• non-photochemical quenching;
  • photosynthesis;
  • photosystem II;
  • tabtoxin;
  • transpiration;
  • variable and maximum fluorescence

Inoculation of Nicotiana benthamiana leaves with virulent and avirulent strains of Pseudomonas syringae pv. tabaci resulted in increasing changes in Fv/Fm, inline image and NPQ over time. Images of these chlorophyll a fluorescence measurements revealed different changes in different zones of the leaf. For the virulent strain, the infiltrated zone and zone directly surrounding it showed decreased Fv/Fm, inline image and NPQ before the appearance of visible symptoms, and these decreases corresponded with increasing bacterial populations and putative tabtoxin activity. Another distinct zone of reduced Fv/Fm and NPQ extended several centimetres from the lesion to the nearest leaf margin, but only very low bacterial populations and no putative tabtoxin activity were detected in this zone. For the avirulent strain, a hypersensitive response occurred, bacterial populations remained low, and there was little detectable putative tabtoxin activity. Decreased Fv/Fm and NPQ, but not inline image, were observed in the infiltrated zone prior to the hypersensitive response, followed by decreased values in a zone directly surrounding it. Following that, no further changes were observed. These results demonstrate that in addition to detecting pre-symptomatic impacts of bacteria, chlorophyll a fluorescence imaging can also show that there are highly distinct regions of affected tissue that can extend considerably beyond the area of bacterial colonization. This should be considered in selecting leaf tissues for examining the effects of pathogens on plants, such as altered host gene expression or protein levels.