Online analysis of volatile organic compound emissions from Sitka spruce (Picea sitchensis)

Volatile organic compound (VOC) emissions from Sitka spruce (Picea sitchensis Bong.) growing in a range of controlled light and temperature regimes were monitored online with a proton transfer reaction–mass spectrometer (PTR–MS) operating at a temporal resolution of ∼1 min. Isoprene emissions accounted for an average of more than 70% of measured VOCs and up to 3.5% of assimilated carbon. Emission rates (E) for isoprene correlated closely with photosynthetic photon flux (PPF) and temperature, showing saturation at a PPF of between 300 and 400 μmol m⁻² s⁻¹ and a maximum between 35 and 38 °C. Under standard conditions of 30 °C and 1000 μmol m⁻² s⁻¹ PPF, the mean isoprene E was 13 μg g_dm⁻¹ h⁻¹, considerably higher than previously observed in this species. Mean E for acetaldehyde, methanol and monoterpenes at 30 °C were 0.37, 0.78 and 2.97 μg g_dm⁻¹ h⁻¹, respectively. In response to a sudden light to dark transition, isoprene E decreased exponentially by ≪ 98% over about 3 h; however, during the first 7 min, this otherwise steady decay was temporarily but immediately depressed to ∼40% of the pre-darkness rate, before rallying during the following 7 min to rejoin the general downward trajectory of the exponential decay. The sudden sharp fall in isoprene E was mirrored by a burst in acetaldehyde E. The acetaldehyde E maximum coincided with the isoprene E minimum (7 min post-illumination), and ceased when isoprene emissions resumed their exponential decay. The causes of, and linkages between, these phenomena were investigated.