Evidence of the Photosynthetic Origin of Monoterpenes Emitted by Quercus ilex L. Leaves by 13C Labeling

Abstract

l h e carbon of the four main monoterpenes emitted by Quercus ilex L. leaves was completely labeled with 13C after a 20-min feeding with 99% "CO,. This labeling time course is comparable with the labeling time course of isoprene, the terpenoid emitted by other Quercus species and synthesized in leaf chloroplasts. It is also comparable with that of phosphoglyceric acid. Our experiment therefore provides evidence that monoterpenes emitted by Q. ilex are formed from photosynthesis intermediates and may share the same synthetic pathway with isoprene. By analyzing the rate and the distribution of labeling in the different fragments, we looked for evidence of differential carbon labeling in the a-pinene emitted. However, the labeling pattern was quite uniform in the different fragments, suggesting that the carbon skeleton of the emitted monoterpenes comes from a unique carbon source.

<sup>13</sup>C labelling study of constitutive and stress-induced terpenoide missions from Norway spruce and Scots pine

“…This finding supports results from previous studies on isoprene and monoterpenes from deciduous species (Loreto et al, 1996;Kreuzwieser et al, 2002;Schnitzler et al, 2004; Biogeosciences Discuss., https://doi.org /10.5194/bg-2017-260 Manuscript under review for journal Biogeosciences Discussion started: 4 July 2017 c Author(s) 2017. CC BY 4.0 License.…”

“…3). In Ghirardo et al (2010), they considered the uncomplete labelling of de novo monoterpenes by assuming that 13 _ of de novo monoterpenes is equal to that for the pure de novo isoprene emission, since monoterpenes share the same carbon sources as isoprene (Delwiche and Sharkey, 1993;Loreto et al, 1996). We determined the values of 13 _ 25 0.62-0.77 for de novo monoterpenes and 0.13-0.68 for sesquiterpenes, indicating their assumption of equal 13 _ may not hold for other terpenoids than isoprene and monoterpenes.…”

“…We assumed that instantaneous mixing of 13 C-atoms and 12 C-atoms occurs throughout the carbon sources. Based on findings from previous studies (Delwiche and Sharkey, 1993;Loreto et al, 1996), we also assumed that all carbon atoms of the isoprene, monoterpene and sesquiterpene molecule skeleton have equal probability to be labelled by 13 C-atoms from the carbon 5 source. The predicted relative ion intensity ( _ ) of the isotope mass M + i (i = 0, 1…n) of the compound can be calculated by a binomial distribution (Karl et al, 2002) (Eq.…”

“…For monoterpenes, the study of Loreto et al (1996) shows that the constitutive monoterpenes from Holm oak (Quercus ilex L.) are mainly de novo emissions and share the same carbon sources as isoprene. Ghirardo et al (2010) Biogeosciences Discuss., https://doi.org /10.5194/bg-2017-260 Manuscript under review for journal Biogeosciences Discussion started: 4 July 2017 c Author(s) 2017.…”

“…13 CO 2 labelling experiments have been also used to determine the fraction of de novo biosynthesis (Shao et al, 2001;Ghirardo et al, 2010). By feeding plants with 99 % 13 CO 2 , most of the isoprene molecules, as well as those of monoterpenes emitted directly from de novo biosynthesis, are fully or partly labelled with 13 C-atom(s) and thus can be identified as de novo emissions (Delwiche and Sharkey, 1993;Loreto et al, 1996).…”