Pedunculate oak (Quercus robur L.) is known as a strong isoprene (2-methyl-1,3-butadiene) emitter. Diurnal changes in isoprene emission were determined by branch enclosure measurements. In contrast to the diurnal cycle in emission rates, specific isoprene synthase activity in the leaves remained unchanged. Based on in vitro enzyme activity and its temperature dependency, an isoprene synthesis capacity at specific leaf temperatures was calculated. The comparison of these 'leaf temperature-dependent enzyme capacities' and the measured emission rates revealed that the enzyme activity of isoprene synthase is comparable to the observed isoprene emission rates. In addition, variation in the isoprene synthase activity of the leaves due to changes in light intensity during leaf development was investigated. A 50% reduction of light intensity by shading of single branches reduced isoprene synthase activity by ≈ 60% compared with full sunlight. The calculation of isoprene synthesis capacities based on enzymatic data obtained under optimum reaction conditions, corrected for actual leaf temperature and related to leaf surface area, provides a sound basis for predicting the isoprene emission potential of plants.

Biogenic emission of hydrocarbons plays an important role in the interactions between plants, especially trees, and the atmosphere. Among these volatile organic compounds isoprene (2-methyl-1.3-butadiene) is the predominant component emitted by many photosynthesizing leaves. Its rapid atmospheric breakdown substantially affects the oxidation potential of the atmosphere. An enzyme, isoprene synthase. extracted from leaves of European oak (Quercus robur L.) was previously found to catalyse the Mg 2+-dependent elimination of diphosphate from dimethylallyldiphosphate to form isoprene. The present paper describes the seasonal variation of this enzyme acitivity in Quercus robur (L.) leaves in 1995.The enzymatic data obtained were used to create an additional term for the isoprene emission algorithm (ISOG93). The addition of this correction term for the seasonality of isoprene synthase to the emission model improved considerably the simulation of seasonal isoprene emission rates in oaks, avoiding over-and underestimations in the current modeling approach.

The emission rate of the volatile reactive compound isoprene, emitted predominantly by trees, must be known before the level of photo-oxidants produced during summer smog can be predicted reliably. The emission is dependent on plant species and local conditions, and these dependencies must be quantified to be included in any empirical algorithm for the calculation of isoprene production. Experimental measurements of isoprene emission rates are expensive, however, and existing data are scarce and fragmentary. To overcome these difficulties, it is promising to develop a numerical model capable of precisely calculating the isoprene emission by trees for diverse ecosystems, even under changing environmental conditions. A basic processbased biochemical isoprene emission model (BIM) has therefore been developed, which describes the enzymatic reactions in leaf chloroplasts leading to the formation of isoprene under varying environmental conditions (e.g. light intensity, temperature). Concentrations of the precursors of isoprene formation, 3-phosphoglyceric acid and glyceraldehyde 3-phosphate, are provided by a published light fleck photosynthesis model. Specific leaf and enzyme parameters were determined for the pedunculate oak (Quercus robur L.), so that the BIM is capable of calculating oak-specific isoprene emission rates as influenced by the leaf temperature and light intensity. High correlation was observed between isoprene emission rates calculated by the BIM and the diurnal isoprene emission rates of leaves measured under controlled environmental conditions. The BIM was even capable of describing changes in isoprene emission caused by midday depression of net photosynthesis.

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Biogenic emission of isoprene (2‐methyl‐1,3‐butadiene) by many plant species plays an important role in atmospheric chemistry. Its rapid breakdown in the atmosphere substantially affects the oxidation potential of the atmosphere. Leaves of Quercus petraea were found to contain an enzyme which catalyses the conversion of dimethylallyl pyrophosphate (DMAPP) to isoprene. A standard enzyme assay was established and the isoprene synthase activity was characterized in purified leaf extracts. Optimum enzyme activity was observed at pH 8.5. The enzyme had an apparent Km of 0.97 mM for its substrate DMAPP, but isopentenyl pyrophosphate (IPP), the isomeric form of DMAPP, was not converted to isoprene by the enzyme extract. The temperature optimum of the enzyme activity was 35 °C. Isoprene synthase activity was strictly dependent on the presence of bivalent cations, with magnesium being most effective. Molecular weight determination by FPLC revealed the presence of a single protein with a native molecular weight of approximately 90–100 kDa.