Infection of plants with pathogens leads not only to the induction of defence reactions but also to changes in carbohydrate metabolism. In this study, the effects of infection by a virulent and an avirulent strain of P. syringae on spatio-temporal changes in photosynthesis were compared using chlorophyll fluorescence imaging. The maximum PSII quantum yield, effective PSII quantum yield and nonphotochemical quenching were decreased in Arabidopsis leaves infected with either strain. At the same time, the quantum yield of nonregulated energy dissipation was increased. These changes could be detected by chlorophyll fluorescence imaging before symptoms were visible by eye. The effects were restricted to the vicinity of the infection site and did not spread to uninfected areas of the leaf. Qualitatively similar changes in photosynthetic parameters were observed in both interactions. Major differences between the responses to both strains were evident in the onset and time course of changes. A decrease in photosynthesis was detectable already at 3 h only after challenge with the avirulent strain while after 48 h the rate of photosynthesis was lower with the virulent strain. In contrast to photosynthesis, the regulation of marker genes for source/sink relations and the activities of invertase isoenzymes showed qualitative differences between both interactions. Inoculation of the virulent but not the avirulent strain resulted in downregulation of photosynthetic genes and upregulation of vacuolar invertases. The activity of vacuolar invertases transiently increased upon infection with the virulent strain but decreased with the avirulent strain while extracellular invertase activity was downregulated in both interactions.