as their synthesis rates. These two new studies lead to the hypothesis that there is not necessarily a specific role for every PVOC emitted, given that their emission is unavoidable as result of their volatility. However, in some cases, natural selection has worked to take advantage of this volatility.