Isoprene emission represents a significant loss of carbon to those plant species that synthesize this highly volatile and reactive compound. As a tool for studying the role of isoprene in plant physiology and biochemistry, we developed transgenic tobacco plants capable of emitting isoprene in a similar manner to and at rates comparable to a naturally emitting species.Thermotolerance of photosynthesis against transient high-temperature episodes could only be observed in lines emitting high levels of isoprene; the effect was very mild and could only be identified over repetitive stress events. However, isoprene-emitting plants were highly resistant to ozone-induced oxidative damage compared with their non-emitting azygous controls. In ozone-treated plants, accumulation of toxic reactive oxygen species (ROS) was inhibited, and antioxidant levels were higher. Isopreneemitting plants showed remarkably decreased foliar damage and higher rates of photosynthesis compared to nonemitting plants immediately following oxidative stress events. An inhibition of hydrogen peroxide accumulation in isoprene-emitting plants may stall the programmed cell death response which would otherwise lead to foliar necrosis. These results demonstrate that endogenously produced isoprene provides protection from oxidative damage.

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 C6-VOCs. Under anoxic conditions, acetaldehyde was the only compound released after switching off the light. This clearly indicated that hexenal and other C6-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.

The spectrum and diurnal course of carbonyl exchange of mature Norway spruce ( Picea abies ) was analysed in a temperate forest and under controlled conditions. In parallel, plant physiological and meteorological parameters were determined. • Spruce emitted acetaldehyde, formaldehyde and acetone under field and laboratory conditions. • Carbonyl emissions were highest at midday, for acetaldehyde amounting up to 100 nmol m − 2 min − 1 . During darkness uptake was also observed. Fumigation of spruce seedlings with acetaldehyde indicated a compensation point of c . 6 ppb. The exchange rates were strongly correlated with temperature and mass flow of ethanol in the xylem sap. The studies further indicated that the height of a twig on the tree affects its carbonyl emission rates. • The present findings support the view that acetaldehyde emission by spruce is related to mass flow of ethanol in the xylem sap, as previously shown for tree seedlings under controlled conditions. The basis of formaldehyde and acetone emissions by spruce is still not clear and remains to be studied in further experiments.

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