Detection of plant volatiles after leaf wounding and darkening by proton transfer reaction "time-of-flight" mass spectrometry (PTR-TOF).
Bottom Line: In the strong isoprene-emitter Populus alba, light-dependent isoprene emission was sustained and even enhanced for hours after photosynthesis inhibition due to leaf cutting.Thus isoprene emission can uncouple from photosynthesis and may occur even after cutting leaves or branches, e.g., by agricultural practices or because of abiotic and biotic stresses.This observation may have important implications for assessments of isoprene sources and budget in the atmosphere, and consequences for tropospheric chemistry.
Affiliation: Ionicon Analytik G.m.b.H., Innsbruck, Austria.
Proton transfer reaction-time of flight (PTR-TOF) mass spectrometry was used to improve detection of biogenic volatiles organic compounds (BVOCs) induced by leaf wounding and darkening. PTR-TOF measurements unambiguously captured the kinetic of the large emissions of green leaf volatiles (GLVs) and acetaldehyde after wounding and darkening. GLVs emission correlated with the extent of wounding, thus confirming to be an excellent indicator of mechanical damage. Transient emissions of methanol, C5 compounds and isoprene from plant species that do not emit isoprene constitutively were also detected after wounding. In the strong isoprene-emitter Populus alba, light-dependent isoprene emission was sustained and even enhanced for hours after photosynthesis inhibition due to leaf cutting. Thus isoprene emission can uncouple from photosynthesis and may occur even after cutting leaves or branches, e.g., by agricultural practices or because of abiotic and biotic stresses. This observation may have important implications for assessments of isoprene sources and budget in the atmosphere, and consequences for tropospheric chemistry.
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Mentions: In D. glomerata only a burst of GLVs (Fig. 4a) but no acetaldehyde emission (Fig. 4d) was measured after light-dark transitions. The transient emission of GLVs was characterized first by a strong and sharp peak of hexenals (ion m/z = 81.070), with smaller peaks attributable to fragments of hexyl acetate (m/z = 43.018) and pentanol (m/z = 71.086); sequentially the emissions of hexenols and hexanal (ion m/z = 83.085) and hexenyl acetate (ion m/z = 143.107) were observed (Fig. 4a). In P. alba, light-dark transitions triggered the burst of GLVs (Fig. 4b), but the emission was associated with a strong release of acetaldehyde (m/z = 45.054) and a drop of isoprene emission (m/z = 69.070) (Fig. 4e) as shown by Graus et al. (2004) . Q. ilex showed a constitutive monoterpene emission in illuminated leaves, as measured by both the intensities of the fragment m/z = 81.070 (Fig. 4c) and of the protonated molecular ion m/z = 137.133 (Fig. 4h). Monoterpene emission declined quickly after the light was switched off, whereas, similarly to P. alba, a burst of GLVs (but only hexenyl acetate, Fig. 4g) and acetaldehyde (Fig. 4f) was also observed in oak leaves upon darkening. The complete drop of photosynthesis and the reduction of stomatal conductance induced by darkening are also presented for the single species (see Fig. 4, g, h, i). Finally, light-dark transition did not stimulate any BVOCs emission in the leaves of the monoterpene-storing species C. limon (data not shown). The constitutive level of emitted isoprenoids (isoprene by poplar or monoterpenes by oak) was directly associated to the maximum value of acetaldehyde detected during the burst (Fig. 5). When comparing the total BVOC emitted by the three species, the emissions of the three species were significantly different. The highest emission was observed in P. alba and the lowest in Q. ilex (Table 2). In any case, the emission of GLV after the light-dark transition was 3 to 4 order of magnitude lower than that of BVOCs measured after wounding (compare Tables 1 and 2). When considering the blend characteristics, the emission of P. alba leaves after darkening was characterized by a high percentage of hexenols + hexenal (m/z = 83.085). On the other hand, half of the low GLV emission of Q. ilex was made of hexenyl acetates (m/z = 143.107).