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The majority of biogenic volatile organic compounds (BVOCs) are emitted from terrestrial sources such as forests, grasslands, shrublands and croplands (Guenther et al., 1995; Peñuelas & Staudt, 2010). Isoprene is among the most abundant BVOCs emitted from vegetation (Sharkey & Yeh, 2001; Sharkey et al., 2008). Its share may reach up to 40% of total BVOC emissions, with estimated yearly totals of 440–660 Tg C yr−1 (Guenther et al., 2006). Previous studies have revealed that isoprene plays an important physiological role in protecting plants from biotic and abiotic stresses (Sharkey & Singsaas, 1995; Behnke et al., 2007; Loivamäki et al., 2008; Vickers et al., 2009; Velikova et al., 2011). In particular, dissipation of excess energy to protect the photosynthetic apparatus (Sanadze, 2010), stabilizing thylakoid membranes at high temperatures (Sharkey & Singsaas, 1995; Singsaas & Sharkey, 1998, 2000; Owen & Peñuelas, 2005; Behnke et al., 2007; Velikova et al., 2011), quenching of reactive oxygen species (Affek & Yakir, 2002; Sharkey et al., 2008; Vickers et al., 2009), and repelling herbivores (Loivamäki et al., 2008) have been reported.
Isoprene also plays a major role in tropospheric photochemistry and contributes to secondary organic aerosol formation, thereby potentially influencing large-scale Earth system processes (Fehsenfeld et al., 1992; Claeys et al., 2004; Hallquist et al., 2009). As a very reactive volatile compound, it strongly affects ozone and secondary organic aerosol formation in the troposphere (Williams et al., 1997; Fuentes et al., 2000; Kroll et al., 2005) and partly controls the lifetime of the glasshouse gas methane by its reaction with hydroxyl radicals (Kaplan et al., 2006).
Isoprene is formed in chloroplasts by isoprene synthase from its immediate precursor dimethylallyldiphosphate (DMADP) via the 1-deoxy-d-xylulose-5-phosphate (DOXP) pathway (Lichtenthaler et al., 1997) and a major part of its carbon skeleton is derived from recently assimilated photosynthates (Lichtenthaler, 1999; Affek & Yakir, 2003; Trowbridge et al., 2012). Thus, DMADP availability and isoprene synthase activity are key factors determining the isoprene emission rate (Calfapietra et al., 2008; Rasulov et al., 2009, 2010; Li et al., 2011), although the physiological regulation mechanisms of isoprene synthesis have still not been fully resolved. The instantaneous isoprene emission rate is strongly light- and temperature-dependent and this response is similar for different plant species. The instantaneous responses result from changes in the supply of intermediates to isoprene synthesis (Loreto & Sharkey, 1993; Schnitzler et al., 2004; Magel et al., 2006; Rasulov et al., 2009, 2010).
Over the long term, prevailing environmental conditions and leaf ontogeny affect the development of isoprene synthesis capacity (Kuzma & Fall, 1993; Sasaki et al., 2005; Loivamäki et al., 2007; Cinege et al., 2009; Niinemets et al., 2010b; Sun et al., 2012a). The isoprene emission capacity starts to develop just before full leaf photosynthetic competence, a pattern observed in velvet bean (Mucuna sp.; Kuzma & Fall, 1993; Harley et al., 1994) and aspen (Populus tremuloides; Monson et al., 1994). After reaching a maximum isoprene emission rate, the isoprene emission capacity starts to decline in senescing leaves (Kuhn et al., 2004; Sun et al., 2012a). These modifications are associated with changes in isoprene synthase gene expression and isoprene synthase protein content (Mayrhofer et al., 2005). Although isoprene synthase gene is ‘constitutively’ expressed, its promoter region contains circadian-, heat-, and stress-dependent elements, and the promoter activity depends on light and temperature over days to weeks (Loivamäki et al., 2007; Cinege et al., 2009).
