Differentiation between de novo synthesized and constitutively released terpenoids from Fagus sylvatica
Introduction
Plants emit a wide range of volatile hydrocarbons into the atmosphere (Fehsenfeld et al., 1992). Numerous studies implicate that monoterpenes, next to isoprene, are the dominant class of released compounds (Guenther et al., 1995). Although the role of volatile terpenoids in secondary metabolism still represents a largely debated issue, models have been proposed describing for example the isoprene release as a means of controlling energy flow in plants (Sharkey, & Singsaas, 1995). The important role of volatile terpenoids in tritrophic plant–insect interactions is well established Dicke, & Sabelis, 1988, Turlings, Tumlinson, & Lewis, 1990. Pare and Tumlinson showed that, in cotton, the volatiles induced by insect herbivore damage are synthesized de novo with little or no release from storage, an observation that was interpreted as an active channelling of energy towards the release of volatiles as a defensive response (Pare, & Tumlinson, 1997a). The induced terpenoids showed significant higher incorporation levels of than those which were released constitutively. Recently, Loreto et al. gave evidence for the photosynthetic origin of released monoterpenes from Quercus ilex by labelling (Loreto et al., 1996b). Terpenoids released from Picea abies plants were partly enriched with isotope label whereas analysis of endogenous terpenoids in the needles indicate that even after a 24 h exposure to 2 there was no incorporation of (Schürmann, Ziegler, Kotzias, Schönwitz, & Steinbrecher, 1993).
In this study we used labelling to distinguish whether the volatile terpenoids from Fagus sylvatica are emitted directly after CO2 assimilation depending on photosynthetic activity (connected with a rapid turnover of carbon) or are emitted from preformed intermediates which are stored in different compartments of the plant (McGarvey, & Croteau, 1995). Recently, we have shown that the rate of emission of certain terpenes from Fagus sylvatica is closely coupled to the rate of biosynthesis (Schuh et al., 1997). Emissions of sabinene, the main compound emitted from Fagus sylvatica, was shown to be light-depended and decreased below the detection limit in darkness whereas emission of another monoterpene, limonene, did not change in darkness (Table 1).
Here we present results, focussing on the monoterpenes α-pinene, sabinene and limonene, which give evidence that different carbon sources are used for the synthesis of the released terpenes and that the contribution of the sources vary depending on light condition and the particular terpenoid structure.
Section snippets
Results
After 66 min of exposure to 2 a fraction of the fragments in the mass spectrum of the emitted sabinene showed complete incorporation of into the carbon skeleton (Fig. 1). The high intensity of m/z 100 indicates that all carbons in the seven-carbon-fragment (C7H9+) are replaced by . Similarly, m/z 130 as well as m/z 146 could only result if all carbons from the nine-carbon-fragment and the ten-carbon fragment were . The rapid shift of all fragments to higher masses shown in Fig. 1
Discussion
Two different carbon sources exist for the de novo biosynthesis of monoterpenes in the leaves of Fagus sylvatica. For sabinene the de novo synthesized intermediates supply all of the carbon inside the monoterpene-skeleton (whereby 90% originates from recently assimilated CO2, 10% from preformed intermediates) whereas 100% of the limonene molecules seem to come from a monoterpene pool. This is the first demonstration for intact plants, which gives a strong evidence that plant volatile terpenoid
Plant material
6 year old Fagus sylvatica plants were from commercial supplier. Plants were grown outside in 5 l pots in commercial soil under natural light conditions. Plants were watered the last time 12 h before the experiment.
Gas exchange and 2 administration
Three 6 year old Fagus sylvatica plants were placed 48 h before the experiment in a controlled plant enclosure chamber. The chamber allows the simulation of typical ambient conditions. The wall materials (PFA and a specific glass material (SANALUX©)) transmit the PAR and partly UV
Acknowledgements
The authors thank the Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie for financial support within in the research program: “cycles of trace constituents”.
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