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21 páginas, 6 figuras. ; The rate of constitutive isoprenoid emissions from plants is driven by plant emission capacity under specified environmental conditions (ES, the emission factor) and by responsiveness of the emissions to instantaneous variations in environment. In models of isoprenoid emission, ES has been often considered as intrinsic species-specific constant invariable in time and space. Here we analyze the variations in species-specific values of ES 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 CO2 concentration) and developmental (leaf age) variations in ES values operating at medium to long time scales. These biological factors can alter species-specific ES 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. ; The authors' studies on BVOC emissions have been funded by the Estonian Ministry of Education and Research (grant SF1090065s07), the Estonian Science Foundation (grant 7645), the US National Science Foundation and the US Environmental Protection Agency, the joint collaborative project between Spanish CSIC and the Estonian Academy of Sciences, the Spanish Government (grants CGL2006-04025/BOS and Consolider-Ingenio Montes CSD2008-00040), the Catalan government (grant SGR2009-458), the Human Frontier Science Programme, the Swedish Research Councils VR and Formas. ; Peer reviewed

61 páginas, 7 figuras, 1 tabla. ; In models of plant volatile isoprenoid emissions, the instantaneous compound emission rate typically scales with the plant's emission capacity under specified environmental conditions, also defined as the emission factor, ES . In the most widely employed plant isoprenoid emission models, the algorithms developed by Guenther and colleagues (1991, 1993), instantaneous variation of the steady-state emission rate is described as the product of ES and light and temperature response functions. When these models are employed in the in atmospheric chemistry modeling community, species-specific ES values and parameter values defining the instantaneous response curves are typically considered as constant. In the current review, we argue that ES is largely a modeling concept, importantly depending on our understanding of which environmental factors affect isoprenoid emissions, and consequently need standardization during ES determination. In particular, there is now increasing consensus that variations in atmospheric CO2 concentration, in addition to variations in light and temperature, need to be included in the emission models. Furthermore, we demonstrate that for less volatile isoprenoids, mono- and sesquiterpenes, the emissions are often jointly controlled by the compound synthesis and volatility, and because of these combined biochemical and physico-chemical properties, specification of ES as a constant value is incapable of describing instantaneous emissions within the sole assumptions of fluctuating light and temperature, as are used in the standard algorithms. The definition of ES also varies depending on the degree of aggregation of ES values in different parameterization schemes (leaf- vs. canopy- or region-level, species vs. plant functional type level), and various aggregated ES schemes are not compatible for different integration models. The summarized information collectively emphasizes the need to update model algorithms by including missing environmental and physico-chemical controls, and always to define ES within the proper context of model structure and spatial and temporal resolution. ; The authors' work on BVOC emissions has been supported by the Estonian Ministry of Education and Research (grant SF1090065s07), the Estonian Science Foundation (grant 7645), the US National Science Foundation and the US Environmental Protection Agency, the join collaborative project between Spanish CSIC and the Estonian Academy of Sciences, the Spanish Government (grants CGL2006-04025/BOS and Consolider-Ingenio BGD 7, 1233–1293, 2010 The emission factor of volatile isoprenoids U. Niinemets et al. ¨ Title Page Abstract Introduction Conclusions References Tables Figures J I J I Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Montes CSD2008-00040), the Catalan government (grant SGR2009-458), the Human Frontier Science Programme, the Swedish Research Councils VR and Formas. ; Peer reviewed