Influence of powdery mildew (Microsphaera alphitoides) on isoprene biosynthesis and emission of pedunculate oak (Quercus robur L.) leaves

The influence of powdery mildew (Microsphaera alphitoides.) infection on is oprene emission rate, IDP isomerase (E.C.5.3.3.2) and isoprene synthase act ivities as well as photosynthetic parameters of pedunculate oak (Quercus ro bur L.) - a dominant isoprene emitter of Central Europe - was investigated in two independent field experiments in September 1998 and 1999. In 1998 th e pathogenic attack by mildew (Microsphaera alphitoides) caused serious vis ible damages of the leaves which were accompanied by a 50% reduction of net assimilation rate and total leaf chlorophyll content. IDP isomerase activi ty, isoprene synthase activity and standard isoprene emission factors were reduced by 70-80% under these conditions. In 1999 the powdery mildew infect ion of the oak leaves was weaker with no chlorotic and senescent leaf areas , although all infected leaves were totally covered by mycelium. In additio n, none of the physiological parameters measured was significantly influenc ed by the Microsphaera infection. The results demonstrate that with respect to the widespread occurrence of powdery mildew on oaks during wet summer p eriods in some years the isoprene emission potential of oaks can be strongl y affected by a serious infection with Microsphaera. This influence has to be considered for a realistic calculation of isoprene fluxes from forest ve getation.