The emission factor of volatile isoprenoids: stress, acclimation, and developmental responses

The rate of constitutive isoprenoid emissions from plants is driven by plant emission capacity under specified environmental conditions (<i>E</i><sub>S</sub>, the emission factor) and by responsiveness of the emissions to instantaneous variations in environment. In models of isoprenoid emission, <i>E</i><sub>S</sub> has been often considered as intrinsic species-specific constant invariable in time and space. Here we analyze the variations in species-specific values of <i>E</i><sub>S</sub> under field conditions focusing on abiotic stresses, past environmental conditions and developmental processes. The reviewed studies highlight strong stress-driven, adaptive (previous temperature and light environment and growth CO<sub>2</sub> concentration) and developmental (leaf age) variations in <i>E</i><sub>S</sub> values operating at medium to long time scales. These biological factors can alter species-specific <i>E</i><sub>S</sub> values by more than an order of magnitude. While the majority of models based on early concepts still ignore these important sources of variation, recent models are including some of the medium- to long-term controls. However, conceptually different strategies are being used for incorporation of these longer-term controls with important practical implications for parameterization and application of these models. This analysis emphasizes the need to include more biological realism in the isoprenoid emission models and also highlights the gaps in knowledge that require further experimental work to reduce the model uncertainties associated with biological sources of variation.