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 isoprene emission rate, IDP isomerase (E.C.5.3.3.2) and isoprene synthase activities as well as photosynthetic parameters of pedunculate oak (Quercus robur L.) - a dominant isoprene emitter of Central Europe - was investigated in two independent field experiments in September 1998 and 1999. In 1998 the pathogenic attack by mildew (Microsphaera alphitoides) caused serious visible damages of the leaves which were accompanied by a 50% reduction of net assimilation rate and total leaf chlorophyll content. IDP isomerase activity, isoprene synthase activity and standard isoprene emission factors were reduced by 70-80% under these conditions. In 1999 the powdery mildew infection of the oak leaves was weaker with no chlorotic and senescent leaf areas, although all infected leaves were totally covered by mycelium. In addition, none of the physiological parameters measured was significantly influenced by the Microsphaera infection. The results demonstrate that with respect to the widespread occurrence of powdery mildew on oaks during wet summer periods in some years the isoprene emission potential of oaks can be strongly affected by a serious infection with Microsphaera. This influence has to be considered for a realistic calculation of isoprene fluxes from forest vegetation.