Long-term dynamics of monoterpene synthase activities, monoterpene storage pools and emissions in boreal Scots pine

Seasonal variations in monoterpene emissions from Scots pine (Pinus sylvestris) are well documented, and emissions are often shown to follow the incident temperatures due to effects on compound volatility. Recent studies have indicated a link between monoterpene emissions and physiological drivers such as photosynthetic capacity during needle development. The complex interplay between the dynamic changes in the biosynthetic capacity to produce monoterpenes and the temperature-dependent evaporation process of volatiles from internal storage reservoirs has not yet been studied under field conditions. In this study, we analysed the relationships between needle monoterpene synthase activities, endogenous monoterpene storage pools and monoterpene emissions of needles in two consecutive years at a boreal forest site in Finland. The results showed changes in the monoterpene synthase activity of needles, linked to seasonality and needle ontogenesis, while the pool of stored monoterpenes (about 0.5% of dry weight) did not change considerably as a function of needle aging. Monoterpene emissions did not correlate directly with enzyme activity or the storage pool size. We observed notably high plant-to-plant variation in the biosynthesis rates of individual monoterpenes, which did not reflect the storage compound mixture. The enzyme activity producing delta-3-carene was only present in the first months after needle flushing, and decreased with needle age, whereas delta-3-carene was abundant in the endogenous monoterpene pool and dominated the needle emissions. This study emphasizes the seasonal, developmental and intraspecific variability of monoterpene biosynthesis and storage, and calls for more in-depth analyses to reveal how such complex interaction affects monoterpene emissions from pine needles in boreal forests.Peer reviewe

Photosynthesis and carbon balance

The requirements for modelling photosynthesis and related processes within the framework of a Functional-Structural Plant Model (FSPM; cf. Vos et al. this volume) are discussed. A combined local gas (carbon dioxide, water vapour) and radiant energy exchange model (GREM) is presented, that was specified to be embedded into an FSPM. The model accounts for the effect of organ nitrogen content (N) on gas exchange expressing certain key model parameters as functions of N. This approach enables the model also to account for the observed effects of growth conditions and organ development on gas exchange, since such effects could to a large part be ascribed to concurrent changes in N. The GREM was parameterized for leaf blades of barley (Hordeum vulgare L.) plants. The combined FSPM-GREM system was successfully applied in simulation studies providing reliable predictions of (1) diurnal time courses of carbon dioxide and water vapour exchange of leaf blades and (2) overall carbon balance and dry-mass accumulation of barley plants during ontogenesis