A further important, and much less understood, driver that affects short- and long-term isoprene emissions is ambient [CO2]. Effects of growth [CO2] on isoprene emissions have been studied in different plant species under various experimental conditions with controversial outcomes. Elevated [CO2] had no or only a moderate effect on the isoprene emission capacity in Populus alba (Loreto & Velikova, 2001; Loreto et al., 2007), P. tremuloides (Calfapietra et al., 2008), and Populus × euramericana (Centritto et al., 2004), while elevated [CO2] resulted in enhanced isoprene emission capacity in Quercus rubra (Sharkey et al., 1991), Quercus pubescens (Tognetti et al., 1998), Gingko biloba (Li et al., 2009) and Populus tremula × P. tremuloides (Sun et al., 2012b). In other studies, elevated [CO2] led to a remarkable depression of isoprene emissions, including P. deltoides (Rosenstiel et al., 2003), Acacia nigrescens (Possell & Hewitt, 2011), Liquidambar styraciflua (Monson et al., 2007; Wilkinson et al., 2009), Populus tremuloides (Sharkey et al., 1991; Darbah et al., 2010), Eucalyptus globulus, P. tremuloides and P. deltoides (Wilkinson et al., 2009), Phragmites australis (Scholefield et al., 2004), and Platanus orientalis (Velikova et al., 2009). However, most studies on the inhibition of isoprene emission by elevated [CO2] were carried out at the leaf level and described mostly the response to instantaneously elevated [CO2], thereby mixing up the instantaneous CO2 response and the long-term acclimation response (see Sun et al., 2012b for a detailed discussion). In fact, the effects of elevated [CO2] on plants are multifaceted, involving instantaneous and acclimation metabolic responses at the leaf scale, and whole-plant processes such as acceleration of plant and leaf growth rates, leading to faster biomass accumulation, but also to alterations in stand development dynamics (Gielen et al., 2003; Rapparini et al., 2004; Arneth et al., 2007; Liberloo et al., 2007; Monson et al., 2007; Niinemets, 2010a). For constructing predictive models of isoprene emission under higher atmospheric [CO2], it is essential to consider plant acclimation and ontogeny.
Atmospheric [CO2] has been rising since the industrial revolution (Long et al., 2004; Rapparini et al., 2004; IPCC, 2007) and is predicted to continue to rise and to affect the global climate (Fuentes et al., 2000; Wiedinmyer et al., 2006). Yet, many models that predict isoprene emission from plants are based on empirical or semi-mechanistic algorithms (Guenther et al., 1993, 2006; Niinemets et al., 1999; Heald et al., 2009). These models usually utilize leaf-scale measurements and rely on meteorological input parameters as driving factors. To account for the effects of elevated [CO2], the models typically use an empirical parameterization based on measurements of instantaneous enhancements of [CO2] (Wilkinson et al., 2009). In several studies, it has been speculated that a possible increase in leaf area might cancel out the declining effect of instantaneous [CO2] elevation on isoprene emission (Rosenstiel et al., 2003; Centritto et al., 2004; Sun et al., 2012b). To our knowledge, this hypothesis has been tested with dense poplar stands at midseason when stand leaf area was the highest; in this study, leaf area increase at higher [CO2] moderated the leaf-level [CO2] effect by 15–50%, but did not fully offset the effect of reduced isoprene emission at leaf scale (Rosenstiel et al., 2003). However, at the canopy scale, the situation becomes further complicated by enhanced shading by increasing leaf area that might also reduce isoprene emission (see earlier). Thus, the stand-scale effect can strongly depend on ontogenetic characteristics. In rapidly developing more open stands, elevated [CO2] effects on leaf area can be more important than in fully closed stands exhibiting a steady-state leaf area index (LAI). Thus, for fast-growing stands, it is important to monitor the stand-level [CO2] response through the start of canopy development to closure.
In this study, we investigated carbon assimilation and isoprene emission in hybrid aspen (P. tremuloides × P. tremula) on canopy and leaf level from the start of canopy development to maturation under different ambient [CO2]. Our main aim was to test the hypothesis that the canopy isoprene emission of hybrid aspen grown under elevated [CO2] is increased even though an instantaneous effect of elevated [CO2] lowers isoprene emission at the level of individual leaves. We have previously demonstrated that growth under elevated [CO2] did not affect isoprene emissions when gauged under the same CO2 concentration (either ambient or elevated) at moderate-high light intensity (Sun et al., 2012b). Here we further use a modeling framework to quantitatively analyze the dynamics of canopy development among plants grown under current ambient and elevated [CO2].