A unifying conceptual model for the environmental responses of isoprene emissions from plants

Abstract

Background and AimsIsoprene is the most important volatile organic compound emitted by land plants in terms of abundance and environmental effects. Controls on isoprene emission rates include light, temperature, water supply and CO2 concentration. A need to quantify these controls has long been recognized. There are already models that give realistic results, but they are complex, highly empirical and require separate responses to different drivers. This study sets out to find a simpler, unifying principle.MethodsA simple model is presented based on the idea of balancing demands for reducing power (derived from photosynthetic electron transport) in primary metabolism versus the secondary pathway that leads to the synthesis of isoprene. This model's ability to account for key features in a variety of experimental data sets is assessed.Key resultsThe model simultaneously predicts the fundamental responses observed in short-term experiments, namely: (1) the decoupling between carbon assimilation and isoprene emission; (2) a continued increase in isoprene emission with photosynthetically active radiation (PAR) at high PAR, after carbon assimilation has saturated; (3) a maximum of isoprene emission at low internal CO2 concentration (ci) and an asymptotic decline thereafter with increasing ci; (4) maintenance of high isoprene emissions when carbon assimilation is restricted by drought; and (5) a temperature optimum higher than that of photosynthesis, but lower than that of isoprene synthase activity.ConclusionsA simple model was used to test the hypothesis that reducing power available to the synthesis pathway for isoprene varies according to the extent to which the needs of carbon assimilation are satisfied. Despite its simplicity the model explains much in terms of the observed response of isoprene to external drivers as well as the observed decoupling between carbon assimilation and isoprene emission. The concept has the potential to improve global-scale modelling of vegetation isoprene emission.

Photochemical reflectance index as an indirect estimator of foliar isoprenoid emissions at the ecosystem level

“…5), fitted well with our hypothesis that isoprenoid emissions could be remotely sensed using PRI at the leaf level. This remote sensing capacity is based on the relationships that exist between isoprenoid emissions and LUE as a result of the greater availability of photosynthetic reducing power for isoprenoid production under lower LUEs, that is under a higher excess of photosynthetically active radiation (PAR) that is not used for fixing carbon 12,21,22 and based on the fact that PRI is already widely tested as a good estimator of LUE at the leaf, canopy and ecosystem levels [23][24][25]27 . The better fit of LUE with percentages of isoprenoid emissions relative to the maximum than with absolute values (Figs 2,3) warrants further efforts to develop the standardization of the signal for different species, ecosystems and conditions by gaining the necessary knowledge on the scaling physiological and structural processes involved.…”

“…This excess absorbed energy is available for isoprenoid production 12,21,22 . As these pigment changes translate into changes in reflectance at 531 nm, and as a reflectance of 570 nm is instead insensitive to short-term changes in these pigments, the PRI was defined as (R 531 À R 570 )/(R 531 þ R 570 ), where R indicates reflectance and numbers indicate wavelength in nanometres 23,24 .…”

“…However, these estimates remain unsatisfactory 11 . Current efforts are now being made to base the modelling on a fundamental understanding of plant biology 11,12 , nevertheless, uncertainty remains high 3,11,12 . For example, MEGAN isoprene flux estimates were within a factor of 2 above-canopy fluxes measured over a growing season in northern Michigan 3 .…”

“…We assume a negative relationship between light use efficiency (LUE; here calculated as the ratio between measured net photosynthetic rates and incident photosynthetic photon flux density (PPFD) in mol CO 2 mol per photons) and isoprenoid emissions as a result of a higher availability of photosynthetic reducing power and substrate for isoprenoid production under lower LUEs 12,21,22 . The photochemical reflectance index (PRI), which is calculated as (R531 À R570)/(R531 þ R570), where Rn is the reflectance of the leaf at n nm is a good estimator of LUE at the leaf, canopy and ecosystem levels [23][24][25][26][27] .…”

Seasonality of isoprenoid emissions from a primary rainforest in central Amazonia

“…Furthermore, as the canopy structure might vary seasonally due to leaf phenology/demography, the pattern of light penetration/absorption and then leaf temperature may change as well; thus, this, together with the differences in emissions among species and among leaf ontogenetic stages, could have an important impact on seasonal changes of local emissions. Besides the effects of light, temperature and leaf phenology/demography, some efforts have been made to include effects of CO 2 variation (Arneth et al, 2007; as well as the link between photosynthesis and emission (Grote et al, 2014;Morfopoulos et al, 2013Morfopoulos et al, , 2014Unger et al, 2013) into isoprene emission models at regional and global scales. However, the current regional and global BVOC emission models predict much smaller seasonal variations (Guenther et al, 2006Müller et al, 2008;Unger et al, 2013) compared to the measurements in Amazonia (Table 1).…”