Transient release of oxygenated volatile organic compounds during light-dark transitions in Grey poplar leaves

Abstract

In this study, we investigated the prompt release of acetaldehyde and other oxygenated volatile organic compounds (VOCs) from leaves of Grey poplar [Populus x canescens (Aiton) Smith] following light-dark transitions. Mass scans utilizing the extremely fast and sensitive proton transfer reaction-mass spectrometry technique revealed the following temporal pattern after light-dark transitions: hexenal was emitted first, followed by acetaldehyde and other C(6)-VOCs. Under anoxic conditions, acetaldehyde was the only compound released after switching off the light. This clearly indicated that hexenal and other C(6)-VOCs were released from the lipoxygenase reaction taking place during light-dark transitions under aerobic conditions. Experiments with enzyme inhibitors that artificially increased cytosolic pyruvate demonstrated that the acetaldehyde burst after light-dark transition could not be explained by the recently suggested pyruvate overflow mechanism. The simulation of light fleck situations in the canopy by exposing leaves to alternating light-dark and dark-light transitions or fast changes from high to low photosynthetic photon flux density showed that this process is of minor importance for acetaldehyde emission into the Earth's atmosphere.

Figure 1.

VOC emission (a) and gas exchange (b) of intact poplar leaves following rapid light-dark transitions. During the time indicated by the gray background, the light was switched off. One typical sequence out of 12 independent experiments is shown.

Figure 2.

Effect of cutting a poplar leaf on VOC release (a) and gas exchange (b). At the time indicated, the leaf was excised and the cut end of the petiole placed in a solution. One typical sequence out of five independent experiments is shown.

Figure 3.

VOC emission (a) and gas exchange (b) of poplar leaves following light-dark transitions under anoxic conditions. At the time indicated by the arrow, the cuvette atmosphere was switched to anoxic conditions (N₂, 360 ppm CO₂). Later the light was switched off (gray bar) and, after 20 min, the light was turned on again and the atmosphere switched back to aerobic conditions (arrow). One typical sequence out of three independent experiments is shown.

Figure 4.

Effects of enzyme inhibitors on acetaldehyde emission of poplar leaves. At the times indicated, the leaf was excised and the cut petiole placed in a solution containing either 0.65 mm of the ALDH inhibitor disulfiram, 1 mm of the PDH inhibitor acetylphosphinate, or both inhibitors. Acetaldehyde emissions after 2 h of inhibitor treatment were determined using DNPH-coated silica gel cartridges. Means ± sd of five independent experiments are shown. Statistically significant differences at P < 0.05, as calculated by Tukey's test under ANOVA, are indicated by different letters.

Figure 5.

Effects of anoxia and the ALDH inhibitor disulfiram on isoprene emission of poplar leaves. An excised poplar leaf was placed into a cuvette and was then fed with 0.65 mm disulfiram (a); fed via the transpiration stream, with 0.65 mm disulfiram under normoxic conditions and then switched to anoxic conditions (b); and exposed to anoxic conditions (N₂, 360 ppm CO₂) and then switched to normoxic conditions (c). Changes of conditions are indicated by vertical lines. One typical experiment out of three is shown.

Figure 6.

Influences of alternating light-dark transitions and dark-light transitions on VOC release (a) and gas exchange (b) of poplar leaves. At times indicated by gray bars, the light was switched off (0 μmol m⁻² s⁻¹ PPFD) and switched on (1,400 μmol m⁻² s⁻¹) again. Duration of dark/light periods: 23 s (dark)/23 s (light) (min 107–114); 46 s (dark)/23 s (light) (min 126–138). One typical sequence out of three independent experiments is shown.

Figure 7.

Influence of changes from high to low light intensities on VOC release (a) and gas exchange (b) of poplar leaves. As indicated by gray bars, PPFD was reduced (395 μmol m⁻² s⁻¹) and increased again (1,400 μmol m⁻² s⁻¹). In a second series, PPFD was decreased to 95 μmol m⁻² s⁻¹ PPFD. Each dark/light phase lasted for 46 s. One typical sequence out of three independent experiments is shown.

Figure 8.

Possible pathways responsible for the production of acetaldehyde by the leaves of trees. Acetaldehyde can be derived from the oxidation of xylem-transported ethanol (Kreuzwieser et al., 1999), from the decarboxylation from pyruvate (pyruvate overflow mechanism; Karl et al., 2002a), and as hypothesized in this study from conversion of acetyl-CoA during light-dark transitions and wounding. Enzymatic catalyzed reactions inhibited in this study are indicated by crosses (ALDH, PDH complex).