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, flu orescence yield in the dark and drought stress indicators (stomatal conduct ance and mid-day water potential) were concurrently measured under similar temperature and illumination in current-year leaves of Quercus ilex L. of p lants 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 monoterp ene 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 stress es, photosynthesis was stimulated with respect to ambient CO2, but that the emission of the three most abundantly emitted monoterpenes (alpha -pinene, sabinene and beta -pinene) was inhibited by approximately 68%. The enzyme activities of the monoterpene synthases catalysing the formation of the thr ee monoterpenes were also inhibited at elevated CO2 and an excellent relati onship was found between monoterpene emission and activity of the correspon ding enzyme both at ambient and elevated CO2. Interestingly, however, limon ene emission was enhanced in conditions of elevated CO2 as it was also the corresponding synthase. The ratio between enzyme activity and emission of t he three main monoterpenes was high (above 20) at ambient CO2 but it was be low 10 at elevated CO2 and, for limonene, on both treatments. Our results i ndicate that the overall emission of monoterpenes at elevated CO2 will be i nhibited because of a concurrent, strong down-regulation of monoterpene syn thase activities. When the enzyme activity does not change, as for limonene , the high photosynthetic carbon availability at elevated CO2 conditions ma y even stimulate emission. The results of the 1997 study show that severe a nd persistent drought, as commonly occurs in the Mediterranean, may inhibit both photosynthesis and monoterpene (a-pinene) emission, particularly at a mbient CO2. Thus, emission is probably limited by photosynthetic carbon ava ilability; the effect of elevated CO2 per se is not apparent if drought, an d perhaps other environmental stresses, are also present.