Isoprene synthase activity and its relation to isoprene emission in Quercus robur L-leaves

Pedunculate oak (Quercus robur L.) is known as a strong isoprene (2-methyl- 1,3-butadiene) emitter. Diurnal changes in isoprene emission were determine d by branch enclosure measurements. In contrast to the diurnal cycle in emi ssion rates, specific isoprene synthase activity in the leaves remained unc hanged. Based on in vitro enzyme activity and its temperature dependency, a n 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% reductio n of light intensity by shading of single branches reduced isoprene synthas e activity by approximate to 60% compared with full sunlight. The calculati on of isoprene synthesis capacities based on enzymatic data obtained under optimum reaction conditions, corrected for actual leaf temperature and rela ted to leaf surface area, provides a sound basis for predicting the isopren e emission potential of plants.