Monoterpene emission and monoterpene synthase activities in the Mediterranean evergreen oak Quercus ilex L. grown at elevated CO2 concentrations

Monoterpene emissions, monoterpene synthase activities, photosynthesis, fluorescence yield in the dark and drought stress indicators (stomatal conductance and mid‐day water potential) were concurrently measured under similar temperature and illumination in current‐year leaves of Quercus ilex L. of plants grown in open‐top chambers at ambient (350 ppm) and elevated (700 ppm) CO₂. The study was undertaken to understand the effect of CO₂ on monoterpene biosynthesis, and to predict and parameterize the biogenic emissions at growing CO₂ concentrations. The results of the 1998 and 1999 studies show that at elevated CO₂, and in the absence of persistent environmental stresses, photosynthesis was stimulated with respect to ambient CO₂, but that the emission of the three most abundantly emitted monoterpenes (α‐pinene, sabinene and β‐pinene) was inhibited by approximately 68%. The enzyme activities of the monoterpene synthases catalysing the formation of the three monoterpenes were also inhibited at elevated CO₂ and an excellent relationship was found between monoterpene emission and activity of the corresponding enzyme both at ambient and elevated CO₂. Interestingly, however, limonene emission was enhanced in conditions of elevated CO₂ as it was also the corresponding synthase. The ratio between enzyme activity and emission of the three main monoterpenes was high (above 20) at ambient CO₂ but it was below 10 at elevated CO₂ and, for limonene, on both treatments. Our results indicate that the overall emission of monoterpenes at elevated CO₂ will be inhibited because of a concurrent, strong down‐regulation of monoterpene synthase activities. When the enzyme activity does not change, as for limonene, the high photosynthetic carbon availability at elevated CO₂ conditions may even stimulate emission. The results of the 1997 study show that severe and persistent drought, as commonly occurs in the Mediterranean, may inhibit both photosynthesis and monoterpene (α‐pinene) emission, particularly at ambient CO₂. Thus, emission is probably limited by photosynthetic carbon availability; the effect of elevated CO₂per se is not apparent if drought, and perhaps other environmental stresses, are also present.