| Summary: | The system <i>Nicotiana tabacum</i> L.—<i>Pseudomonas syringae</i> VAN HALL pv. <i>tomato</i> (Pto) DC3000 was investigated at a low inoculation level (c. 5 × 10<sup>5</sup> colony-forming units (CFU) mL<sup>–1</sup>) such as it occurs in the field. The aim of this study was to test the hypothesis that <i>N. tabacum</i>, a non-host of Pto DC3000, improved the PSII efficiency in inoculated leaves compared with control detached leaves. Visible symptoms at the infected area were not detected within 14 days. Chlorophyll (Chl) <i>a</i> fluorescence was measured 6–7 days after inoculation of detached leaves. Compared with the control, the actual photochemical quantum yield of photosystem (PS) II was higher in the inoculated leaves at the expense of the fraction of heat dissipated by photo-inactivated non-functional centers. In addition, the fraction of open PSII reaction centers (RCs) was higher in inoculated leaves. Maximum fluorescence in the dark-adapted detached inoculated leaves, as a measure of the absorbed energy, was lower than in control leaves. The lower capacity to absorb energy in combination with a higher fraction of open PSII RCs is interpreted as an acclimation to limit over-excitation and to reduce heat dissipation. This should limit the production of reactive oxygen species and reduce the probability of a hypersensitive response (HR), which represents an expensive cell-death program for the plant.
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