Photosynthetic processes of sweet orange plants [Citrus sinensis (L.) Osbeck cv. Pera] infected with Xylella fastidiosa were investigated. Simultaneous measurements of leaf gas exchanges and chlorophyll a fluorescence were taken in healthy and infected leaves before development of symptoms. The photosynthetic oxygen evolution response to different light intensities and curves of photosynthesis induction were carried out in leaf discs extracted from the same leaves used for gas exchange measurements. Infected plants showed decreased CO2 assimilation, transpiration and stomatal conductance. No differences between healthy and infected intact leaves regarding chlorophyll a fluorescence parameters were observed, except for the potential quantum yield of photosystem II which was higher in infected plants. Values of photosynthetic oxygen evolution, effective quantum yield of photosystem II, apparent electron transport rate, and photochemical fluorescence quenching were higher in healthy leaf discs than in infected ones. Our results suggest that low photosynthetic rates of non-symptomatically sweet orange leaves infected with X. fastidiosa were caused by low stomatal conductance, biochemical alterations of photosynthetic machinery and increase in alternative electron sinks. (C) 2003 Elsevier Ltd. All rights reserved.