Figure 3.

Plant-derived VOCs are the active signals. Volatile compounds of BTH do not elicit airborne resistance induction (A), while VOCs emitted by biologically induced plants do (B). A, Bacterial spot numbers per leaflet are depicted for plants to which BTH was directly applied, for the respective water-treated controls (Direct/Plant), and for plants exposed to plant-free pots where BTH or water was applied to the soil (Indirect/Soil). B, Bacterial spot numbers on plants that had been induced with the hrp(−) mutant P. syringae 61-18 prior to infection with P. syringae on plants that had been exposed to induced neighbors (Indirect) and on controls that were infected without prior treatment. Bars represent means ± se. Sample size was n = 5 plants per treatment, and different letters indicate significant differences among treatments (P < 0.05 according to lsd). The experiment was repeated three times with similar results.