Trees have various mechanisms for avoiding and mitigating biotic and abiotic stresses. Resin is one such mechanism, and it is essential for conifer trees. Conifer resin is also a large pool of monoterpenes that – similarly to other biogenic volatile organic compounds (BVOCs) produced by plants, e.g. methanol, acetone and acetaldehyde – play important roles in tree signalling and atmospheric chemistry once emitted to ambient air. BVOC emissions from various tree parts and resin dynamics depend on environmental variables, with intrinsic effects on conifer defence.
This thesis aims to clarify the environmental and physiological drivers of resin dynamics and BVOC emissions from the shoots and stem of mature boreal Scots pines (Pinus sylvestris) in field conditions, with special attention given to the effect of tree water relations. Resin pressure dynamics were studied using pressure transducers and BVOC emissions using an online mass spectrometer and dynamic chamber system. Resin and monoterpene emission compositions were analysed based on gas chromatography measurements.
Temperature explained resin pressures and BVOC emissions from both the shoots and stems of Scots pine in the short term. Over a longer period, resin pressures and stem monoterpene emissions decreased with decreasing soil water availability and water potential in stem. In addition, the emission dynamics of water-soluble acetaldehyde, methanol, and acetone from the shoots and stem were connected to transpiration rate and soil water content, indicating an important effect of their transport in the xylem sap.
These results show that although often overlooked, tree stems are an important source of BVOCs and that even relatively small changes in water availability may alter BVOC and resin dynamics despite their strong short-term temperature control. This information may help to understand the potential susceptibility of conifer trees to biotic stresses in various environmental conditions and improve BVOC emission modelling by accounting for stem emission dynamics
Plants synthesise thousands of biogenic volatile organic compounds (BVOCs) as part of their secondary metabolism. Scots pine (Pinus sylvestris) particularly produces mono- and sesquiterpenes, which are mainly stored in oleoresin in resin ducts. In this study, the monoterpene emission rate from stems was found to increase as a function of increasing resin pressure, which was positively correlated with the air temperature and foliage transpiration rate.
Monoterpene synthase activity describes the maximum monoterpene production potential. The seasonal cycle and needle age were observed to explain the majority of the variation in needle monoterpene synthase activities, monoterpene storage pools and monoterpene emissions from shoots. Variation in the monoterpene concentration between seasons, different needle age classes and different trees was observed to be minor. Monoterpene synthase activity was higher in <1-year-old needles compared to older ones. Within a single tree, the compound-specific composition of monoterpene synthase activities and monoterpene storages was not reflected in the composition of emissions. For example, the share of δ-3-carene was substantially higher in the emissions than in the storage pools and synthase activities.
An automated enclosure measurement system including a proton transfer reaction mass spectrometer was utilized to follow the VOC emissions from the woody compartments of trees over several years. This was the first study to quantify such emissions for an extended period. Scots pine stems were observed to emit monoterpenes and methanol into the ambient air. The fluxes displayed a seasonal cycle: methanol emissions were highest in the midst of the growing season, whereas monoterpene emissions peaked not only on the hottest summer days, but also in the spring when the photosynthetic capacity of trees recovered. The emissions of some monoterpenes exhibited distinct diurnal patterns in their enantiomeric compositions. The above-canopy air terpene concentrations reflected the emission rates from trees, the atmospheric reactivities of the compounds, the tree species composition of the measurement site and the abundances of different tree chemotypes.