PHOTOSYNTHETIC ELECTRON-TRANSPORT IS DIFFERENTIALLY AFFECTED DURING EARLY STAGES OF CULTIVAR RACE-SPECIFIC INTERACTIONS BETWEEN POTATO AND PHYTOPHTHORA-INFESTANS/

The influence of the early stages of fungal infection on chloroplast m etabolism was studied in cultivar/ race-specific interactions between potato (Solanum tuberosum L. cv. Datura) and the late-blight fungus Ph ytophthora infestans. The accumulation of several mRNAs encoding compo nents of the photosynthetic apparatus was not affected, either in comp atible or in incompatible interactions, However, within 3 h after inoc ulation of potato leaves with fungal spores, a change in the photochem istry of photosystem II was detectable by measuring chlorophyll a fluo rescence. Characteristic fluorescence parameters, such as maximum fluo rescence yield (F-m), variable fluorescence yield (F-v) and photochemi cal efficiency (F-v/F-m), were specifically reduced in the compatible host/pathogen interaction. Analyses of photochemical and nonphotochemi cal fluorescence quenching showed an increase in the photochemical fra ction. The amounts of two selected thylakoid membrane proteins and of total chlorophyll remained unchanged during this process, suggesting t hat the functional modification of the electron-transport system was n ot correlated with a change in the composition of the photosynthetic a pparatus. The alterations of photosynthetic electron transport represe nt a rapidly detectable and sensitive physiological marker for compati ble interactions in the potato/Phytophthora infestans pathosystem.