Introduction

Terpenoids constitute the largest and most diverse class of chemical substances that play major roles in plant primary and secondary metabolisms (Gershenzon & Dudareva, 2007; Nagel, Berasategui, Paetz, Gershenzon & Schmidt, 2014; Trapp & Croteau, 2001). Two spatially separated pathways exist for the biosynthesis of terpenoids; the mevalonate (MVA) pathway takes place in cytosol and peroxisomes, whereas the methylerythritol phosphate (MEP) pathway is active in plastids (Dudareva, Klempien, Muhlemann, & Kaplan, 2013; Nagegowda, 2010). Both pathways form the C5 compounds isopentenyl diphosphate (IPP) and its allylic isomer, dimethylallyl diphosphate (DMAPP), which act as precursors for the biosynthesis of all terpenoids (McGarvey & Croteau, 1995). Whereas the MVA pathway produces sesquiterpenes (C15), irregular terpenes and geranyllinalool, the MEP pathway forms hemiterpenes (C5), monoterpenes (C10), diterpenes (C20) and volatile carotenoid derivatives (McGarvey & Croteau, 1995; Muhlemann, Klempien & Dudareva, 2014). Terpenoids can be stored in specialized structures such as resin ducts in many conifers (Gershenzon & Croteau, 1991; Wu & Hu, 1997) or directly emitted into the environment (Loreto et al., 2001). In storing plants, terpenoids usually make up 1-2% of plant dry weight, in some special cases shares of up to 15-20% were observed (Blanch, Penuelas, Sardans & Llusia, 2009). Terpenoids exert key ecological functions for plants, such as defence against predators, pathogens or competitors, inter- and intraspecific communication as well as protection against abiotic stress (Gershenzon & Dudareva, 2007; Loreto & Schnitzler, 2010; Loreto, Pollastri, Fineschi & Velikova, 2014; Martin, Gershenzon & Bohlmann, 2003; Vickers et al., 2009).
According to present knowledge, there are two sources driving terpenoid emission from leaves of plants. (i) De novo biosynthesis controls emission of isoprene and terpenoids from non-storing plants such asQuercus ilex . Production of these compounds is tightly coupled to photosynthesis and therefore shows similar dependencies on environmental parameters such as light availability and temperature (Ghirardo et al., 2010; Loreto et al., 1996; Loreto, Nascetti, Graverini & Mannozzi, 2000; Tingey, Manning, Grothaus & Burns, 1979). On the other hand, (ii) the release of leaf-internally stored terpenoids is purely temperature dependent because temperature controls the corresponding saturation vapour pressures of the emitted compounds (Ghirardo et al., 2010; Guenther, Zimmerman, Harley, Monson & Fall, 1993; Wu et al., 2017).
As a typical conifer, Norway spruce (Picea abies ) stores large amounts of terpenoids in resin ducts of various tissues, which are at least partially produced in the epithelial cells of the resin ducts and mesophyll tissue (Schürmann, Ziegler, Kotzias, Schönwitz & Steinbrecher, 1993). 13C-labelling approaches demonstrated that about one third of the emitted monoterpenoids in spruce are derived from de novo biosynthesis whereas the remainder is released from storage pools in a temperature depended manner (Ghirardo et al., 2010; Grabmer et al., 2006). Recent studies provided indirect hints that besides de novo biosynthesis and/or release from storage pools, xylem-transported terpenoids might contribute to emission from Norway spruce needles. Bourtsoukidis et al. (2012, 2014) observed that sesquiterpene (SQT) emission from Norway spruce strongly correlated with relative humidity. At high relative humidity - and consequently reduced transpiration rates - SQT emission was slowed down and vice versa when relative humidity was below 50% (Bourtsoukidis et al., 2012). In agreement with this finding, Filella, Wilkinson, Llusia, Hewitt & Peñuelas (2007) demonstrated that the emissions of most volatile organic compounds including monoterpenes from Norway spruce were well related to stomatal conductance and transpiration. This effect may in part be explained by a contribution of the tree’s sap flow to terpenoid emissions. Moreover, Bäck et al. (2012) found the largest SQT pools in Pinus sylvestris not in needles but below the bark, where SQTs constitute a toxic barrier for bark beetles and function as stress defense agents. Such terpenes might partially be released into the xylem sap. This assumption is in consistence with observations of Kuroda (1991) who observed a strong link between xylem cavitation in pine wilt disease infected Pinustrees and terpene abundance in the xylem. The author hypothesizes that cavitation is caused by higher terpene content in the xylem sap and the thereby diminished adhesion forces. A strong induction of terpene biosynthesis in the developing xylem by different stresses has also been demonstrated in Norway spruce (Martin, Tholl, Gershenzon & Bohlmann, 2002).
In this study, we aim at clarifying whether or not terpenoids are transported in the xylem sap of Norway spruce. Moreover, we aimed to obtain information on possible sources of xylem-transported terpenoids. In addition, we tried to find hints if xylem-transported terpenoids might contribute to terpenoid emission from Norway spruce needles.Therefore, we analyzed terpenoid emission rates as well as terpenoid contents and compositions in root, bark and wood of the same Norway spruce trees for comparison with xylem sap terpenoids.